Hypersensitivity reactions (types I to IV)

Adverse drug reactions represent a broad spectrum of unwanted clinical events that occur during medical treatment. These events are traditionally divided into two main categories based on their underlying predictability and mechanism. Type A reactions are predictable and related to an exaggeration of the normal pharmacological action of the drug^[1]. In contrast, Type B reactions, also known as hypersensitivity reactions, are unpredictable and not expected from the known pharmacological action of the agent^[1]. This distinction is fundamental in clinical practice, as Type B reactions require entirely different investigative and management strategies compared to simple dose-dependent toxicities.

Hypersensitivity reactions to contrast media are a prime example of this unpredictable Type B phenomenon. These reactions can occur either within the first 60 minutes following intravascular administration, which are classified as immediate reactions, or at a later time point, which are termed non-immediate reactions^[1]. In the context of medical imaging, the majority of these events are mild or moderate in severity, while severe reactions remain very rare, occurring in less than 1 in every 10,000 administrations^[1].

The classical framework for understanding these immune-mediated events is the Gell and Coombs classification, which divides hypersensitivity into four distinct types based on the effector mechanisms involved. Types I, II, and III are antibody-mediated, meaning they involve the production of specific immunoglobulins that drive the pathological response. Type IV is cell-mediated, relying on the activation of specific T-lymphocytes rather than circulating antibodies. It is crucial to recognise that while many immediate reactions are truly immunoglobulin E mediated, others may utilise entirely different pathways, such as direct receptor activation on innate immune cells^[1].

Understanding whether a reaction is immediate or non-immediate dictates the entire clinical pathway, from the choice of diagnostic tests to the selection of alternative agents for future procedures. A standardized risk assessment and close patient monitoring are crucial for all individuals undergoing procedures involving potential allergens^[1]. Furthermore, detailed documentation and correct classification of the severity of such reactions will help improve risk management in future clinical encounters^[1]. Regular hands-on training for all personnel involved in managing these reactions is mandatory to ensure rapid and appropriate intervention when unpredictable events occur^[1].

Type I hypersensitivity is the classic immediate allergic reaction, driven by immunoglobulin E antibodies. The pathophysiology begins with a sensitisation phase, during which the immune system encounters an allergen and produces specific immunoglobulin E antibodies. These antibodies then bind with high affinity to Fc epsilon RI receptors located on the surface of mast cells and basophils^[1]. The individual remains asymptomatic during this sensitisation period, but the effector cells are now primed and armed with allergen-specific receptors.

Upon re-exposure to the same allergen, the antigen cross-links the adjacent immunoglobulin E molecules on the cell surface^[1]. This cross-linking triggers a rapid intracellular signalling cascade that leads to immediate degranulation of the mast cells and basophils^[1]. The pre-formed mediators stored within the granules, most notably histamine, are released into the surrounding tissues and systemic circulation^[1]. This sudden release causes the classic clinical features of immediate hypersensitivity, including vasodilation, increased vascular permeability, smooth muscle contraction, and mucous gland secretion.

In addition to histamine, the activated cells rapidly synthesise and release newly formed lipid mediators, including leukotrienes and prostaglandins, as well as various cytokines. These secondary mediators prolong the inflammatory response and recruit additional effector cells, such as eosinophils and neutrophils, to the site of the reaction. The clinical manifestation depends on the route of exposure and the target organs involved. Systemic release of these mediators leads to anaphylaxis, characterised by cardiovascular collapse, bronchospasm, and widespread cutaneous manifestations.

Histamine H1-receptor activation specifically causes nasal congestion, flushing, headache, pruritus, as well as coronary and bronchial constriction^[1]. H1-antihistamines work by blocking these receptors on various effector cells, selectively counteracting the effects of histamine via competitive inhibition^[1]. However, because mast cells and basophils secrete various other active mediators that are not blocked by antihistamines, these drugs alone are insufficient to reverse the full spectrum of a severe systemic reaction.

The speed of onset in Type I reactions is typically within minutes, making it the most acute and potentially life-threatening of the four classifications. In the context of intravascular contrast media, immediate hypersensitivity reactions can present with varying degrees of severity, ranging from isolated mild urticaria to profound distributive shock. Recognising the rapid timeline and the underlying mast cell-driven mechanism is essential for initiating life-saving therapy, primarily through the administration of intramuscular adrenaline, which counteracts the profound vasodilation and bronchoconstriction caused by the released mediators.

Type II hypersensitivity reactions are driven by immunoglobulin G or immunoglobulin M antibodies directed against antigens that are intrinsic to the patient's own cells or extracellular matrix. Unlike Type I reactions, where the antibodies target soluble environmental allergens, Type II reactions involve antibodies binding directly to fixed tissue antigens or cell surface receptors. This binding initiates tissue damage through three primary pathophysiological mechanisms: opsonisation and phagocytosis, complement-mediated inflammation, and antibody-mediated cellular dysfunction.

In the first mechanism, the antibodies bind to the cell surface and act as opsonins, marking the cells for destruction by phagocytes in the spleen and liver. The Fc portion of the antibody is recognised by Fc receptors on macrophages, leading to engulfment and destruction of the targeted cell. This mechanism is classically seen in autoimmune haemolytic anaemia and immune thrombocytopenia, where red blood cells or platelets are inappropriately targeted for phagocytic clearance.

The second mechanism involves the activation of the classical complement pathway. When immunoglobulin G or immunoglobulin M binds to a cell surface antigen, it triggers the complement cascade, leading to the formation of the membrane attack complex. This complex creates pores in the target cell membrane, resulting in direct cell lysis. Additionally, the complement split products, particularly C3a and C5a, act as potent anaphylatoxins and chemotactic factors, recruiting neutrophils and macrophages to the site, which then release destructive enzymes and reactive oxygen species, causing significant collateral tissue damage.

