What laboratory test can determine the cause of an abnormal PT or aPTT result?

Mixing studies are a crucial laboratory test used to investigate the underlying causes of an abnormal prothrombin time (PT) or activated partial thromboplastin time (aPTT). When a patient presents with prolonged PT or aPTT, mixing studies can help differentiate between a factor deficiency and the presence of an inhibitor.

In a mixing study, the patient's plasma is mixed with normal plasma in a specified ratio. If the PT or aPTT corrects to normal after mixing, it suggests a factor deficiency, since the normal plasma provides the missing factor(s). Conversely, if the PT or aPTT remains prolonged, it indicates the presence of an inhibitor, which could be due to autoimmune disorders such as lupus anticoagulant or factor-specific inhibitors.

This test is particularly valuable because it directs further diagnostic workup and treatment decisions based on the nature of the coagulation abnormality observed in the patient.

Other tests, like coagulation factor assays, are used to measure specific coagulation factor levels but do not initially identify the presence of inhibitors. The platelet aggregation test evaluates platelet function rather than the coagulation pathway, and thrombin time assesses the final common pathway rather than the causes of prolonged PT or aPTT. Thus, while these tests provide