The activated partial thromboplastin time (aPTT) is a clotbased test that measures the time for recalcified plasma to clot in the presence of anionic phospholipids and an activator of the contact system. It assesses the integrity of the so-called intrinsic and common pathways (factors XI, X, IX, VIII, V, and II, and fibrinogen), as well as the contact pathway (factors XII, prekallikrein, and high-molecularweight kininogen).

The prothrombin time (PT) measures the ability of recalcified plasma to clot in the presence of anionic phospholipids and tissue factor. It is primarily sensitive to deficiency of the vitamin K-dependent factor VII and is widely employed to monitor the oral anticoagulant warfarin.

Any abnormality of aPTT or PT warrants further investigation.

The first step in evaluating a prolonged PT or PTT is a mixing study in which patient plasma is mixed 1:1 with pooled normal plasma. Complete correction of the PT or PTT indicates a factor deficiency whereas persistent prolongation or only partial correction indicates a circulating factor inhibitor or lupus anticoagulant.

An initial panel of tests is advised to evaluate a bleeding diathesis, which includes: a complete blood count with differential, an examination of the peripheral blood smear cells, and a PT and aPTT to evaluate the coagulation cascade.

von Willebrand disease (vWD) is the most common inherited bleeding disorder of primary hemostasis, affecting up to 1% of the general population. It is caused by quantitative or qualitative abnormalities in von Willebrand factor (vWF).