The third mechanism occurs when antibodies bind directly to cell surface receptors, either stimulating or blocking their normal function without necessarily causing immediate cell death. This is known as antibody-mediated cellular dysfunction. A classic example is Graves disease, where antibodies stimulate the thyroid stimulating hormone receptor, leading to hyperthyroidism. Conversely, in myasthenia gravis, antibodies block or destroy acetylcholine receptors at the neuromuscular junction, resulting in profound muscle weakness.

Type II reactions typically develop over hours to days, depending on the rate of antibody production and the specific mechanism of tissue injury. The clinical presentation is highly variable and depends entirely on the specific cell type or tissue targeted by the autoantibodies. Diagnosis generally relies on detecting the presence of specific antibodies on the cell surface or in the circulation, often using direct immunofluorescence or specialised flow cytometry techniques. Treatment focuses on suppressing the underlying immune response, removing the pathogenic antibodies through plasmapheresis, or blocking the specific receptor interaction.

Type III hypersensitivity is characterised by the formation and deposition of immune complexes, which are aggregates of soluble antigens bound to immunoglobulin G or immunoglobulin M antibodies. The pathogenic potential of these complexes depends heavily on their size and the ratio of antigen to antibody. Large complexes are typically cleared rapidly by the mononuclear phagocyte system, while very small complexes fail to activate complement efficiently. It is the medium-sized complexes that are most pathogenic, as they circulate in the blood and eventually deposit in various tissues, particularly the walls of blood vessels, the glomerular basement membrane of the kidneys, and the synovial lining of joints.

Once deposited, these immune complexes activate the classical complement pathway. The generation of C3a and C5a attracts large numbers of neutrophils to the site of deposition. The neutrophils attempt to phagocytose the complexes but are often unable to do so because the complexes are embedded in the tissue matrix. This frustrated phagocytosis leads to the release of lysosomal enzymes, reactive oxygen species, and inflammatory cytokines, which cause severe local tissue destruction and vasculitis.

The clinical timeline for systemic Type III reactions, such as serum sickness, typically ranges from one to three weeks after initial exposure to the antigen, allowing sufficient time for antibody production and complex formation. In contrast, the Arthus reaction occurs much more rapidly, usually within four to twelve hours, in individuals who have already been sensitised and possess high titres of circulating precipitating antibodies.

The morphological hallmark of Type III hypersensitivity is necrotising vasculitis with a prominent neutrophilic infiltrate and fibrinoid necrosis of the vessel walls. Immunofluorescence microscopy typically reveals a granular pattern of immunoglobulin and complement deposition along the basement membranes, distinguishing it from the linear pattern seen in Type II reactions.

Classic examples of Type III hypersensitivity include systemic lupus erythematosus, where antibodies target nuclear antigens, leading to widespread multi-organ involvement. Rheumatoid arthritis also involves immune complex deposition in the synovial joints, driving chronic inflammation and joint destruction. Post-streptococcal glomerulonephritis is another well-known example, where complexes of streptococcal antigens and antibodies deposit in the glomeruli, causing acute renal impairment. Management involves treating the underlying cause, reducing inflammation with corticosteroids or immunosuppressants, and providing supportive care for the affected organs.

Type IV hypersensitivity is fundamentally different from the first three types because it is entirely cell-mediated and does not involve antibodies. Instead, the reaction is driven by the activation of specific T-lymphocytes, which can be either CD4 positive helper T-cells or CD8 positive cytotoxic T-cells. In the context of cutaneous non-immediate hypersensitivity reactions, an infiltrate in the dermis consisting of activated CD4 positive or CD8 positive T-cells and eosinophils is usually found^[1].

The pathophysiology involves two distinct phases. During the sensitisation phase, antigen-presenting cells process the offending agent and present it to naive T-cells in the regional lymph nodes, leading to the clonal expansion of memory T-cells. This phase is entirely asymptomatic and typically takes one to two weeks. During the elicitation phase, upon re-exposure to the antigen, the memory T-cells migrate to the site of exposure and release a cascade of cytokines, recruiting macrophages and other inflammatory cells, resulting in visible tissue damage.

Recent immunological research has identified specific pathways for antigen recognition in delayed reactions. The traditional hapten concept suggests that the offending agent undergoes uptake and processing by antigen-presenting cells, followed by presentation to T-cells via major histocompatibility complex class II molecules^[1]. Alternatively, the p-i concept involves the direct binding of unprocessed drug molecules to the T-cell receptor or major histocompatibility complex molecule^[1]. This hapten-independent pathway acts as a direct pharmacological interaction with immune receptors and helps explain cross-reactivity between structurally similar agents.

The most common clinical manifestation of Type IV hypersensitivity is a maculopapular exanthema, which usually occurs 2 to 10 days after first exposure to an agent and 1 to 2 days after re-exposure^[1]. Most of these skin reactions are mild to moderate in severity, are usually self-limiting, and resolve within 1 week^[1]. However, discrimination must be made between these mild reactions and the very rare severe cutaneous adverse reactions with danger signs, such as acute generalized exanthematous pustulosis, drug reaction with eosinophilia and systemic symptoms, generalized bullous fixed drug eruptions, Stevens-Johnson syndrome, and toxic epidermal necrolysis^[1].

Because the reaction relies on the clonal expansion and migration of T-cells, the onset of symptoms is inherently delayed, typically taking at least 48 to 72 hours to become clinically apparent after re-exposure. Diagnosis often relies on delayed reading of intradermal tests or patch testing, which directly assesses the local T-cell response in the skin. Treatment involves immediate withdrawal of the offending agent and the use of topical or systemic corticosteroids to suppress the T-cell driven inflammation.

The management of hypersensitivity reactions requires a clear understanding of the underlying mechanism, as treatments differ vastly between immediate and non-immediate events. For immediate reactions, intramuscular adrenaline 1 mg/mL remains the first-line drug in severe cases, and most guidelines recommend an adult dose of 0.5 mg administered intramuscularly^[1]. Adrenaline acts on alpha and beta-adrenergic receptors to reverse vasodilation, reduce mucosal oedema, and relieve bronchospasm. Because of the diminished frequency of these events and lack of experience, major guidelines recommend restricting adrenaline administration by non-experienced users to the intramuscular route^[1].

