- Partial thromboplastin time
The partial thromboplastin time (PTT) or activated partial thromboplastin time (aPTT or APTT"') is a performance indicator measuring the efficacy of both the "intrinsic" (now referred to as the
contact activation pathway) and the common coagulation pathways. Apart from detecting abnormalities in blood clotting, it is also used to monitor the treatment effects with heparin, a major anticoagulant. It is used in conjunction with the prothrombin time(PT) which measures the "extrinsic pathway".
Blood is collected, by a
phlebotomist, with oxalateor citratewhich arrest coagulation by binding calcium. This specimen is delivered to the laboratory. In order to activate the intrinsic pathway, phospholipid, an activator (such as silica, celite, kaolin, ellagic acid), and calcium(to reverse the anticoagulant effect of the oxalate) are mixed into the plasma sample . The time is measured until a thrombus(clot) forms. This testing is performed by a medical technologist.
The test is termed "partial" due to the absence of
tissue factorfrom the reaction mixture.
Values below 25
seconds or over 39 s (depending on local normal ranges) are generally abnormal. Shortening of the PTT has little clinical relevance. 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.
To distinguish the above causes,
mixing tests are performed, in which the patient's plasma is mixed (initially at a 50:50 dilution) with normal plasma. If the abnormality does not disappear, the sample is said to contain an "inhibitor" (either heparin, antiphospholipid antibodies or coagulation factor specific inhibitors), while if it does correct a factor deficiency is more likely. Deficiencies of factors VIII, IX, XI and XII and rarely von Willebrand factor(if causing a low factor VIII level) may lead to a prolonged aPTT correcting on mixing studies.
The aPTT was first described in 1953 by researchers at the University of North Carolina at Chapel Hill. [cite journal |author=Langdell RD, Wagner RH, Brinkhous KM |title=Effect of antihemophilic factor on one-stage clotting tests; a presumptive test for hemophilia and a simple one-stage antihemophilic factor assy procedure |journal=J. Lab. Clin. Med. |volume=41 |issue=4 |pages=637–47 |year=1953 |pmid=13045017 |doi=]
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