The partial thromboplastin time or activated partial thromboplastin time is a blood test that characterizes coagulation of the blood. A historical name for this measure is the kaolin-cephalin clotting time, reflecting kaolin and cephalin as materials historically used in the test. Apart from detecting abnormalities in blood clotting, partial thromboplastin time is also used to monitor the treatment effect of heparin, a widely prescribed drug that reduces blood's tendency to clot. Partial thromboplastin time measures the overall speed at which blood clots by means of two consecutive series of biochemical reactions known as the intrinsic pathway and common pathway of coagulation. PTT measures the following coagulation factors: I, II, V, VIII, X, XI, and XII. The partial thromboplastin time is often used in conjunction with another measure of how quickly blood clotting takes place called the prothrombin time. The prothrombin time measures the speed of clotting by means of the extrinsic'' pathway.
A sample of the plasma is extracted from the test tube and placed into a measuring test tube.
Next, an excess of calcium is mixed into the plasma sample.
Finally, in order to activate the intrinsic pathway of coagulation, an activator is added, and the time the sample takes to clot is measured optically. Some laboratories use a mechanical measurement, which eliminates interferences from lipemic and icteric samples.
Interpretation
The typical reference range is between 30 seconds and 50 s . Shortening of the PTT is considered to have little clinical relevance, but some research indicates that it might increase risk of thromboembolism. Normal PTT times require the presence of the following coagulation factors: I, II, V, VIII, IX, X, XI and XII. Notably, deficiencies in factors VII or XIII will not be detected with the PTT test. Prolonged APTT may indicate:
To distinguish the above causes, mixing tests are performed, in which the patient's plasma is mixed with normal plasma. If the abnormality does not disappear, the sample is said to contain an "inhibitor", while if it does disappear a factor deficiency is more likely. Deficiencies of factors VIII, IX, XI and XII and rarely von Willebrand factor may lead to a prolonged aPTT correcting on mixing studies.
