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Thrombin Time


At a Glance

Why Get Tested?

As part of an investigation of excessive bleeding or inappropriate blood clot formation (thrombotic episode), particularly to evaluate the level and function of fibrinogen; to detect heparin contamination

When to Get Tested?

When you have bleeding or thrombotic episodes, or recurrent miscarriages; when a PT and/or PTT test is prolonged, particularly if abnormal fibrinogen level or function is considered; when heparin contamination of a blood sample is suspected

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?

None

The Test Sample

What is being tested?

Thrombin is an enzyme in blood that acts on the clotting factor fibrinogen to form fibrin, helping blood to clot.The thrombin time (TT) assesses the activity of fibrinogen.

When an injury occurs and bleeding begins, the body begins to form a clot at the injury site to help stop the bleeding. Small cell fragments called platelets adhere to, aggregate, and are activated at the injury site. At the same time, the coagulation cascade begins and proteins called coagulation factors, including fibrinogen, are activated. Fibrinogen is then converted by thrombin into insoluble threads called fibrin that crosslink together to form a fibrin net that adheres to the injury site. Along with the platelets adhering, this forms a stable blood clot and prevents additional blood loss, remaining in place until the injury has healed.

For a stable clot to form, there must be enough normally functioning platelets and coagulation factors. If there are dysfunctional factors or platelets, or too few of them, it can lead to bleeding episodes and/or to inappropriate blood clotting (thrombosis).

The thrombin time evaluates that part of the hemostatic process where soluble fibrinogen is changed into fibrin threads. It measures the time required for a fibrin clot to form following the addition of a standard amount of thrombin to plasma. It is affected by the level and/or function of fibrinogen and the presence of inhibitors (e.g., heparin, fibrinogen/fibrin degradation products, direct thrombin inhibitor). With the addition of thrombin to the test sample, the thrombin time bypasses the rest of the coagulation factors and focuses on the function of fibrinogen.

It is now understood that blood coagulation tests are based on what happens artificially in the test setting (in vitro) and thus do not necessarily reflect what actually happens in the body (in vivo(). Nevertheless, there are several laboratory tests used to evaluate specific components of the hemostasis system. (For more on this, see the explanation of the Coagulation Cascade).

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

How is it used?

The thrombin time (TT) may sometimes be used as part of an investigation of a possible bleeding disorder or inappropriate blood clot formation (thrombotic episode), particularly to evaluate the level and function of fibrinogen.

The clotting-based functional fibrinogen assay is now routinely available in clinical laboratories and has largely replaced the need for thrombin time for evaluation of fibrinogen.

This test is very sensitive to the anticoagulant heparin.Because of this, it was once used to monitor unfractionated heparin therapy and to detect heparin contamination in a blood sample.While it is still sometimes used for these purposes, other tests and heparin neutralization procedures have largely replaced it.

When is it ordered?

A thrombin time may be ordered by itself or along with a combination of other tests when a person has bleeding or clotting episodes, experiences recurrent miscarriages, or has unexplained prolonged results on primary coagulation tests such as prothrombin time (PT) or partial thromboplastin time (PTT).

This test may be ordered when a health practitioner suspects that a person may have a disorder associated with decreased or dysfunctional fibrinogen. However, as mentioned previously, the functional fibrinogen assay (with or without a fibrinogen antigen assay) has largely replaced thrombin time for evaluating fibrinogen.

A thrombin time may sometimes be ordered when heparin contamination of a sample is suspected or when a person is receiving heparin therapy, although these uses have declined.

What does the test result mean?

Significantly prolonged thrombin times are most commonly found when there is contamination of the blood sample by the anticoagulant heparin, direct thrombin inhibitor, and may be seen with heparin-like substances and inhibitors (e.g., fibrinogen/fibrin degradation products).Contamination can occur when a person is on heparin therapy and/or when heparin is used as a periodic flushing agent to keep intravenous catheters from clotting.

A prolonged thrombin time may indicate decreased fibrinogen level (hypofibrinogenemia or afibrinogenemia) and/or abnormal fibrinogen function (dysfibrinogenemia). Conditions such as disseminated intravascular coagulation (DIC) or abnormal fibrinolysis impair fibrin formation and can also lead to a prolonged thrombin time, as can conditions such as end stage liver disease or malnutrition.

If thrombin time is abnormal, additional tests may be needed, including fibrinogen activity assay (now routinely available in clinical laboratories), fibrinogen antigen assay, and liver function tests.

Is there anything else I should know?

The thrombin time is just one component of the battery of tests typically required to evaluate a bleeding or thrombotic disorder.

A significant percentage of people with decreased or dysfunctional fibrinogen will have no symptoms or mild symptoms and may only be diagnosed if the abnormality is discovered during testing for another reason, or because they have unexpected or prolonged bleeding following a surgical procedure or trauma.

Fibrinogen is one of several blood factors that are called acute phase reactants. Blood levels of fibrinogen along with other acute phase reactants rise sharply with conditions causing acute tissue inflammation or damage.

Genetic molecular testing is occasionally performed on those with inherited dysfibrinogenemia, hypofibrinogenemia, or afibrinogenemia to identify the genetic mutation responsible.Testing for this mutation may also be performed on other family members.

Some recurrent miscarriages are thought to be related to inappropriate clotting in placental blood vessels.



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