What this test measures
The Thrombin Time (TT) measures the time for plasma to clot after adding standardised thrombin. It bypasses the entire upstream coagulation cascade and tests only the final step — conversion of fibrinogen to fibrin. Normal range is typically 14–21 seconds (lab-specific).
Prolonged TT can reflect: (1) Heparin presence (unfractionated heparin is a very potent inhibitor of thrombin — even tiny contamination from a heparinised line can dramatically prolong TT); (2) Dabigatran (direct thrombin inhibitor — significantly prolongs TT); (3) Low fibrinogen (DIC, severe liver disease, congenital hypofibrinogenaemia); (4) Dysfibrinogenaemia (qualitatively abnormal fibrinogen — fibrinogen level is normal but function is impaired); (5) Fibrin degradation products in DIC.
Why it matters
Thrombin Time is not part of the routine coagulation screen but is added in specific situations: (1) Suspected dabigatran effect — a normal TT essentially excludes therapeutic dabigatran; this is useful before emergent surgery in patients who may have taken it; (2) Suspected heparin contamination of a sample (especially from a line) — TT is dramatically prolonged; (3) Investigation of prolonged PT and APTT — if TT is also prolonged, the defect is in fibrinogen; if TT is normal, the defect is in an upstream factor; (4) Workup of dysfibrinogenaemia — TT is prolonged but functional fibrinogen (Clauss) may be normal or low; antigenic fibrinogen is normal.
In the Indian context, where dabigatran is increasingly prescribed for atrial fibrillation, a normal TT before emergent surgery provides rapid reassurance that dabigatran is not contributing to bleeding risk.
How to prepare
No fasting required. Disclose all current medications, particularly dabigatran (Pradaxa), unfractionated heparin, low molecular weight heparin (which has lesser effect on TT), and any recent fibrinogen-affecting therapy. The sample must NOT be drawn from a heparinised line. Citrate tube; process within 4 hours.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Thrombin Time (seconds)[1][2] | 14–21 seconds (lab-specific) | Shortened TT — uncommon; usually not clinically significant. | Prolonged TT. Differential: heparin (UFH most potent), dabigatran (Pradaxa), low fibrinogen (<150 mg/dL — DIC, severe liver disease, congenital hypofibrinogenaemia), dysfibrinogenaemia (abnormal function), high fibrin degradation products. A "reptilase time" (which is insensitive to heparin and dabigatran) helps differentiate. |
TT interpretation in combination with other coagulation tests
| PT | APTT | TT | Likely cause |
|---|---|---|---|
| Normal | Normal | Prolonged | Heparin contamination, dabigatran, dysfibrinogenaemia |
| Prolonged | Prolonged | Prolonged | Severe liver disease, DIC, low fibrinogen, common pathway defect |
| Prolonged | Prolonged | Normal | Common pathway factor deficiency (II, V, X) — not fibrinogen |
| Normal | Prolonged | Normal | Intrinsic pathway defect (VIII, IX, XI) or lupus anticoagulant |
Frequently asked questions
When is Thrombin Time ordered?
When dabigatran or heparin effect needs to be assessed, when the cause of a prolonged PT/APTT is unclear, or when dysfibrinogenaemia is suspected.
Do I need to fast?
No.
My TT is prolonged but PT and APTT are normal — what does that mean?
Almost always heparin contamination, dabigatran, or dysfibrinogenaemia. The clinical context (current medications, recent line draw) usually clarifies. A reptilase time, which is unaffected by heparin and dabigatran, can confirm dysfibrinogenaemia.
Is TT useful for monitoring dabigatran?
TT is very sensitive to dabigatran — any therapeutic dabigatran will significantly prolong it. A normal TT effectively rules out clinically meaningful dabigatran effect. For quantifying dabigatran, the dilute thrombin time (Hemoclot) or ecarin clotting time is more precise.
Can a heparinised IV line affect TT?
Yes — and dramatically. Even tiny amounts of heparin contamination from a flush line can prolong TT to >100 seconds. Always draw the sample from a clean peripheral vein, not a heparinised line.
What is dysfibrinogenaemia?
A condition where fibrinogen is structurally abnormal — antigen level is normal but function is impaired. TT is prolonged; functional (Clauss) fibrinogen is low; antigenic fibrinogen is normal. Inherited forms exist; acquired forms occur in liver disease and some malignancies.
Does LMWH affect TT?
Much less than unfractionated heparin. Therapeutic LMWH typically does not significantly prolong TT.
How long does the report take?
Most NABL labs report Thrombin Time within 2–4 hours.
Related Coagulation tests
Tests commonly ordered alongside THROMBIN TIME, or that help interpret an unexpected result.
Sources & references
- NCBI StatPearls — Thrombin Time · accessed 2026-05-30T00:00:00.000Z
- ISTH — Coagulation Testing Standards · accessed 2026-05-30T00:00:00.000Z
- BSH — Investigation of Bleeding Disorders · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — Thrombin Time · accessed 2026-05-30T00:00:00.000Z
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