Thrombin time

The range depends on the reagent (some laboratories use 11 to 16 or 13 to 19 seconds); use the range your lab printed.

The thrombin time measures how many seconds it takes plasma to clot when thrombin, the final enzyme of the clotting cascade, is added directly. It tests only the last step, the conversion of fibrinogen to fibrin, so it is a focused check on fibrinogen and on anything that interferes with that step, such as heparin.

Doctors request it as part of a fuller clotting screen when the PT or APTT is unexpectedly long, when checking whether a sample contains heparin, and when looking into problems with fibrinogen. It is also very sensitive to the direct thrombin blocking medicine dabigatran.

• Heparin, including contamination of the sample from a heparin-flushed line
• Dabigatran, a direct oral anticoagulant
• Low or abnormally working fibrinogen
• Liver disease
• A severe illness that uses up clotting factors
• High levels of fibrin breakdown products, as after clot-dissolving medicines

• There is no common cause of a short thrombin time; a slightly short result is not usually of any importance

• Heparin contamination from a line is the commonest reason for a long result in hospital
• An under-filled or delayed citrate tube reads long
• Different reagents give different times, so results are not comparable between laboratories

No fasting needed. Citrate tube, ideally not taken from a heparin-flushed line.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.