In addition to adrenaline, the most frequently used medications in Europe for immediate reactions include H1-antihistamines like clemastine or chlorpheniramine, short-acting beta-2 agonists like salbutamol, and occasionally corticosteroids^[1]. However, corticosteroids have a delayed onset of action and play no role in the acute management of immediate hypersensitivity after contrast media^[1]. There is no evidence from high-quality studies for the use of corticosteroids in the emergency management of severe immediate reactions^[1].

For non-immediate reactions, which are predominantly T-cell mediated, the traditional mainstay of treatment follows that of cutaneous reactions to other drugs: withdrawal of the agent combined with symptomatic treatment^[1]. More than 90 percent of non-immediate reactions involve the skin only^[1]. Usually, oral H1-antihistamines or corticosteroids, in combination with topical corticosteroid creams or foams, can be used to alleviate maculopapular exanthema reactions^[1]. Any patient with suspicion of a severe cutaneous adverse reaction should be promptly referred to a dermatologist or drug allergy specialist for diagnosis, treatment, and advice on future agent use^[1].

Diagnostic testing is crucial for identifying the specific culprit and selecting safe alternatives. The basophil activation test is a flow-cytometry-based cellular assay valuable in diagnosing immediate reactions, showing a specificity that exceeds 90 percent, although its sensitivity is typically only slightly above 50 percent^[1]. Skin tests, including prick and intradermal tests, are also employed, with intradermal tests being more sensitive than prick tests for diagnosis^[1]. For delayed reactions, delayed intradermal test readings are the most sensitive diagnostic tool, and patch tests are preferred for severe cutaneous adverse reactions^[1].

When preventing recurrent reactions, changing the culprit contrast medium has emerged as a superior alternative to premedication^[1]. Comparative studies have shown that an alternative agent was more effective than premedication, with the risk of recurrent hypersensitivity being 67.1 percent lower in cases where the implicated agent was changed^[1]. Premedication with corticosteroids and antihistamines does not significantly impact the risk of severe recurrent reactions^[1]. Therefore, selecting an alternative contrast medium based on the results of an allergy evaluation is safer and more effective than premedication for preventing recurrent hypersensitivity reactions^[1].

Understanding the epidemiology of hypersensitivity reactions is essential for risk stratification and clinical preparedness. Immediate hypersensitivity reactions to iodinated contrast media occur with frequencies of 0.15 to 0.69 percent, with severe reactions occurring in 0.005 to 0.013 percent of administrations^[1]. For gadolinium-based contrast agents, recent studies in large adult patient cohorts have shown low incidence rates of 0.06 to 0.17 percent, with severe reactions occurring in 0.003 to 0.006 percent^[1]. In the largest Korean cohort for gadolinium agents, the incidence was slightly higher, with 0.4 percent for immediate reactions and 0.04 percent for non-immediate reactions^[1].

The most consistent factor predicting a future hypersensitivity reaction to iodinated contrast media is a history of a previous reaction to the same class of agents^[1]. This prior history carries a massive odds ratio ranging from 27.6 to 198.8, a finding that is reflected in all major clinical guidelines^[1]. For gadolinium-based agents, a prior hypersensitivity reaction also carries the highest risk, resulting in a 12-fold higher frequency of recurrence^[1].

A history of other drug or allergic diseases has a lower and less consistent predictive value, with odds ratios ranging from 1.21 to 6.8 for iodinated agents^[1]. For gadolinium agents, a history of allergic diseases has a moderate predictive value with an odds ratio of 2.83^[1]. Numerous minor factors, such as female sex, age below 50 years, history of uncontrolled asthma or chronic urticaria, and multiple co-morbidities, have weaker predictive values ranging from 1.45 to 3.0 but are still considered in specific clinical guidelines^[1]. Minor factors for gadolinium agents include female sex, age below 50 years, type of magnetic resonance imaging, and outpatient status, with weaker predictive values of 1.4 to 2.0^[1].

The specific contrast agent used also influences the risk profile. For gadolinium-based agents, the specific molecule seems more important, with higher frequencies or predictive values ranging from 3.0 to 8.0 for agents like gadobutrol, gadoteridol, gadobenate, and gadoxetate^[1]. In a meta-analysis, the overall rate of immediate reactions to gadolinium agents was 0.09 percent, with severe reactions occurring in 0.005 percent^[1]. Linear non-ionic gadolinium agents had a lower rate of 0.015 percent, while non-ionic macrocyclic agents and linear ionic agents with protein-binding had higher immediate reaction rates of 0.16 percent and 0.17 percent, respectively^[1].

It is a common misconception that these reactions are triggered by the iodine or gadolinium atoms themselves. In reality, hypersensitivity reactions are not triggered by these elemental atoms; instead, they are induced by specific epitopes present in one or more contrast media molecules, generally considered to be located in the carbamoyl sidechain^[1]. This molecular understanding explains why switching to an agent lacking the specific epitope is highly effective in preventing recurrent reactions, reducing the risk by an odds ratio of 0.329 compared to premedication strategies^[1].

The aetiology of hypersensitivity reactions to contrast media is multifactorial, involving a complex interplay between the patient's immune system, the chemical properties of the agent, and the procedural context. Risk factors for hypersensitivity reactions to iodinated contrast media can be categorised into patient-related, contrast media-related, and procedure-related factors^[1]. It is important to recognise that these factors are not yet fully defined and remain a topic of ongoing debate within the radiological and allergological communities^[1]. Most available data do not clearly distinguish between general adverse drug reactions and true hypersensitivity reactions, and are often derived from non-controlled, retrospective observational studies that lack comparisons with appropriately matched control groups^[1].

