The Activated Partial Thromboplastin Time (aPTT) test measures the adequacy/functionality of factors that comprise the intrinsic and common pathways, including:
Table 2. Intrinsic (and Common) Pathway. | Factor | Pathway |
| Factor I (Fibrinogen) | Common |
| Factor II (Prothrombin) | Common |
| Factor V | Common |
| Factor VIII | Intrinsic |
| Factor IX | Intrinsic |
| Factor X | Common |
| Factor XI | Intrinsic |
| Factor XII | Intrinsic |
If one or more of the factors above is deficient, the aPTT will be prolonged. In this case, an aPTT mixing study may be ordered.
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 before performing a mixing study.
A deficiency in any factor component of the common pathway in the coagulation cascade will cause a prolonged PT and aPTT. This includes factors I, II, V, and X.
