Coagulation tests: PT, INR, aPTT, fibrinogen and D-dimer

• PT (prothrombin time) tests the extrinsic and common pathways of coagulation: factors VII, X, V, II (prothrombin) and fibrinogen (factor I). INR is the PT converted with the laboratory's thromboplastin sensitivity (ISI) so results can be compared between reagents and laboratories.
• aPTT (activated partial thromboplastin time) tests the intrinsic and common pathways: factors XII, XI, IX, VIII, X, V, II and fibrinogen. It is the test used to monitor unfractionated heparin.
• Thrombin time tests the final step of clotting, the conversion of fibrinogen to fibrin by thrombin; it is markedly prolonged by heparin and direct thrombin inhibitors.
• Fibrinogen is the substrate for the clot and an acute-phase reactant. D-dimer is a fragment produced when cross-linked fibrin is broken down; it rises in thrombosis and DIC, but also after surgery, in pregnancy and with age.
• D-dimer is used only with a pre-test probability (for example the 2-level Wells score): a low pre-test probability together with a negative D-dimer excludes venous thromboembolism; a high probability needs imaging regardless of the result.
• Mixing studies tell deficiency apart from an inhibitor: correction within the reference range by normal pooled plasma suggests factor deficiency; non-correction suggests an inhibitor, such as a lupus anticoagulant.

By the end of this page you should be able to:

1. State what PT, INR and aPTT each measure, and which factors belong to the extrinsic, intrinsic and common pathways.
2. Read standard adult reference ranges for PT, INR, aPTT, fibrinogen and D-dimer, and state why the reporting laboratory's range is authoritative.
3. Recognise the four common patterns of prolonged PT and/or aPTT, including when platelets are low.
4. Interpret three short sample result sheets and give a clinical conclusion and next step.
5. Explain warfarin and heparin monitoring with target ranges, DIC, liver disease, vitamin K deficiency and lupus anticoagulant. ^[1]
6. Recognise mixing studies and the pitfalls of LA testing.

A routine coagulation screen asks three questions on one blood sample: how quickly can the plasma form a fibrin clot (PT), how quickly does the intrinsic pathway do so (aPTT), and how much substrate (fibrinogen) and clot breakdown (D-dimer) are present. The two clotting times are global tests of groups of factors, so an abnormal result tells you which group is affected and points to the next specific assay. Every number, threshold and rule on this page is taken from a cited source, and ranges differ so much by reagent, age, sex and population that the printed interval on the report is the only one that matters for a patient.

This page assumes no prior knowledge of haemostasis. It is written for a global reader: it gives both conventional and SI-style units, uses generic drug names, and reminds you where guidelines differ (for example BSH versus ISTH versus NICE).

Coagulation in one picture. Two cascades run in parallel. The extrinsic pathway starts when tissue factor meets factor VII. The intrinsic pathway starts by contact activation and runs through factors XII, XI, IX and VIII. Both feed the common pathway: factor X, factor V, prothrombin (factor II) and fibrinogen (factor I), ending with thrombin converting fibrinogen into fibrin. Factor VII has the shortest half-life of the vitamin K-dependent factors, which is why PT lengthens before aPTT in early vitamin K deficiency or at the start of warfarin. ^{[4] [11] [12]}

Prothrombin time (PT). PT measures the extrinsic and common pathways: factors VII, X, V, II and fibrinogen. It is performed by adding tissue factor (thromboplastin) and calcium to citrated plasma and measuring the clot time in seconds. A prolonged PT therefore localises a problem to factor VII (early or vitamin K-related) or to the common pathway. PT is sensitive to liver synthetic failure and to oral anticoagulation because all the measured factors except V are vitamin K-dependent. ^{[1] [4] [12]}

International normalized ratio (INR). PT results depend on the responsiveness of the thromboplastin used, so laboratories convert PT to an INR using the reagent's international sensitivity index (ISI): INR = (patient PT / normal mean PT)^ISI. With a low-ISI reagent (about 1.0) the ratio is close to the raw PT ratio and monitoring is more precise. The INR is what is used to monitor warfarin. ^[4]

Activated partial thromboplastin time (aPTT). aPTT measures the intrinsic and common pathways: factors XII, XI, IX, VIII, X, V, II and fibrinogen. It is begun with a contact activator (such as silica or ellagic acid) and then excess calcium and phospholipid, and the clot time is read in seconds. It is the test used to monitor unfractionated heparin and to investigate unexplained bleeding. A prolonged aPTT with a normal PT points to the intrinsic pathway (factors VIII, IX, XI, XII, von Willebrand factor) or to an inhibitor such as a lupus anticoagulant. ^{[2] [5] [9]}

