The term partial thromboplastin is used to distinguish the reagent from that used in the prothrombin time, since the APTT reagent lacks the apoprotein component of the complete tissue thromboplastin. The APTT is the main test for screening for intrinsic clotting defects including haemophilia. It is also used for detection of lupus anticoagulant and for laboratory monitoring of heparin administration. The presence of an activator in the test system, to accelerate the PTT test by effecting maximum activation, also increases its precision and reproducibility by eliminating the variable effects of contact with glass surfaces.