Partial Thromboplastin Time (PTT) / Activated Partial Throboplastin Time (aPTT)

The partial thromboplastintime (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

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.

Interpretation

Values below 25 and over 39 seconds (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)

Sources: DiaPharma, Wikipedia