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Partial thromboplastin time
The partial thromboplastin time (PTT) or activated partial thromboplastin time (aPTT) is a commonly performed test for the system of coagulation, especially its intrinsic pathway. Apart from detecting abnormalities in blood clotting, it is also used to monitor the effectivity of treatment with heparin, a major anticoagulant.
The PTT is measured by using a citrated sample, which arrests coagulation by binding calcium. In order to activate the intrinsic pathway, phospholipid and another activator are mixed into the plasma sample (such as silica, celite, kaolin, ellagic acid ), and calcium (to revert the anticoagulant effect of the citrate). The time is measured until a thrombus (clot) forms.
The test is termed "partial" due to the absence of tissue factor from the reaction mixture.
Values below 25 seconds and over 39 s (depending on local normal ranges) are generally abnormal. Shortening of the PTT has little clinical relevance, as most thrombosis patients have normal coagulation studies. Prolonged APTT may indicate:
- use of heparin (or contamination of the sample)
- antiphospholipid antibody (especially lupus anticoagulant, which paradoxically increases propensity to thrombosis)
- coagulation factor deficiency (e.g. hemophilia)
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