Thrombin time (TT). Thrombin time adds thrombin directly to plasma and measures how quickly fibrinogen is converted to fibrin. It therefore tests the final step only, independent of the cascade above it. It is very sensitive to heparin and to direct thrombin inhibitors, and it is prolonged when fibrinogen is low or dysfunctional or when fibrin degradation products interfere. In contrast to PT and aPTT, thrombin time is not used as a routine screening test and has no universal reference interval; it is compared with the control plasma of the method. ^{[2] [11]}

Fibrinogen (factor I). Fibrinogen is a plasma glycoprotein made in the liver that is cleaved by thrombin to form the fibrin mesh, and it is consumed in clot formation. It is also an acute-phase reactant, so it rises with inflammation, pregnancy and tissue injury, and falls in DIC and in significant liver disease. A low fibrinogen prolongs both PT and aPTT, and causes a marked prolongation of the thrombin time. A severe deficiency should be considered whenever a patient with a bleeding history has both an extended PT and an extended aPTT, because mild deficiency may not lengthen either test. ^{[7] [12]}

D-dimer. D-dimer is a specific degradation product formed when plasmin cleaves cross-linked fibrin. It is a marker of both clot formation and fibrinolysis, so it is high whenever the body is forming and dissolving clots. It is used to assess possible thrombosis, to exclude venous thromboembolism when the pre-test probability is low, and to monitor DIC. ^{[3] [6] [10]}

Mixing studies. When a PT or aPTT is prolonged, mixing the patient's plasma 1:1 with normal pooled plasma and repeating the test separates deficiency from inhibition. Correction into the reference range suggests a deficiency of one or more factors, because the added plasma supplies more than 50 percent of every factor. Non-correction or incomplete correction suggests an inhibitor in the patient's plasma, such as a lupus anticoagulant or a specific factor antibody. Weak inhibitors and minimal deficiencies can both be overcome by mixing, so the test is only informative when the prolongation is clear (more than about 3 seconds above the upper reference limit in the Labcorp method), and time- and temperature-dependent inhibitors, such as some factor VIII inhibitors, only show after incubation at 37 degrees C. A lupus anticoagulant may correct on the mix, so a mixing study that corrects does not exclude LA. ^[8]

Ranges are method-specific and age-, sex- and population-specific. Only the reporting laboratory's own interval is authoritative for a patient; where textbook and laboratory disagree, the laboratory wins. Ranges for PT and aPTT also differ between neonates, children and adults (children's values are often longer), so adult intervals should not be applied to infants.

A D-dimer result is read against the patient's pre-test probability, not against a single universal cutoff. In a person with low clinical probability of venous thromboembolism (VTE), a D-dimer under 0.5 mg/L FEU makes further imaging unnecessary; a result above that cutoff, or any result in a high-probability patient, needs imaging to confirm or exclude the clot. ^{[6] [13]}

Results are read in pairs. A long result points at a different set of problems from a short one, and many results only make sense when the platelet count and clinical context are added.

Two details that save diagnostic errors:

• Factor VII has the shortest half-life, so PT becomes prolonged while aPTT is still normal in early vitamin K deficiency and in the first days of warfarin. A prolonged PT with a normal aPTT and normal platelets therefore suggests an early, isolated factor VII problem. ^{[11] [12]}
• Bleeding can occur with a fibrinogen that is still inside the normal range in severe consumption. Fibrinogen is a consumable substrate; a falling trend matters as much as a low value. ^{[7] [10]}

Named combinations are more useful than single results. Look at PT, aPTT and platelets together, and always add the clinical picture.

Warfarin monitoring and target ranges. Warfarin is a vitamin K antagonist: it blocks carboxylation of factors II, VII, IX and X and of proteins C and S, so the balance shifts only once the long-half-life prothrombin (factor II) has fallen. Because PT is sensitive to these factors, PT is reported as INR, and warfarin is dose-adjusted to keep the INR in range. For atrial fibrillation and venous thromboembolism the usual target is 2.0 to 3.0, with a mid-point of about 2.5. ^[14] Patients with mechanical mitral valves (or selected high-thrombosis-risk valves) generally use a higher target, about 2.5 to 3.5, under cardiology guidance. ^{[4] [14]} If the INR is above range the bleeding risk rises; if below, the thrombosis risk rises, so regular INR monitoring with time in therapeutic range (TTR) is part of good care. Anti-factor Xa or a direct thrombin inhibitor (for example argatroban, dabigatran) prolongs the screen differently; a chromogenic factor X assay may be needed to separate warfarin effect from interference. ^[4]

