Pleural fluid analysis: Light criteria and patterns

A pleural effusion is an excess of fluid in the pleural space, the thin gap between the lung and its covering pleura. Fluid normally enters that space continuously and is drained by the lymphatics, so a drained tap is never a "normal" result by itself. What matters is its character: is it an ultrafiltrate of plasma driven by pressure and protein changes (a transudate), or is it leaked by inflamed or involved pleural surfaces, carrying protein, cells and enzymes (an exudate)? Light's criteria are the standard way of making that split ^[1]. ^[7]

That split is the first decision in every pleural tap. Transudates (heart failure, cirrhosis, nephrotic syndrome and similar) usually need the underlying illness treated; exudates (infection, cancer, thromboembolism, tuberculosis, rheumatoid disease and many more) need an active search for the cause, and often drainage ^[2]. Before Light's criteria, the same three measurements ask five different questions of the fluid: ^[4][5]

• What does it look like? Straw, pus, milky chyle, blood or a viscous mucin.
• Is it acid? pH separates complicated from uncomplicated infection and carries prognostic weight in cancer.
• How much protein is there? Protein and the serum-to-pleural gradient tell you how leaky the pleura is.
• How much LDH? Lactate dehydrogenase is a marker of turnover and cell breakdown.
• How much glucose, how many cells, what kind? Glucose, a total and differential cell count and cytology narrow the cause.

This page is an investigation topic rather than a disease: it walks through the standard panel, the Light criteria and their known blind spots, the patterns that individual combinations make, and three worked sample result sheets.

Appearance. Colour and clarity are surprisingly informative. A clear straw effusion is typical of a transudate. Frank pus is an empyema. A milky white fluid that does not clear on centrifugation and is high in triglycerides is chylothorax. Pseudochylothorax looks similar but is low in triglycerides and high in cholesterol. Blood-tinged fluid can occur in any effusion and is non-specific; a true hemothorax is confirmed only when the pleural fluid hematocrit is at least 50% of the peripheral blood hematocrit ^[2]. ^[1][5]

pH. Pleural fluid is normally mildly alkaline, with a pH of about 7.6 ^[6]. pH falls when cells and bacteria generate acid locally, so it is the key measurement for complicated parapneumonic effusion and empyema, and it has prognostic value in malignant effusion ^[1].

Protein and the serum-to-pleural protein gradient. Protein rises when capillary permeability or lymphatic flow change. Comparing pleural with serum protein, as a ratio or as a gradient, is the heart of Light's criteria and is also the usual correction for effusions misclassified when a patient is on diuretics ^[1].

LDH. LDH reflects cell turnover, inflammation and malignancy. It appears twice in Light's criteria: as a ratio to serum LDH and as an absolute value above two-thirds of the serum upper limit of normal. Correction for red cell lysis is needed when a tap is bloody: measured LDH minus 0.0012 times the red cell count per microlitre ^[5].

Glucose. Glucose crosses into pleural fluid by diffusion, so a normal effusion has a glucose close to serum. It falls when metabolism by bacteria, neutrophils or malignant cells outpaces diffusion. A glucose below 40 mg/dL (about 2.2 mmol/L) is typical of empyema, rheumatoid pleurisy and some tuberculous and malignant effusions ^[2]. ^[5][6]

Cell count and differential. A low total nucleated cell count favours a transudate and high counts an exudate, with overlap. Neutrophils dominate in acute infection; lymphocytes dominate in cancer, tuberculosis and chronic inflammation ^[2]. In empyema, paradoxically, the count can be extremely low because neutrophils have undergone autolysis in an acid, hypoxic environment ^[2].

Cytology. Cytology is the direct proof of a malignant effusion, but its yield is only about 60 to 70% on a first tap; a second thoracentesis improves this somewhat. Immunocytochemistry helps distinguish adenocarcinoma from mesothelioma and from benign mesothelial cells ^[2]. ^[1]

Other measurements that change the answer. Adenosine deaminase (ADA) supports tuberculosis, especially in lymphocyte-predominant exudates in high-prevalence settings ^[4]. Triglycerides (and chylomicrons) separate chylothorax from pseudochylothorax. Amylase is useful when pancreatitis or oesophageal rupture is possible, and microbiology with Gram stain and culture confirms infection ^{[2] [5]}.

There is no universal pleural fluid range. Reference intervals differ by laboratory, instrument, units, age, sex and method, and the same illness can produce different-looking numbers in different hospitals. The laboratory's own range, printed beside the result, always beats any textbook number ^{[1] [6]}. The values below are a teaching frame of the sort published in open reviews and lab user guides; they are illustrations, not your patient's range.

