The aPTT is a measure of intrinsic pathway activity. It assesses factors XII, XI, IX, VIII, and the final common pathway.
Causes of prolonged aPTT
- Haemophilia A, B, or C
- von Willebrand disease
- Heparin
- Severe hepatosis (see above)
- DIC
- Antiphospholipid syndrome (APS) causes a paradoxical increase in aPTT in vitro despite producing a hypercoagulable state*
Heparin complexes with antithrombin and accelerates its inactivation of thrombin; a heparinised sample (eg post-bolus or sampling from cannulae or heparin-locked lines) may falsely elevate the aPTT. Warfarin has a weak effect on aPTT, however supratherapeutic doses may exacerbate the heparin effect.
The most common factor inhibitor is to factor VIII producing an acquired haemophilia A. Differentiation between VIII inhibition and APS is important in the setting of prolonged aPTT due to potentially significant bleeding in the former, and the hypercoagulable state of the latter.