Thrombin Time Interpreter
Thrombin Time Interpreter
A prolonged thrombin time has two classic causes and one modern one: heparin in the sample, a problem with fibrinogen, and dabigatran. This page works out which, from the thrombin time, the reptilase time and the fibrinogen — and a normal result is worth as much as an abnormal one.
Thrombin time
TT + reptilase + fibrinogen → causeThrombin time 45 s (upper limit 15 s), reptilase time normal, fibrinogen not done, not on dabigatran
How the test works
Reptilase (batroxobin) does the same job but is not inhibited by heparin or by direct thrombin inhibitors.
- what it measures
- only the last step of coagulation — the conversion of fibrinogen to fibrin. It is blind to every factor above it, to factor XIII and to platelets
- reference range
- in the region of 13 to 15 seconds, but thrombin-concentration dependent and genuinely laboratory-specific. A low-thrombin method is more sensitive to heparin, a high-thrombin method less so
- unfractionated heparin
- prolongs the thrombin time in a dose-dependent, semi-logarithmic fashion, and is the commonest cause of an unexpected result
- low-molecular-weight heparin
- does not usually prolong the thrombin time at therapeutic doses, which is why a normal thrombin time does not exclude it
- dabigatran
- prolongs the thrombin time more than tenfold at peak. Sensitive enough to exclude the drug, far too sensitive to quantify it
Worked example
Thrombin time 45 s (upper limit 15 s), reptilase time normal, fibrinogen not done, not on dabigatran
45 seconds against an upper limit of 15 — three times prolonged
The reptilase time is normal, so fibrinogen clots perfectly well when thrombin is bypassed
That localises the abnormality to inhibition of thrombin itself, not to fibrinogen
In practice: heparin in the sample until proven otherwise — most often a line draw or a heparinised flush rather than true anticoagulation
Resample from a clean peripheral venepuncture. If the repeat is normal, the first sample was contaminated and there is nothing to investigate
The two classic causes, and how to tell them apart
| Cause | Thrombin time | Reptilase time | Clauss fibrinogen | What to do |
|---|---|---|---|---|
| Heparin contamination of the sample | Prolonged, often markedly | Normal | Normal | Resample from a clean peripheral vein. This is by far the commonest cause of an unexpected prolonged thrombin time |
| Therapeutic unfractionated heparin | Prolonged | Normal | Normal | Expected. Monitor with an anti-Xa or a calibrated aPTT ratio, not with the thrombin time |
| Dabigatran | Markedly prolonged, more than tenfold at peak | Normal | Normal | Confirms exposure. Quantify with a dilute thrombin time or an ecarin-based assay if the level matters |
| Hypofibrinogenaemia — DIC, dilution, thrombolysis, liver disease | Prolonged | Prolonged | Low | Replace against the local threshold if bleeding, and treat the cause |
| Dysfibrinogenaemia | Prolonged | Prolonged | Low by Clauss, but fibrinogen antigen normal | The Clauss-to-antigen discrepancy is the finding. Refer for genotyping and a bleeding and thrombosis history |
| High fibrin degradation products, paraproteins, amyloid | Prolonged | Prolonged | Variable | Fibrin polymerisation is interfered with rather than fibrinogen being absent. Interpret alongside the underlying disease |
Which anticoagulants prolong the thrombin time
| Drug | Thrombin time | Note |
|---|---|---|
| Unfractionated heparin | Prolonged, dose-dependent | Semi-logarithmic response. Very sensitive to contamination |
| Low-molecular-weight heparin | Usually normal at therapeutic dose | A normal thrombin time does not exclude it. Use an anti-Xa level |
| Dabigatran | Markedly prolonged | A normal thrombin time effectively excludes a clinically relevant level |
| Argatroban, bivalirudin | Prolonged | Direct thrombin inhibitors, same mechanism as dabigatran |
| Rivaroxaban, apixaban, edoxaban | Normal | Factor Xa inhibitors. The thrombin time says nothing about them |
| Warfarin | Normal | Affects factors II, VII, IX and X, not the fibrinogen-to-fibrin step |
One step of the cascade, three questions it answers
The thrombin time measures only the final step of coagulation. Thrombin is added to platelet-poor plasma and the time to a fibrin clot is recorded, which means the test is blind to every factor above fibrinogen, blind to factor XIII, and blind to platelets. That narrowness is what makes it useful: a prolonged result has a short list of explanations, and the list divides cleanly in two. Either something is inhibiting the thrombin that has just been added, or there is something wrong with the fibrinogen it is acting on.
