Hemophilia A is a deficiency of factor VIII. Notice from the coagulation cascade that factor VIII is part of the intrinsic pathway. A deficiency of factor VIII will cause prolongation of the PTT or aPTT. Hemophilia A is more common than hemophilia B. About 80% of all people diagnosed with hemophilia have hemophilia A. Hemophilia A is relatively common, about 1 in 5,000 males born in the United States have the condition. Interestingly, about 30% of cases are not inherited but arise from a spontaneous gene mutation.
Hemophilia is usually first suspected from the patient's history and reports of prolonged bleeding episodes. A routine aPTT will usually be elevated in hemophilia, although the aPTT can be normal in those with only mild factor deficiencies (when factor VIII activity is greater than 15%). The platelet count and the PT will be normal in hemophilia A.
A key for laboratory testing is that in patients with hemophilia, the aPTT will correct to normal when the patient's sample is mixed with normal plasma. So unless an inhibitor is present, a mixing study followed by a factor-VIII activity assay is sufficient to diagnose hemophilia A. Measurement of factor VIII activity less than <40% is typical in hemophilia A.
