Article: Partial thromboplastin time

The partial thromboplastin time (PTT) or activated partial thromboplastin time (aPTT) is a performance indicator measuring the efficacy of both the intrinsic 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.

Methodology

Blood is collected with citrate which arrests coagulation by binding calcium. 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 citrate) are mixed into the plasma sample . 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.

Interpretation

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)

To distinguish the above causes, mixing studies 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.