What coagulation test helps differentiate between a factor deficiency and the presence of a circulating anticoagulant?

The mixing study is utilized to differentiate between a factor deficiency and the presence of a circulating anticoagulant, such as lupus anticoagulant. In this test, patient plasma is mixed with normal plasma. If the coagulation issue is due to a factor deficiency, then the addition of normal plasma will correct the clotting time, indicating that the patient's plasma lacks a specific factor that is present in the normal plasma. Conversely, if the clotting time does not correct with the addition of normal plasma, this suggests the presence of an inhibitor or circulating anticoagulant affecting the clotting process.

This approach is critical in clinical settings, as it guides treatment by identifying whether patients require factor replacement therapy for a deficiency or further investigation and potential immunosuppressive therapy for an underlying condition related to anticoagulants.