Thrombin time: normal range, high and low

In shortThrombin time measures how quickly fibrinogen turns into a fibrin clot. Prolongation points to low or abnormal fibrinogen, heparin contamination, or circulating fibrin degradation products. It is interpreted together with fibrinogen and D-dimer.

Reference ranges

Group	Reference range
Result — typical range (international reference, not a national standard)	compared with the laboratory's reference interval in seconds

Lab Tests Online describes the test qualitatively: the result is a clotting time in seconds read against the laboratory's own interval.

Check your value

Enter your result — we will show how it compares with a typical range. Match the units of the selected row.

Approximate comparison with a typical range, not a diagnosis. Reference intervals vary by laboratory and method — check your report; result interpretation is done by a doctor.

What this marker is

Thrombin is added directly to the sample, so the test isolates the last step of coagulation. Anything that interferes with fibrin formation — too little fibrinogen, a defective fibrinogen molecule, or inhibitors such as heparin — prolongs the time.

The test is especially sensitive to heparin: even trace amounts from a line flush can prolong it, which is a common cause of a surprising result.

When it is high

Heparin effectThe most common cause — therapy or sample contamination from a heparinized line.

Low or abnormal fibrinogenInherited or acquired fibrinogen problems slow fibrin formation.

Fibrin degradation productsElevated in DIC and after thrombolytic therapy; they interfere with clot assembly.

When it is low

Short thrombin timeNot a recognized clinical finding.

What can skew the result

These factors can shift the value without any disease — worth considering before a retest:

Heparin (even trace amounts)Fibrinogen level and qualityD-dimer and fibrin degradation products

How to prepare

Blood from a vein; the team avoids drawing from heparinized lines. No special patient preparation.

When a retest makes sense

A prolonged result is repeated after heparin exposure is excluded, together with a fibrinogen level.

Who usually reviews this result

A result outside the reference range is usually reviewed by: your GP or primary care doctor; a hematologist for clotting disorders. Bring your lab report and any previous results to the appointment.

Names used on lab reports

Your report may use any of these names:
Thrombin timeTTThrombin clotting time

Units

Measured in: seconds

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Frequently asked questions

Why did my thrombin time come back long when I feel fine?

The most frequent explanation is laboratory-related — trace heparin in the sample. A clean repeat draw often normalizes it.

Is thrombin time the same as prothrombin time?

No. Prothrombin time tests the extrinsic pathway; thrombin time tests only the final fibrin step.

What is reptilase time?

A related test using a snake-venom enzyme insensitive to heparin — it helps tell heparin effect apart from a true fibrinogen problem.

Sources

Thrombin time: Lab Tests Online — Testing.com (AACC). Accessed: August 31, 2026.

Thrombin time