Heparin monitoring. Unfractionated heparin (UFH) potentiates antithrombin and is monitored with the aPTT. The usual therapeutic target is an aPTT 1.5 to 2.5 times the control (equivalent to an activated clotting time as per local protocol), but continuous infusions in thrombotic DIC are sometimes run at "sub-therapeutic" heparin intensity because the goal there is to slow thrombosis without worsening bleeding. ^[10] Low molecular weight heparin (LMWH) is usually given in fixed weight-adjusted doses without routine monitoring; anti-Xa levels are reserved for renal failure, pregnancy, obesity or children. Too much heparin produces bleeding and a markedly prolonged aPTT, and is reversed with protamine. ^{[5] [10]}

Disseminated intravascular coagulation (DIC). DIC is the simultaneous generation and consumption of clot and anticoagulant factors, usually secondary to sepsis, trauma, malignancy or obstetric catastrophe. The screen typically shows prolonged PT and aPTT, a low platelet count, a low or falling fibrinogen and, almost always, a markedly raised D-dimer. The ISTH (International Society on Thrombosis and Haemostasis) overt DIC score makes this objective: platelet count, INR (or PT), fibrinogen and D-dimer are scored; 5 or more means overt DIC. BSH guidance and the ISTH criteria are the named guidelines for the panel. ^{[9] [10]} Management rests on treating the underlying condition, plus:

• Bleeding with low platelets (under 50 x 10^9/L) or before a high-risk procedure: platelet transfusion. ^[10]
• Bleeding with prolonged PT and aPTT: fresh frozen plasma, if there is active bleeding or an invasive procedure; not on numbers alone. ^[10]
• Persistent severe hypofibrinogenaemia (under 1 g/L) after plasma: fibrinogen concentrate or cryoprecipitate. ^[10]
• Thrombosis-predominant DIC: therapeutic heparin; continuous UFH is attractive when bleeding is also a risk, because it has a short half-life and is reversible. ^[10]
• Critically ill, non-bleeding patients: prophylactic heparin or LMWH for VTE. ^[10]
• Primary hyperfibrinolysis with severe bleeding: a lysine analogue such as tranexamic acid (for example 1 g every 8 hours). Antifibrinolytics are otherwise avoided. ^[10]

Tests should be repeated to follow the dynamic course, because the first sample only captures one moment. ^[10]

Liver disease. The liver makes most coagulation factors, including fibrinogen and the vitamin K-dependent factors. Liver disease therefore prolongs PT (and, in severe failure, aPTT), lowers fibrinogen, and can give thrombocytopenia through portal hypertension and splenic pooling. Early liver disease may show only a prolonged PT, like early vitamin K deficiency, because factor VII has the shortest half-life. Vitamin K will not correct a true synthetic failure, and anticoagulant or procedure decisions need senior review. ^{[12] [1] [2]}

Vitamin K deficiency. Vitamin K is required for carboxylation of factors II, VII, IX and X and of proteins C, S and Z. Deficiency is suspected when bleeding occurs in a patient at risk: fat malabsorption, prolonged broad-spectrum antibiotics, or breast-fed infants. The classic pattern is a prolonged PT with a raised INR, while aPTT, thrombin time, platelet count, bleeding time, fibrinogen, fibrin degradation products and D-dimer are normal. Early deficiency may show only a prolonged PT. Diagnosis is confirmed by the response to vitamin K (oral or parenteral, depending on risk). Warfarin is an iatrogenic vitamin K antagonist and produces the same pattern, with PT lengthening first. ^{[11] [12]}

Lupus anticoagulant (LA) and antiphospholipid syndrome (APS). Lupus anticoagulant is an antiphospholipid antibody. It has no specific factor target: it binds to the phospholipid-dependent reactions in the test, so it prolongs in-vitro tests that are phospholipid-dependent, most often the aPTT and the dilute Russell viper venom time (dRVVT). It is therefore a cause of a prolonged aPTT with no bleeding at all, and the patient may instead have thrombosis or recurrent pregnancy loss. The ISTH Scientific and Standardisation Committee guideline (2020 update) recommends parallel testing with dRVVT and aPTT; the diagnosis needs phospholipid dependence (correction of the prolonged clotting time on re-addition of phospholipid) and evidence of an inhibitor. A negative or only mildly prolonged screen on a sensitive reagent must be interpreted by the laboratory. LA testing should not be done during acute thrombosis, severe inflammation or anticoagulation, because these cause false results; retest positive patients at least 12 weeks later. For clinical APS, LA must be present on 2 or more occasions at least 12 weeks apart, together with a clinical event (thrombosis or qualifying pregnancy morbidity). ^[9]