Patient-related factors play a significant role in determining individual susceptibility. The risk from co-morbidities may be attributable to the fact that these patients often undergo multiple imaging procedures with iodinated contrast media, rather than an inherent immunological predisposition, or may be due to the use of immunomodulatory medications^[1]. Furthermore, the risk of a history of multiple well-documented allergies or minor risk factors can be additive, leading to a more severe clinical presentation, which may be particularly relevant in unstable patients^[1].

Contrast media-related factors are heavily influenced by the molecular structure of the agents. The potential high reactivity of contrast media complicates a change based on practical experience alone^[1]. Several studies have attempted to assess the cross-reactivity between different iodinated contrast media, but their findings are contradictory, making it difficult to draw clear conclusions^[1]. The 2D chemical distinctions between different non-ionic iodinated contrast media seem insufficient for deducing their level of cross-reactivity^[1]. In various classifications, one group sharing a common N-(2,3-dihydroxypropyl)-carbamoyl sidechain is defined, while other authors discriminate another group sharing an N-(2,3-dihydroxypropyl)-N-methyl-carbamoyl sidechain and a residual group lacking these carbamoyl sidechains^[1].

For gadolinium-based contrast agents, cross-reactivity between linear and macrocyclic agents appears to be very low^[1]. Conversely, cross-reactivity seems to be higher among macrocyclic gadolinium-based contrast agents^[1]. Consequently, a decision after a hypersensitivity reaction to a macrocyclic agent would traditionally be to recommend a linear agent^[1]. However, this option has been complicated by the current European restriction on the use of linear gadolinium-based contrast agents, related to concerns about their risk for nephrogenic systemic fibrosis and long-term gadolinium deposition^[1].

The pathophysiology of immediate hypersensitivity reactions revolves around the rapid activation of mast cells and basophils, leading to the systemic release of potent inflammatory mediators. Understanding the receptor-level interactions is crucial for comprehending why certain prophylactic strategies fail in severe reactions. Modern H1-antihistamines possess a significantly higher binding affinity (nanomolar Ki) for the H1 receptor compared to histamine itself (micromolar Kd). The limited efficacy of premedication in severe anaphylaxis occurs because massive histamine release outcompetes these inverse agonists by mass action, and because multiple non-histaminergic mediators are also released. When massive quantities of histamine and other mediators are released during anaphylaxis, the overwhelming concentration of the endogenous ligands outcompetes the competitive inhibition provided by H1-antihistamines, allowing the pathological cascade to proceed unchecked.

The structural basis of the immune response is also critical in understanding cross-reactivity. The exact region of the contrast media molecule that acts as an epitope is generally considered to be located in the carbamoyl sidechain^[1]. The presence of specific sidechains, such as the N-(2,3-dihydroxypropyl)-carbamoyl sidechain or the N-(2,3-dihydroxypropyl)-N-methyl-carbamoyl sidechain, dictates the immunological recognition by T-cells or specific antibodies^[1]. Switching to an iso-osmolar iodinated contrast media may significantly increase the risk of inducing non-immediate hypersensitivity reactions, especially between iohexol and its dimeric form iodixanol^[1]. This highlights that osmolality and dimeric structures can act as independent triggers for delayed, T-cell mediated pathways, distinct from the immediate IgE-mediated mast cell activation.

Elevated tryptase levels usually indicate IgE-mediated mast cell activation and mediator release, correlating with the clinical severity of the reaction^[1]. Tryptase is a pre-formed enzyme stored in the secretory granules of mast cells. Its release into the circulation is a direct quantitative marker of the extent of mast cell degranulation. In contrast, histamine is also released but is degraded quickly, is less specific, and is more difficult to measure using commercial assays^[1]. Therefore, tryptase serves as the superior pathophysiological biomarker for confirming the underlying mechanism of an immediate reaction.

The classification of hypersensitivity reactions to contrast media is primarily based on the temporal relationship between the administration of the agent and the onset of symptoms. Hypersensitivity reactions are classified based on timing: immediate hypersensitivity reactions occur within 1 h of contrast media administration (rarely up to 6 h), while non-immediate hypersensitivity reactions occur between 1 h and 1 week (rarely up to 8 weeks) after administration^[1]. It is vital to understand that both immediate and non-immediate hypersensitivity reactions can be either allergic or non-allergic in nature^[1]. This distinction separates the clinical timeline from the underlying immunological mechanism, meaning a delayed reaction is not automatically a T-cell mediated allergy, just as an immediate reaction is not always IgE mediated.

Staging the severity of the reaction is equally critical for guiding subsequent management and prophylactic strategies. Several studies have investigated combination premedication with H1-antihistamines and corticosteroids to prevent recurrent events^[1]. These regimens are frequently stratified according to the severity of the previous hypersensitivity reaction, with H1-antihistamines only prescribed for mild hypersensitivity reactions and H1-antihistamines plus corticosteroids for moderate-to-severe hypersensitivity reactions^[1]. Alternatively, these protocols have been adapted according to clinical preference, though evidence-based guidelines provide a more standardised approach.

Due to a lack of well-designed comparative studies, management recommendations can only be based on results of observational studies and reviews, and on the recommendations in other major international guidelines^[1]. These include the American College of Radiology Manual on Contrast Media v2024, the European Society of Urogenital Radiology Contrast Media Safety Committee guidelines v10, the World Allergy Organization Anaphylaxis Guidelines 2020, and the AAAAI/ACAAI Anaphylaxis Practice Parameter Update 2024^[1]. Additional guidance is provided by the International Consensus On Drug Allergy 2014, the guideline Safe Use of Contrast Media, Part 2, of the Radiological Society of the Netherlands, the European Association for Allergy and Clinical Immunology Anaphylaxis Guidelines 2021, and the Resuscitation Council UK guidelines on anaphylaxis^[1]. It is important to realise that such guidelines are regularly updated to reflect new evidence regarding risk stratification and prophylactic efficacy.