A few ranges carry extra teaching weight:

• Transudate cut-off. Effusions with total cell counts below 1000 per microlitre are transudates, while those with higher counts are exudates; when polymorphonuclear cells predominate, the process is acute ^[2].
• Parapneumonic threshold. Nucleated cell counts above 10,000 per microlitre make parapneumonic effusion likely, but counts can collapse to fewer than 200 neutrophils in an empyema because of autolysis ^[2].
• Glucose. A ratio of pleural-fluid glucose to serum glucose below 0.5 marks an exudate from infection, rheumatoid disease, lupus, tuberculosis, malignancy or oesophageal rupture ^[2].
• pH of a tap. Normal pleural fluid pH runs between 7.60 and 7.66; a lower value is the signature of infection and inflammation, and a value under 7.3 in malignant effusion predicts a shorter survival ^[6].
• ADA caveat. Values above 70 U/L make tuberculosis highly likely, values below 40 U/L make it unlikely in a lymphocyte-predominant exudate, and very high values of 250 U/L or more are more typical of empyema, complicated parapneumonic effusion or lymphoma than tuberculosis ^[3].

Transudates versus exudates. A transudate forms when hydrostatic pressure is high or plasma oncotic pressure is low: cardiac failure (the commonest cause), cirrhosis with ascites, hypoalbuminaemia in nephrotic syndrome, constrictive pericarditis, peritoneal dialysis and venous obstruction, among others ^{[2] [5]}. An exudate forms through local inflammation or pleural involvement: pneumonia and parapneumonic effusion, cancer (above all lung, breast, ovary and lymphoma), pulmonary embolism, tuberculosis, viral infection, rheumatoid arthritis and other autoimmune disease, pancreatitis and drug effect ^{[2] [5]}.

Appearance clues from the bedside. LDH above 1000 U/L in the right setting, a total protein of 70-80 g/L suggesting paraproteinaemia ^[8], eosinophils, or pH and glucose patterns point toward narrower causes. Rheumatoid effusion gives a glucose under 40 mg/dL, pH around 7.0 to 7.13 and a high ADA; lupus effusion, by contrast, has a pH above 7.36 and glucose above 60 mg/dL with an LDH under about 600 U/L. The contrast between those two tables is the kind of detail that turns a tap into a diagnosis ^[2].

• Normal (no significant effusion). Clear straw fluid, pH about 7.6, protein around 1 g/dL, LDH low, glucose matching serum, under 1000 cells per microlitre, macrophages about 75 percent and lymphocytes about 23 percent, negative cytology, ADA under 10 U/L ^[6].
• Uncomplicated transudate. Clear yellow, low protein, LDH low, pH normal, glucose close to serum, low cell count with macrophages predominating. Treat the underlying condition; no further pleural testing is needed ^{[2] [5]}.
• Uncomplicated exudate. Clear or slightly turbid, protein and LDH above the Light cut-offs, normal or near-normal pH, glucose close to serum. Needs a search for cause; the clinical context drives it. ^[2]
• Discordant but Light-positive "transexudate." A patient with heart failure or cirrhosis who is on diuretics can have a fluid that satisfies Light's criteria only just, because diuretics concentrate pleural protein and LDH. If the margin is small and nothing suggests an exudative cause, measure the serum-to-pleural protein (or albumin) gradient: almost all true transudates have a serum albumin level more than 1.2 g/dL above the pleural albumin level, or a serum protein level more than 3.1 g/dL above the pleural protein level ^[1]. ^[5]
• Chylothorax. Milky white fluid from thoracic duct disruption (trauma, surgery, lymphoma, superior vena cava syndrome), exudative, lymphocyte-predominant. Triglycerides above 110 mg/dL, chylomicrons present, pleural-to-serum triglyceride ratio above 1 and cholesterol ratio below 1 are diagnostic ^[2].
• Pseudochylothorax (chyliform effusion). Also milky, but with triglycerides below 110 mg/dL and cholesterol above 200 mg/dL and rhomboid cholesterol crystals. Usually tuberculosis, rheumatoid arthritis or trapped lung ^[2].
• Empyema and empyema-like fluid. Thick, turbid, odorous pus, polymorphonuclear predominance, Gram stain may show organisms, glucose below 40 mg/dL, LDH above 1000 U/L, pH below 7.2 ^[2]. The American Thoracic Society phases are exudative, fibrinopurulent and organising.
• Complicated parapneumonic effusion. In the setting of pneumonia, a pH below 7.2, or a glucose below 3.4 mmol/L, or a positive Gram stain or culture, or frank pus ^[1]. These need prompt drainage, traditionally with an intercostal drain.
• Tuberculous effusion. A lymphocyte-predominant exudate, often with a total protein frequently above 5 g/dL, glucose somewhat reduced but usually similar to serum, pH usually above 7.3, adenosine deaminase (ADA) of 30 to 40 U/L or above, and values below 40 U/L making tuberculosis unlikely. Cytology is not diagnostic ^{[2] [3]}.- Malignant effusion. An exudate, often hemorrhagic and lymphocyte-predominant, with protein above 5 g/dL in some cases, cytology positive in only about 60 to 70 percent on a first tap, and a low pH below 7.3 predicting a shorter survival. Leung's criteria (circumferential, nodular, mediastinal or parietal pleural thickening above 1 cm) support it on imaging, and the glucose is usually close to serum ^[2].
• Rheumatoid effusion. Glucose under 30 mg/dL, pH about 7.0 to 7.13, ADA above 50 U/L, and a rheumatoid factor above 1:320 with low C4. Self-limiting, but fibrothorax may follow ^[2].
• Lupus effusion. Exudative, polymorphonuclear predominance, pH above 7.36, glucose above 60 mg/dL, LDH under about 600 U/L, positive for lupus erythematosus cells and antinuclear antibody above 1:160 in the fluid ^[2].
• Pancreatitis-associated effusion. Usually left-sided, small, haemorrhagic, with a pH of 7.32 to 7.50 and glucose matching serum but raised amylase; in chronic pancreatitis the amylase in fluid is high while serum amylase is normal ^[2].

