||aPTT mixing study. The prolongation of the PTT may be due to either factor deficiencies or a circulating inhibitor (usually an antibody). In the case of a factor deficiency, the 1:1 mix of patient and normal should show correction of the PTT equal to or less than the 1:1 mix of normal and buffer. An inhibitor will prolong the 1:1 mix of the patient and normal compared to the 1:1 mix of the normal and buffer. If the factor concentrations are adequate, the 1:1 mix of patient and buffer may have a shorter clotting time than the original patient specimen because the inhibitor has been diluted. In some cases, the circulating anticoagulant is time dependent and will become more evident after the one-hour incubation. This is often seen in specific factor inhibitors like the factor VIII inhibitor. Other inhibitors are immediate acting and will be evident at both the five minute and one hour incubations as is usually seen with the “Lupus Like” inhibitor and factors II, V, IX, X, and XI inhibitors. Proper identification of inhibitors is essential for appropriate clinical management. Patients with specific factor inhibitors have a significant risk of hemorrhage and often require specialized therapy for effective management. In contrast, patients with lupus anticoagulants are not at increased bleeding risk; rather a subset of these individuals has increased thrombosis risk or risk of fetal loss.