What is the thrombin time?
Thrombin time is a laboratory value that checks a part of blood clotting. It is defined as the time it takes to convert fibrinogen into fibrin.
When a blood vessel is injured, the body attempts to stop the bleeding that has occurred. Hemostasis, also known as primary hemostasis, is the first step in this process: special messenger substances (mediators) activate the blood platelets (thrombocytes), which form a plug on site and thus seal the leak.
However, this clot is still quite unstable and must first be consolidated. This is where the so-called secondary hemostasis, or blood clotting, kicks in. It consists of a reaction chain of several clotting factors. At the end of the reaction chain is the fibrous protein fibrin, which covers the platelet plug as a network structure and thus stabilizes it. The precursor of fibrin is fibrinogen – thrombin is responsible for its conversion into fibrin.
When is the thrombin time determined?
Thrombin time: What value is normal?
The thrombin time is determined from the blood plasma, which is mixed with citrate during collection. This prevents the blood from clotting until the time of the test. In the laboratory, the laboratory physician then adds a small amount of thrombin. He then determines the time it takes for fibrin to form, which is normally around 20 to 38 seconds. However, the normal value may differ depending on the amount of thrombin added.
When is the thrombin time shortened?
A shortened thrombin time has no significance. At most, it can be an indication of large amounts of fibrinogen in the blood (hyperfibrinogenemia).
When is the thrombin time prolonged?
A prolonged thrombin time occurs in the following cases:
- Cirrhosis of the liver
- Collagenoses (connective tissue diseases)
- Plasmocytoma (multiple myeloma)
- Nephrotic syndrome
- Neonates (here PTZ prolongation has no pathological value but is normal)
- increased degradation of fibrin (hyperfibrinolysis) with consequent fibrinogen deficiency
- increased consumption of blood clotting factors due to consumption coagulopathy (for example due to shock or sepsis = “blood poisoning”)
Another common reason for prolonged PTZ is the use of certain drugs such as penicillin, thrombin inhibitors such as hirudin, or heparin. Even a small dose of heparin results in a prolongation of the thrombin time, which is why the laboratory value is a very good test to check the heparin therapy or to detect an overdose.
What to do in case of altered thrombin time?
In the case of a prolonged plasma thrombin time, the physician must find out the cause and clarify the possible diseases. For this purpose, it is often necessary to determine further laboratory values if they have not already been measured during the initial examination.