D-dimer and pre-test probability. D-dimer is sensitive but not specific: most raised results are not thrombosis. Pre-test probability is therefore mandatory. With the 2-level Wells score, PE is "unlikely" at 4 points or fewer and "likely" above 4. In unlikely patients a negative quantitative D-dimer excludes PE, so no imaging is needed; a positive result leads to imaging (CT pulmonary angiography in most centres). In likely patients imaging is requested directly, and an interim therapeutic dose of anticoagulant may be given if imaging will be delayed. The same principle applies to DVT. Age-adjusted D-dimer cutoffs (about 0.01 mg/L FEU more for each year over 50 in the Labcorp assay) reduce false positives in older adults. D-dimer is also high in DIC and is used to follow response to treatment. ^{[6] [13] [3]}

The values below are invented for teaching and are labelled illustrative; they are not real patient data. The ranges and rules used in each interpretation are claims from the cited sources.

Interpretation:

1. PT 12.8 s is inside the adult interval of 12.2 to 14.9 s and the INR 1.0 is inside 0.9 to 1.2, so the extrinsic and common pathways are normal. ^[4]
2. aPTT 28 s is inside 23 to 34 s, so the intrinsic pathway is normal. ^[5]
3. Fibrinogen 3.2 g/L sits within 2.09 to 4.64 g/L, and D-dimer 0.22 mg/L FEU is below the 0.5 mg/L FEU exclusion cutoff in a low-probability patient. ^{[6] [7]}
4. Platelets are within the laboratory's reported interval, and no line is abnormal.
5. Conclusion: a normal coagulation screen; no haematological action is required, though the D-dimer only excludes thrombosis when the pre-test probability is low. ^[3]

Interpretation:

1. PT 18.6 s and INR 1.6 are above the adult interval of 12.2 to 14.9 s and 0.9 to 1.2, while the aPTT 29 s is normal. This isolates the problem to factor VII or the extrinsic pathway. ^{[4] [5]}
2. Thrombin time and fibrinogen are normal, so fibrinogen is not consumed and final clotting is intact. ^{[7] [11]}
3. Normal platelets and a normal D-dimer make DIC unlikely at this point. ^[10]
4. The pattern matches early vitamin K deficiency or the first days of warfarin, because factor VII has the shortest half-life and falls first; liver disease starting (incipient hepatic failure) can give the same result. Drugs, diet and alcohol should also be reviewed. ^{[11] [12]}
5. Conclusion: an isolated extrinsic-pathway defect, most consistent with early vitamin K deficiency or incipient liver disease; next step is a drug and diet review, liver tests, and a therapeutic trial of vitamin K with a repeat screen. If the patient is on warfarin, check dosing and adherence.

Interpretation:

1. Both PT and aPTT are prolonged and the INR (1.9) is above 1.6, so the problem is in the common pathway or in consumption, not a single factor. ^{[4] [5]}
2. Platelets are 58 x 10^9/L, that is below 100 (one point) and above 50; under 50 would be two points on the ISTH score. Thrombin time is prolonged and fibrinogen 1.1 g/L is just above the 1.0 g/L threshold. ^[10]
3. D-dimer is 4.8 mg/L FEU, well above the 0.8 mg/L level that adds two or three points on the ISTH score. ^[10]
4. The ISTH overt DIC score is therefore at least 5 (platelets 50 to 100 = 1; INR/at least 1.6 = 2; fibrinogen at least 1.0 = 0; D-dimer well above 0.8 = 2 to 3), which is compatible with overt DIC. ^{[9] [10]}
5. Conclusion: consumptive coagulopathy (most consistent with DIC on these invented values); next step is to find and treat the cause, give platelets only if bleeding or a high-risk procedure is planned, give FFP only with active bleeding or an invasive procedure, and consider fibrinogen replacement if it falls below 1 g/L. Repeat the screen to follow the course. Do not treat on laboratory numbers alone. ^[10]