Every sheet below is an illustrative sample invented for teaching. None of it is a real patient, a real tap or a real report, and none of the sample values are claims. The thresholds and rules used to interpret them are the claims, and each is cited.

Sample sheet 1 - a normal/low protein fluid (illustrative only). 42-year-old, no infection, cardiac function normal.

Step-by-step interpretation:

1. pH 7.62 sits inside the normal pleural range of 7.60 to 7.66 ^[6].
2. Protein, LDH and the (illustrative) ratios are far under the Light cut-offs for an exudate ^[1].
3. Glucose 96 mg/dL is close to typical serum, as expected for a normal ultrafiltrate ^[2].
4. The cell count of 140 per microlitre is under 1000, consistent with a transudate-level or essentially normal fluid ^[2].
5. ADA is negligible, cytology negative, and appearance is clear straw, so infection and malignancy are not supported ^[2].

Conclusion: this is a normal, non-inflammatory pleural fluid; in a real clinical setting such a fluid is an incidental finding and needs only the clinical context, not pleural treatment ^[5].

Sample sheet 2 - a parapneumonic/empyema pattern (illustrative only). 58-year-old with fever, pleuritic chest pain and a right-sided effusion behind a lobar pneumonia.

Step-by-step interpretation:

1. A pH of 6.90 sits below the 7.20 cut-off that defines a complicated parapneumonic effusion in the British Thoracic Society literature ^[1].
2. Glucose 32 mg/dL is below 40 mg/dL, the level classed as low in empyema ^[2].
3. LDH at 1280 U/L is above the 1000 U/L threshold and protein 3.9 g/dL is above the 3.0 g/dL threshold, both typical of an infected pleura ^{[2] [5]}.
4. The total cell count of 24,000 per microlitre with 88 percent neutrophils is an acute suppurative picture, not the low-count picture of autolysis ^[2].
5. The positive Gram stain with diplococci identifies a bacterial aetiology; cultures will confirm it ^[2].

Conclusion: this is an empyema (complicated parapneumonic effusion) and it needs prompt drainage plus antibiotic therapy, following the local pathway, rather than observation alone ^{[1] [5]}.

Sample sheet 3 - a malignant effusion (illustrative only). 67-year-old with known lung cancer, progressive dyspnoea and a large left effusion.

Step-by-step interpretation:

1. Protein 4.6 g/dL and a raised LDH of 720 U/L satisfy Light's criteria for an exudate ^[1].
2. The lymphocyte predominance (74 percent), a normal-to-low pH of 7.35 and near-normal glucose fit the classical malignant-effusion profile ^[2].
3. ADA 11 U/L is below the tuberculosis threshold, making tuberculous pleurisy unlikely; this is the kind of context in which ADA acts as a triage test ^[3].
4. Cytology is positive for adenocarcinoma, which is definitive even though first-tap cytology is positive in only 60 to 70 percent of cases ^[2].
5. Because cytology can be falsely negative, imaging and pleural biopsy remain sensible if cytology had been negative ^{[1] [2]}.

Conclusion: this is a malignant (adenocarcinoma) pleural effusion; management is therapeutic drainage for symptoms, discussion with oncology and a palliative plan, with low pH recognised as a marker of shorter survival ^[2].