The clinical features of hypersensitivity reactions span a wide spectrum, from mild, self-limiting cutaneous manifestations to life-threatening multi-organ failure. The severity of the clinical presentation is closely tied to the underlying pathophysiology and the patient's baseline risk profile. In mild or moderate immediate hypersensitivity reactions, tryptase levels typically remain normal, and the absence of elevation does not exclude the possibility of a genuine immediate hypersensitivity reaction^[1]. This is a crucial clinical pearl; relying solely on biomarker elevation to diagnose or rule out a reaction in its milder forms will lead to significant under-recognition and inappropriate future management.

Patients with additive risk factors, such as multiple co-morbidities or a history of multiple allergies, may experience a more severe clinical presentation, which may be particularly relevant in unstable patients^[1]. The haemodynamic reserve of the patient dictates how well they can tolerate the profound vasodilation and capillary leak induced by mast cell mediators.

When considering the clinical features related to specific agent classes, the safety profile of gadolinium-based contrast agents must be contextualised within current regulatory restrictions. A decision after a hypersensitivity reaction to a macrocyclic gadolinium-based contrast agent would traditionally be to recommend a linear agent^[1]. However, this option has been complicated by the current European restriction on the use of linear gadolinium-based contrast agents, related to concerns about their risk for nephrogenic systemic fibrosis and long-term gadolinium deposition^[1]. Therefore, the clinical feature of nephrogenic systemic fibrosis, a severe scleroderma-like systemic illness, must be weighed against the risk of recurrent hypersensitivity when selecting an alternative agent in patients with severe renal impairment.

Furthermore, the clinical tolerance of specific agents can vary significantly. Several studies reported greater tolerance for iobitridol when it is chosen based on practical experience as an alternative iodinated contrast media^[1]. This suggests that the clinical features of the reaction, including the severity and the specific cutaneous or systemic manifestations, may be less likely to recur if an agent with a distinct sidechain profile is selected, even in the absence of formal allergological testing.

Differentiating true hypersensitivity reactions from other adverse events is a fundamental challenge in clinical practice. Most available data do not clearly distinguish between general adverse drug reactions and true hypersensitivity reactions, and are derived from non-controlled, retrospective observational studies that lack comparisons with appropriately matched control groups^[1]. An adverse drug reaction encompasses any noxious and unintended response to a drug, including predictable pharmacological toxicities, whereas a hypersensitivity reaction implies an immunologically mediated mechanism. Failing to distinguish between the two can lead to inappropriate labelling of a patient as allergic, unnecessarily restricting their access to essential diagnostic imaging.

The differential diagnosis of an immediate reaction must also consider non-allergic mast cell activation. Elevated tryptase levels usually indicate IgE-mediated mast cell activation and mediator release, correlating with the clinical severity of the reaction^[1]. However, because histamine is degraded quickly, is less specific, and is more difficult to measure using commercial assays, it is not routinely used to differentiate between allergic and non-allergic immediate pathways in standard clinical practice^[1].

When evaluating delayed cutaneous eruptions, the differential diagnosis must account for the complex nature of cross-reactivity. The 2D chemical distinctions between different non-ionic iodinated contrast media seem insufficient for deducing their level of cross-reactivity^[1]. A new, non-validated classification of iodinated and gadolinium-based contrast media has recently been published, based on previous clinical data on contrast media tolerance and integrating 2D and 3D conformation^[1]. This highlights the difficulty in clinically differentiating a true allergic recurrence from a reaction to a structurally similar but immunologically distinct agent.

Additionally, the differential diagnosis of a suspected recurrent reaction must consider the efficacy of prophylactic measures. There is no evidence that premedication reduces the risk of severe immediate hypersensitivity reactions to contrast media^[1]. Therefore, if a patient experiences a severe reaction despite receiving corticosteroid and antihistamine premedication, the clinician must differentiate whether this represents a failure of the prophylactic regimen or a completely independent, non-immunological adverse event. Evidence for the role of premedication in previous mild to moderate immediate hypersensitivity reactions remains weak and conflicting, and premedication is not recommended for previous non-immediate hypersensitivity reactions^[1].

The investigation of hypersensitivity reactions aims to confirm the immunological mechanism, identify the specific culprit agent, and select a safe alternative for future procedures. Histamine and tryptase are both biomarkers to confirm immediate hypersensitivity reactions^[1]. However, histamine is degraded quickly, is less specific, and is more difficult to measure using commercial assays^[1]. Consequently, serum tryptase is the preferred biomarker. Elevated tryptase levels usually indicate IgE-mediated mast cell activation and mediator release, correlating with the clinical severity of the reaction^[1]. To accurately interpret tryptase levels, clinicians must obtain an acute phase sample as soon as possible after the reaction, and a baseline sample at least 24 hours after symptoms have resolved, to differentiate acute degranulation from constitutively elevated baseline levels.

Skin testing remains a cornerstone of in vivo investigation. The interpretation of skin tests can guide the selection of alternative agents. Several studies reported a lower risk of skin test positivity for iobitridol in case of positive skin test for iomeprol (OR 0.2; 95% CI 0.06–0.85)^[1]. This statistical evidence supports the preferential selection of iobitridol when iomeprol has been implicated, although it remains to be determined if this is directly related to the presence of specific N-(2,3-dihydroxypropyl)-N-methyl-carbamoyl sidechains^[1].

For gadolinium-based agents, drug provocation tests would facilitate the selection of an alternative agent^[1]. Given the very low cross-reactivity between linear and macrocyclic agents, and the higher cross-reactivity among macrocyclic agents, drug provocation testing provides a robust, albeit resource-intensive, method to confirm tolerance to a specific alternative molecule when skin testing is inconclusive or contraindicated.

