A prolonged APTT corrected with factor VIII-deficient plasma but not with factor IX-deficient plasma indicates which factor is deficient?

A prolonged activated Partial Thromboplastin Time (APTT) indicates a problem within the intrinsic pathway of the coagulation cascade. When the APTT is corrected with factor VIII-deficient plasma, it suggests that factor VIII is functioning correctly and is present in sufficient quantities. This means that the prolongation of the APTT is not due to a deficiency in factor VIII.

However, when the APTT is not corrected with factor IX-deficient plasma, it indicates that the factor involved in the APTT prolongation is directly linked to factor IX. In this case, this points toward a deficiency of factor IX itself, as the presence of factor IX-deficient plasma would mean that the intrinsic pathway is still impaired without it; thus, the APTT remains prolonged.

This understanding aligns with the behavior of hemophilia B (factor IX deficiency), where APTT is affected but normalizes upon treatment with factor VIII because the thrombin generation pathway is predominantly influenced by factor IX rather than factor VIII in this scenario.