The Test Sample
What is being tested?
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.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.