Regarding the interpretation of prophylactic strategies, the evidence strongly dictates current investigative pathways. There is no evidence that premedication reduces the risk of severe immediate hypersensitivity reactions to contrast media^[1]. Evidence for its role in previous mild to moderate immediate hypersensitivity reactions remains weak and conflicting, and premedication is not recommended for previous non-immediate hypersensitivity reactions^[1]. Therefore, the Contrast Media Safety Committee, along with other major international guidelines, has decided not to recommend routine premedication^[1]. Premedication is optional in emergency situations where an unidentified culprit contrast media led to a severe hypersensitivity reaction, as recommended by the European Association for Allergy and Clinical Immunology^[1]. Ultimately, changing contrast media based on practical experience remains inferior to allergological evaluation, and most studies on cross-reactivity do not allow the choice of the safest contrast media on a robust scientific basis^[1].

The accurate diagnosis of hypersensitivity reactions to contrast media relies on a combination of clinical presentation, temporal relationship to administration, and specific immunological biomarkers. An adverse drug reaction after contrast media administration, whether iodine-based, gadolinium-based, or ultrasound contrast agents, is fundamentally defined as a noxious and unintended response to a contrast medium^[1]. This definition requires that the response suggests a reasonable possibility of a causal relationship between the contrast medium and the adverse event, distinguishing it from coincidental pathological events occurring during the imaging procedure^[1].

Anaphylaxis represents the most severe end of this spectrum. It is defined as a life-threatening allergic reaction, occurring rapidly after exposure to an antigen to which the patient presents some kind of intolerance^[1]. This is a multifactorial response in which multiple immunologic mechanisms are involved, with the typical mechanism resulting from immunoglobulin E-mediated activation of mast cells and basophils^[1]. Recognising the early warning signs of a progressive hypersensitivity reaction is critical for timely intervention. These early signs include erythema, rash, or angioedema, which may be followed by a rise of respiratory frequency, increase of the heart rate, and loss of blood pressure^[1].

Biochemical confirmation is a cornerstone of the diagnostic criteria for immediate reactions. An acute-over-baseline elevation of tryptase levels of at least 2 ng/mL + (1.2 × baseline tryptase level) during or within 4 h after symptoms is suggestive of an immediate hypersensitivity reaction^[1]. This specific mathematical threshold helps differentiate acute mast cell degranulation from a patient's constitutively elevated baseline tryptase level.

For non-immediate reactions, in vitro testing such as the lymphocyte transformation test has specific diagnostic criteria. The lymphocyte transformation test should not be performed in the acute stage of a non-immediate hypersensitivity reaction, but rather 4 to 8 weeks after remission^[1]. Furthermore, skin testing protocols must adhere to strict criteria to ensure validity. Skin tests for the evaluation of hypersensitivity reactions to contrast media should adhere to European Association for Allergy and Clinical Immunology standards and have been shown to be cost-effective^[1]. The testing protocol should include the broadest possible panel of iodine-based or gadolinium-based contrast agents, including the culprit agent if known^[1]. The sensitivity of skin testing is highest when performed within 2 to 6 months of the reaction, but decreases when performed within the first month or beyond 12 months of the reaction^[1]. The procedure should begin with skin prick testing using undiluted contrast media and, if negative, proceed with intradermal testing using a 1:10 dilution^[1].

The acute management of hypersensitivity reactions is dictated by the severity of the clinical presentation and the underlying pathophysiological mechanisms. Adrenaline is the cornerstone of therapy for severe immediate reactions. It is a potent alpha- and beta-adrenergic agonist with multiple systemic effects^[1]. Stimulation of alpha-1 receptors leads to reduced pulmonary oedema, vasoconstriction, and increased peripheral vascular resistance, counteracting the profound distributive shock seen in anaphylaxis^[1]. Concurrently, beta-1 receptor activation leads to increases in inotropic and chronotropic effects on the heart, while beta-2 receptor activation leads to bronchodilatation, vasodilatation, and reduced mediator release^[1]. The dosing is critical: in low doses, adrenaline activates primarily beta-adrenergic receptors with mainly cardiovascular effects, while in higher doses (0.5 mg and higher) both alpha- and beta-adrenergic receptors are activated^[1].

For respiratory symptoms, an inhaled short-acting beta-2 agonist such as salbutamol or albuterol is a potent beta-2 receptor agonist that is rapidly distributed through the body^[1]. It has a local bronchodilator but also a systemic effect, and it can effectively decrease dyspnoea and wheezing from allergic bronchospasm and reduce histamine release in the lung^[1]. The administration may be easier using nebulization to avoid stress for the patient who should inhale deeply^[1].

Special populations require tailored management strategies. Studies with paediatric populations under 18 years were excluded from major recent reviews since there is very limited information available for this group, and the existing data do not differ significantly from those obtained in adults^[1]. For this reason, decisions for children are generally based on the same principles as those for adults, with weight-based dose adjustments and a recommendation to avoid the use of parenteral corticosteroids whenever possible^[1]. However, for certain indications, higher contrast media doses per kilogram of body weight and higher injection rates may be used in children^[1].

The clinical setting heavily influences management capabilities. Given the varying levels of access to drug allergy specialists across Europe, situations may arise where timely allergy analysis is not possible, particularly in emergency or inpatient imaging^[1]. In these settings, reliance on clear, standardised acute management protocols and high-fidelity hands-on simulation training programmes for all personnel becomes paramount, as these are more effective than didactic lectures or computer-based e-learning^[1].

The most severe complication of a hypersensitivity reaction is anaphylaxis, which is a life-threatening allergic reaction that can lead to cardiovascular collapse and respiratory failure if not rapidly reversed^[1]. The physiological complications of anaphylaxis include profound distributive shock, bronchospasm, and upper airway oedema, requiring immediate intervention with intramuscular adrenaline to prevent fatal outcomes. Beyond the acute physiological derangement, a major long-term complication is the development of complex cross-reactivity patterns, which severely limits future diagnostic imaging options for the patient. Because the exact molecular region of contrast media capable of inducing a hypersensitivity reaction remains unidentified, a switch based on practical experience may not entirely prevent a new reaction, mainly due to the high and variable cross-reactivity between molecules^[1].