• Drugs masquerade as deficiency. Warfarin, heparin, the direct oral anticoagulants and direct thrombin inhibitors all prolong the screen; failing to take a full drug history is the commonest cause of a mislabelled abnormal result. ^{[1] [2] [4] [5]}
• A normal or near-normal aPTT does not exclude bleeding. Fibrinogen is consumed before PT or aPTT rises, and individual factor levels have to fall below about 25 to 45 percent of normal before the aPTT moves. A bleeding patient with normal times may still have a problem. ^{[7] [8]}
• Mixing studies can fool you. Weak inhibitors and small factor deficiencies both correct when mixed with normal plasma. A corrected mix does not exclude a lupus anticoagulant or a mild inhibitor; a non-corrected result is the only clear positive, and LA should be excluded by dedicated testing. ^{[8] [9]}
• LA is an in-vitro finding, not bleeding disease. A prolonged aPTT in an otherwise well person may be a lupus anticoagulant and may carry a thrombotic, not a haemorrhagic, risk. Testing patients on anticoagulants produces false results, and a positive result needs confirmation on a repeat sample at least 12 weeks later. ^[9]
• Factor VII falls first in warfarin and vitamin K deficiency. Reading only the aPTT misses the first sign. ^{[11] [12]}
• Timing of LA testing. Samples drawn during acute thrombosis, severe inflammation or anticoagulation are unreliable; retest positives at 12 weeks. ^[9]
• Tube and collection errors. Underfilled citrate tubes (less than 90 percent full), wrong anticoagulant, EDTA contamination, haemolysis or refrigeration change the result. A discordant result needs a fresh sample before decisions are made. ^{[4] [8]}
• D-dimer is not a screening test for clots in high-probability patients. A negative D-dimer does not exclude thrombosis when the pre-test probability is high, and a raised D-dimer is not Specific for VTE. ^{[3] [6]}
• Age, sex, altitude and local method. Paediatric coagulation intervals are different and are not reliably captured by adult ranges. ^[4]
• Fibrinogen failure. A fibrinogen just inside the normal range can still fall fast in consumption; the trend is more informative than the single value, and an isolated fibrinogen assay is needed whenever bleeding is present. ^{[7] [10]}

• PT measures the extrinsic and common pathways (factors VII, X, V, II and fibrinogen); aPTT measures the intrinsic and common pathways (XII, XI, IX, VIII, X, V, II and fibrinogen). INR normalises PT across thromboplastins. ^{[1] [2] [4]}
• Thrombin time tests the final conversion of fibrinogen to fibrin and is very sensitive to heparin and direct thrombin inhibitors; fibrinogen is both substrate and acute-phase reactant. ^{[2] [7]}
• Adult reference examples (one laboratory): PT 12.2 to 14.9 s, INR 0.9 to 1.2, aPTT 23 to 34 s, fibrinogen 2.09 to 4.64 g/L, D-dimer under 0.50 mg/L FEU in low probability. Always use the reporting laboratory's own interval. ^{[4] [5] [6] [7]}
• Factor VII has the shortest half-life of the vitamin K-dependent factors, so PT lengthens before aPTT in early vitamin K deficiency and early warfarin. ^{[11] [12]}
• Vitamin K deficiency gives a prolonged PT/INR with normal aPTT, thrombin time, platelets, fibrinogen and D-dimer; liver failure looks similar but does not correct with vitamin K. ^[11]
• Liver disease lowers synthesis of most factors and lowers fibrinogen; both PT and (in severe failure) aPTT prolong. ^[12]
• Warfarin is monitored with INR; target 2.0 to 3.0 for atrial fibrillation and VTE, and higher (about 2.5 to 3.5) for some mechanical valves under specialist guidance. ^[14]
• Unfractionated heparin is monitored with aPTT, usually 1.5 to 2.5 times the control; LMWH is given in fixed weight-adjusted doses and monitored only in special groups. ^[10]
• DIC shows prolonged PT and aPTT, low platelets, low or falling fibrinogen and a markedly raised D-dimer; the ISTH score is 5 or more. Treatment is of the cause, with platelets, plasma, fibrinogen replacement and, when thrombosis predominates, heparin. ^[10]
• Lupus anticoagulant is a phospholipid-directed antibody that prolongs in-vitro times (aPTT and dRVVT) without bleeding; it is diagnosed by 2-screen testing, phospholipid dependence and persistence at 12 weeks, and is linked to thrombosis or pregnancy loss. ^[9]
• D-dimer is only used with pre-test probability: a low probability plus a negative D-dimer excludes VTE; high probability needs imaging. ^{[6] [13]}
• Mixing studies tell deficiency from an inhibitor by correction or non-correction, but do not exclude LA. ^[8]
• Every result must be read with the clinical picture, the trend, and the reporting laboratory's own interval.

Related topics, as plain text until they are linked in this build: abnormal liver function tests and jaundice; anaemia approach and iron deficiency; anticoagulant and antithrombotic drugs; disseminated intravascular coagulation; full blood count and blood film; hypercoagulable states and HIT; liver function tests; platelet tests; protein C and S; renal function and urea and electrolytes; sickle cell disease and other haemoglobinopathies.