An assay that evaluates blood coagulation by measuring the time required for the appearance of fibrin strands following the mixing blood or plasma with phospholipid platelet substitute (e.g., crude cephalins, soybean phosphatides). It is a test of the intrinsic pathway (factors VIII, IX, XI, and XII) and the common pathway (fibrinogen, prothrombin, factors V and X) of blood coaguation . It is used as a screening test and to monitor heparin therapy.
There is also the 'partial thromboplastin assay', which some people use synonymously to aPPT, but historically was done slightly differently (without adding a coagulant).