Cross-reactivity is formally defined in this clinical context as when patients experience a hypersensitivity reaction following exposure to two or more different contrast media, or when there are positive skin tests for two or more contrast media^[1]. This complication is further exacerbated by the fact that when using ionic contrast media, ions like meglumine may also cause hypersensitivity reactions independently of the main molecule^[1]. Furthermore, certain excipients of contrast media, like trometamol, may very rarely cause hypersensitivity reactions, adding another layer of diagnostic complexity and unpredictability^[1].

Diagnostic complications also arise from the inherent limitations of available testing modalities. For instance, high doses of corticosteroids and other immune-modulatory agents may interfere with lymphocyte transformation test results, rendering the test unreliable in patients recently treated for severe reactions^[1]. Additionally, the negative predictive value of the lymphocyte transformation test for non-immediate hypersensitivity reactions is unknown, and it is not widely available, restricting its use to experienced specialist centers^[1]. Even with skin testing, establishing clear cross-reactivity patterns is challenging if the culprit skin test remains negative, as hypersensitivity reactions may still recur after administration of an alternative iodine-based contrast medium with negative skin test results^[1]. This creates a difficult clinical dilemma where a patient remains at risk for recurrent reactions despite seemingly reassuring negative test results. This often necessitates a drug provocation test, which is the most potentially harmful and laborious procedure, available only in specialized centers for carefully selected patients^[1]. The psychological impact on patients who experience severe reactions can also lead to significant anxiety regarding future medical imaging, potentially resulting in the avoidance of necessary diagnostic procedures and worse clinical outcomes for their underlying conditions.

The prognosis for patients who have experienced a hypersensitivity reaction to contrast media is heavily dependent on the strategy employed to prevent recurrence. Selecting an alternative contrast medium based on practical experience is challenging due to its high and variable cross-reactivity; therefore, the best option is to choose an alternative based on the results of an allergy evaluation^[1]. This approach is safer and more effective than premedication for preventing recurrent hypersensitivity reactions^[1]. The safest method for switching contrast media involves defining an alternative agent guided by a comprehensive allergy evaluation^[1]. In a meta-analysis on the efficacy of skin testing, it was proposed that skin testing could serve as a potentially valuable diagnostic tool to confirm reactive iodine-based contrast media and help in identifying alternative agents without cross-reactivity^[1]. This capacity of skin testing to identify safe alternative contrast media in patients with a history of immediate hypersensitivity reactions has been confirmed in clinical practice^[1]. Consequently, contrast media with negative skin test results are highly likely to be safe alternatives for future imaging^[1].

Statistical evidence strongly supports the superiority of switching agents over prophylactic premedication. A large retrospective study demonstrated that patients who received a different iodine-based contrast medium had a significantly lower rate of recurrent hypersensitivity reactions compared to those who received the same contrast medium with corticosteroid premedication^[1]. Specifically, the recurrence rate for the same iodine-based contrast medium and steroid premedication was 80 of 423 examinations (19%), whereas for a different iodine-based contrast medium and no steroid premedication, it was only 10 of 322 examinations (3%)^[1]. This represents a highly significant odds ratio of 0.14, with a 95% confidence interval of 0.06 to 0.33, and a p-value of less than 0.001^[1]. Furthermore, in a multivariate analysis it was shown that the risk of recurrent hypersensitivity reactions was 67.1% lower in cases where the implicated iodine-based contrast medium was changed to another one (odds ratio: 0.329; 95% confidence interval, 0.168–0.644, p = 0.001), while steroid premedication did not significantly affect the risk of recurrent hypersensitivity reactions (odds ratio 0.907; 95% confidence interval, 0.429–1.918, p = 0.799)^[1]. Switching contrast media appears to be particularly useful in moderate-to-severe hypersensitivity reactions (odds ratio, 0.30; 95% confidence interval: 0.16–0.55; p < 0.001) compared to mild reactions (odds ratio, 0.51; 95% confidence interval: 0.37–0.69; p < 0.001)^[1].

For gadolinium-based contrast agents, the prognosis for avoiding recurrence is similarly improved by strategic switching. The recurrence rate of hypersensitivity reactions decreases by 75% when a gadolinium-based contrast agent with a chelate with a different molecular structure is administered^[1]. Furthermore, greater safety is observed when using an alternative skin test-negative gadolinium-based contrast agent, reinforcing the prognostic value of thorough allergological investigation prior to re-exposure^[1].

The paradigm of preventing recurrent hypersensitivity reactions has shifted significantly away from routine pharmacological prophylaxis. It has long been known that the evidence for administering premedication to prevent hypersensitivity reactions to contrast media is very weak, and its use is recently getting reduced in multiple countries^[1]. Historically, corticosteroid monotherapy was used in studies involving high-osmolar contrast media, but these results cannot be extrapolated to current clinical practice using low-osmolar contrast media^[1]. There are no comparative randomized trials available, and several studies indicate that corticosteroids provide no additional benefit in preventing a recurrent hypersensitivity reaction^[1].

Furthermore, the utility of premedication is lower in non-immediate hypersensitivity reactions, and the European Association for Allergy and Clinical Immunology guideline explicitly does not recommend premedication for non-immediate hypersensitivity reactions^[1]. Traditional protocols from the era of high-osmolar contrast media and the early days of low-osmolar contrast media continue to be widely utilized, including shorter intravenous options for hospitalized patients, despite lacking robust contemporary evidence^[1].

Prevention now prioritises alternative imaging strategies and systemic preparedness. In patients with a previous history of hypersensitivity reactions to a contrast medium, an alternative imaging modality that does not require a contrast medium of a similar class, or performing the examination without a contrast medium, should be considered^[1]. However, this is only recommended if the diagnostic yield is sufficient for correct patient management^[1]. The decision to omit a contrast medium is more compelling when the previous hypersensitivity reaction was severe, but the patient should never be denied a well-indicated examination if a diagnostic alternative is not available^[1]. For milder reactions, opting for an alternative imaging modality is less pertinent if it results in substantially lower image quality^[1].

