The aPTT test measures the adequacy/functionality of factors that comprise the intrinsic and common pathways including:
- Factor I (fibrinogen)
- Factor II
- Factor V
- Factor VIII
- Factor IX
- Factor X
- Factor XI
- Factor XII
If the level of one or more of the factors above is deficient, the aPTT will be prolonged. An aPTT mixing study may be ordered in this case.
Since heparin inhibits the activity of multiple coagulation factors in the aPTT test, it is advisable to perform a thrombin time (TT) to rule out heparin contamination prior to performing a mixing study.
A deficiency in any factor that is a component of the common pathway in the coagulation cascade will cause both a prolonged PT and aPTT. This includes factors I, II, V, and X.