• Light's criteria overcall transudates. Between 15 and 30 percent of true transudates are misclassified as exudates by Light's criteria, and up to one-third of patients with heart failure or cirrhosis on diuretics fall into this trap. If the margin above the cut-off is small, measure the serum-to-pleural protein or albumin gradient before labelling an effusion exudative ^[1].
• Thresholds are not absolute. There is no fixed normal LDH or protein for pleural fluid by itself; results are read against the local serum upper limit of normal. Using another hospital's range is a common mistake ^{[1] [6]}.
• Pre-analytical errors matter especially for pH. Air trapped in the syringe, delayed processing beyond 4 hours, or contamination with local anaesthetic can falsely raise or lower the pH. Expel air, process promptly, and document the analyser method ^[2].
• Use a heparinised blood gas syringe for pH. A routine chemistry tube is not adequate for an accurate pH, and the anaerobic handling used for a blood gas should be mirrored.
• Blood in the tap is not a true haemothorax. Confirm a haemothorax by a pleural fluid hematocrit of at least 50 percent of peripheral blood; otherwise the blood may just be the needle's contribution ^[2].
• Autolysis mimics a non-infective effusion. Empyema can present with fewer than 200 neutrophils because of autolysis in acid, hypoxic fluid; a low cell count never excludes empyema when pH and glucose look wrong ^[2].
• ADA is not a TB test on its own. Empyema, complicated parapneumonic effusion, rheumatoid disease and some lymphomas push ADA high; values above 250 U/L are more typical of these than of tuberculosis ^[3].
• TB in HIV is different. HIV-positive patients with tuberculous pleuritis may have mesothelial cells in the fluid, fewer lymphocytes and positive acid-fast stains in 10 to 20 percent of effusions, so ADA and cell pattern are less reliable ^[2].
• Chyle and pseudochyle look identical to the eye. Only a triglyceride above 110 mg/dL with chylomicrons confirms chylothorax; low triglycerides with cholesterol above 200 mg/dL and cholesterol crystals confirm pseudochylothorax ^[2].
• Cytology fails often and fails early. A single tap is positive in only 60 to 70 percent; a second tap, image-guided biopsy or thoracoscopy is needed when cancer is still suspected ^{[1] [2]}.
• Leung's criteria and clinical picture matter. A cytology-negative exudate is not a finished diagnosis; contrast-enhanced CT and clinical correlation are part of the panel, not an afterthought ^{[1] [2]}.
• Exudate plus transudate mimicry. Parapneumonic effusion can masquerade as a simple transudate before it evolves; a rapidly worsening unilateral effusion with fever always earns a tap even if an early projection looked transudative.

• The first job of a pleural tap is to classify the fluid as transudate or exudate with Light's criteria, then hunt the cause of an exudate ^[1].
• A transudate comes from pressure or protein changes (heart failure, cirrhosis, nephrotic syndrome) and is treated at source; an exudate comes from local pleural disease or inflammation ^{[2] [5]}.
• Light's criteria are a pleural-to-serum protein ratio above 0.5, a pleural-to-serum LDH ratio above 0.6, or a pleural LDH more than two-thirds of the serum upper limit of normal ^{[1] [2]}.
• Light's criteria have sensitivity close to 98 percent for exudates but lower specificity, misclassifying 15 to 30 percent of transudates ^{[1] [4]}. ^[2][5]
• Diuretics concentrate pleural protein and LDH and can cause this misclassification; the serum-to-pleural protein or albumin gradient corrects it ^[1].
• Normal pleural fluid has a pH of 7.60 to 7.66, protein around 1 g/dL and under 1000 cells per microlitre, mainly macrophages ^[6].
• A complicated parapneumonic effusion is a pneumonia-associated effusion with pH below 7.2, or glucose below 3.4 mmol/L, or positive culture or pus ^[1].
• Empyema is frank pus with polymorphonuclear predominance, LDH above 1000 U/L, and glucose below 40 mg/dL ^[2].
• A glucose below 40 mg/dL (below 2.2 mmol/L) or a pleural-to-serum glucose ratio below 0.5 occurs in empyema, rheumatoid, lupus, tuberculous and malignant effusions ^[2].
• Triglycerides above 110 mg/dL with chylomicrons confirm chylothorax; low triglycerides with cholesterol above 200 mg/dL and crystals indicate pseudochylothorax ^[2].
• ADA of 30 to 40 U/L or above supports tuberculosis in a lymphocytic exudate; above 70 U/L makes it highly likely and below 40 U/L makes it unlikely ^[3].
• Very high ADA above 250 U/L is more often empyema, complicated parapneumonic effusion or lymphoma than tuberculosis ^[3].
• Cytology is diagnostic in only about 60 to 70 percent of malignant effusions on a first tap, and a negative result never excludes cancer ^{[1] [2]}.
• In malignant effusion a pH below 7.3 carries a worse prognosis ^[2].
• Every result is read against the local range; different laboratories differ ^{[1] [6]}.