The first branch is dominated by heparin, and in hospital practice much more often by heparin that should not be in the sample at all than by heparin the patient is receiving. A specimen drawn from a central line, an arterial line or any lumen that has seen a heparinised flush carries enough heparin to prolong the thrombin time dramatically while the patient is not anticoagulated in the slightest. The reptilase time settles it: reptilase, a snake venom enzyme, clots fibrinogen directly and is not inhibited by heparin, so a prolonged thrombin time with a normal reptilase time confirms a thrombin inhibitor and a resample from a clean peripheral venepuncture usually ends the investigation. The other occupants of this branch are the direct thrombin inhibitors — dabigatran, argatroban, bivalirudin — which behave identically because they work the same way.
The second branch is fibrinogen. If both the thrombin time and the reptilase time are prolonged, the problem is the substrate. A low Clauss fibrinogen means a quantitative deficiency: disseminated intravascular coagulation, dilution during major haemorrhage, thrombolytic therapy, advanced liver disease, asparaginase. A Clauss fibrinogen that reads low while the immunological fibrinogen antigen is normal means a dysfibrinogenaemia — enough fibrinogen, of the wrong shape — and that discrepancy between the functional and the antigenic assay is the diagnostic finding rather than either number alone.
The modern reason the test is requested, though, is dabigatran. The conventional thrombin time is so sensitive to it — more than a tenfold prolongation at peak concentrations — that a thrombin time inside the reference range effectively rules out a clinically relevant level, which is a genuinely useful thing to be able to do at three in the morning before an urgent procedure on an obtunded patient with an unclear drug history. The corollary is that it cannot quantify: the assay saturates long before the concentration does, so a markedly prolonged thrombin time confirms exposure and no more. If the actual level matters, ask for a dilute thrombin time or an ecarin-based assay calibrated against dabigatran. And remember what a normal thrombin time does not exclude: the factor Xa inhibitors leave it entirely untouched, and so, at therapeutic doses, does low-molecular-weight heparin.
Frequently asked questions
What causes a prolonged thrombin time?
Two classic causes and one modern one. Heparin — most often contamination of the sample from a line rather than true anticoagulation — and a fibrinogen problem, either too little fibrinogen or a structurally abnormal one. The modern cause is dabigatran, which prolongs it more than tenfold. High fibrin degradation products and some paraproteins also interfere.
Does a normal thrombin time exclude dabigatran?
Yes, at clinically relevant concentrations. The conventional thrombin time is extremely sensitive to dabigatran, so a result inside the reference range effectively excludes a level that would matter for surgery or bleeding. It excludes nothing about rivaroxaban, apixaban or edoxaban, which are factor Xa inhibitors and leave the thrombin time normal.
What does a normal reptilase time with a prolonged thrombin time mean?
That something is inhibiting thrombin rather than anything being wrong with fibrinogen. Reptilase clots fibrinogen directly and is not inhibited by heparin or by direct thrombin inhibitors. In practice this means heparin in the sample, therapeutic unfractionated heparin, or a direct thrombin inhibitor such as dabigatran.
How do you tell dysfibrinogenaemia from hypofibrinogenaemia?
By comparing the functional and immunological assays. Both conditions prolong the thrombin and reptilase times, but hypofibrinogenaemia gives a low result on both the Clauss assay and the fibrinogen antigen, whereas dysfibrinogenaemia gives a low Clauss with a normal or near-normal antigen. That discrepancy is the diagnostic finding.
Can the thrombin time be used to monitor heparin?
No. It is too sensitive and its dose response is semi-logarithmic, so it saturates within the therapeutic range. Monitor unfractionated heparin with an anti-Xa level or with an aPTT ratio calibrated against anti-Xa in your own laboratory, and low-molecular-weight heparin with an anti-Xa level where monitoring is indicated at all.
Related calculators
References
- Mackie I, Casini A, Pieters M, et al. International Council for Standardisation in Haematology recommendations on fibrinogen assays, thrombin clotting time and related tests in the investigation of bleeding disorders. Int J Lab Hematol. 2024;46(1):20–32.
- Douxfils J, Ageno W, Samama CM, et al. Laboratory testing in patients treated with direct oral anticoagulants: a practical guide for clinicians. J Thromb Haemost. 2018;16(2):209–219.
- Funk DM. Coagulation assays and anticoagulant monitoring. Hematology Am Soc Hematol Educ Program. 2012;2012:460–465.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.