use mixing studies to differentiate deficiency vs inhibitors

Prolonged aPTT and normal PT (INR) suggest an intrinsic pathway defect.  Deficiencies of factor XII, prekallikrein, or high molecular weight kininogen all cause a prolonged aPTT without clinical bleeding.  Prolonged aPTT with bleeding can occur in congenital or acquired von Willebrand disease and isolated deficiencies of factors VIII, IX, and XI.  Antiphospholipid antibodies (lupus inhibitor) cause prolonged aPTT with thromboses rather than bleeding.

Mixing studies that mix the patient’s plasma in a 1:1 ratio with normal plasma can help differentiate between a factor deficiency and an inhibitor as the cause of the prolonged aPTT.  Factor deficiencies are corrected during a mixing study, resulting in normalization of the clotting time; while the presence of an inhibitor will prevent correction of the clotting time.  This patient’s bloody diarrhea resolved quickly with antibiotic therapy and is unlikely to have caused her prolonged aPTT.  She should have a mixing study performed with further evaluation based on the results.

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