To ensure staff are prepared to prevent complications from acute reactions, personnel who work in departments where contrast media is administered to patients need to be regularly trained in the management of hypersensitivity reactions and other emergency situations^[1]. High-fidelity hands-on simulation training programmes are demonstrably more effective than didactic lectures or computer-based e-learning for building this critical competence^[1].

Effective follow-up after a hypersensitivity reaction is essential to prevent future life-threatening events and to ensure the patient can still access necessary diagnostic imaging. The role of the allergologist is expanding in this domain, and recently, allergy assessments have shown to be cost-effective and have considerably improved the prevention of recurrent hypersensitivity reactions to contrast media, allowing for the selection of safe alternative agents for follow-up imaging^[1].

A critical component of follow-up is meticulous record-keeping. As the allergic evaluation by a drug allergy specialist is becoming more important in the prevention of recurrent hypersensitivity reactions, accurate agent-specific documentation is more important than ever^[1]. Currently, documentation in the electronic health record is far from standardised and is often done by physicians without any experience in the administration of contrast media^[1]. Therefore, it is highly recommended that the physician responsible for the administration of the contrast medium or the drug allergy specialist ensures that the allergy is accurately documented in the electronic health record^[1]. Furthermore, communication with the referring specialist is recommended in all cases to ensure that the chosen alternative imaging strategy aligns with the patient's overall clinical management plan.

The safest method for switching contrast media involves defining an alternative agent guided by a comprehensive allergy evaluation^[1]. In a meta-analysis on the efficacy of skin testing, it was proposed that skin testing could serve as a potentially valuable diagnostic tool to confirm reactive iodine-based contrast media and help in identifying alternative agents without cross-reactivity^[1]. This capacity of skin testing to identify safe alternative contrast media in patients with a history of immediate hypersensitivity reactions has been confirmed in clinical practice^[1]. Consequently, contrast media with negative skin test results are highly likely to be safe alternatives for future imaging^[1].

For patients requiring definitive confirmation of tolerance, a drug provocation test is the gold standard follow-up procedure. A drug provocation test, carefully administered by an allergologist with incremental contrast media doses, is a state-of-the-art allergological procedure^[1]. It is also the most potentially harmful and laborious procedure, available only in specialized centers for selected patients^[1]. However, it can be highly useful to verify the involvement of a contrast medium in a hypersensitivity reaction and to confirm the tolerance of an alternative contrast medium, especially when safe alternatives cannot be identified using skin testing or basophil activation testing^[1]. A strict safety rule governs this follow-up step: only contrast media with negative skin test results should be administered during a provocation test^[1]. In situations where timely allergy analysis is not possible, particularly in emergency or inpatient imaging, reliance on clear institutional protocols and careful risk-benefit discussions becomes paramount^[1].

Clinicians must be vigilant against several common pitfalls in the diagnosis and management of contrast media hypersensitivity. A primary pitfall is the false assumption that premedication guarantees safety. Traditional protocols continue to be widely utilized, but evidence indicates that corticosteroids provide no additional benefit in preventing a recurrent hypersensitivity reaction, and relying on them can create a dangerous false sense of security^[1].

Another significant pitfall is relying on practical experience alone to switch contrast agents. Since the exact molecular region of contrast media capable of inducing a hypersensitivity reaction remains unidentified, a switch based on practical experience may not entirely prevent a new reaction, mainly due to the high and variable cross-reactivity between molecules^[1]. Clinicians may incorrectly assume that agents from different chemical classes are entirely safe, ignoring the potential for shared epitopes or excipient reactions.

A third pitfall involves the misinterpretation of negative skin test results. Establishing cross-reactivity patterns is challenging if the culprit skin test remains negative, as hypersensitivity reactions may still recur after administration of an alternative iodine-based contrast medium with negative skin test results^[1]. A negative test does not equate to absolute safety, and drug provocation testing should then be considered to confirm tolerance^[1].

Finally, mistiming the allergological follow-up is a frequent error. The sensitivity of skin testing is highest when performed within 2 to 6 months of the reaction, but decreases when performed within the first month or beyond 12 months of the reaction^[1]. Testing too early may yield false negatives due to transient mast cell refractoriness, while testing too late may miss the window of optimal immunological memory recall.

In clinical practice, the epidemiology of hypersensitivity reactions varies significantly depending on the type of contrast agent used. The incidence of all hypersensitivity reactions is highest after iodine-based contrast media and lowest after ultrasound contrast agents^[1]. Fortunately, the incidence of immediate hypersensitivity reactions has declined considerably after the introduction of low-osmolar and iso-osmolar iodine-based contrast media and the use of lower mean contrast media doses^[1].

Despite this decline, hypersensitivity reactions to nonionic contrast media remain a heterogeneous disease in daily radiological practice^[1]. These reactions can develop from multiple mechanisms, such as immunoglobulin E-dependent pathways, complement-dependent pathways, direct membrane effects of the contrast media, and possibly other mechanisms that have not been identified yet^[1]. This heterogeneity necessitates a flexible, mechanism-aware approach to clinical decision-making rather than a rigid protocol.

In patients with a previous history of hypersensitivity reactions to a contrast medium, an alternative imaging modality that does not require a contrast medium of a similar class, or performing the examination without a contrast medium, should be considered, but only if the diagnostic yield is sufficient for correct patient management^[1]. The decision to omit a contrast medium is more compelling when the previous hypersensitivity reaction was severe, but the patient should never be denied a well-indicated examination if a diagnostic alternative is not available^[1]. For milder reactions, opting for an alternative imaging modality is less pertinent if it results in substantially lower image quality, and communication with the referring specialist is recommended^[1]. Additionally, given the varying levels of access to drug allergy specialists across Europe, situations may arise where timely allergy analysis is not possible, particularly in emergency or inpatient imaging, requiring radiologists to make immediate, evidence-based decisions regarding agent selection^[1].