The FVIII:C inhibitor is best diagnosed by which description?

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Multiple Choice

The FVIII:C inhibitor is best diagnosed by which description?

The key idea is that an acquired factor VIII inhibitor acts as a neutralizing antibody against FVIII, leading to an abnormal coagulation pattern that is revealed by mixing and incubation tests. Factor VIII inhibitors cause prolongation of the intrinsic pathway test (APTT) because they block FVIII activity. When you mix the patient's plasma with normal plasma, if the issue were a simple FVIII deficiency, the added FVIII from the normal plasma would correct the APTT. With an inhibitor present, however, the antibody continues to neutralize FVIII in the mixture, so the APTT remains prolonged—this is why mixing does not correct the abnormal APTT. Moreover, many FVIII inhibitors are time- and temperature-dependent. If you incubate the mixed plasma, the inhibitor has more time to neutralize FVIII, and the APTT lengthens further. The PT would usually be normal because the extrinsic pathway (which PT measures) is not involved. So the described pattern—prolonged APTT that fails to correct with mixing and that lengthens with incubation—is the best description of an FVIII:C inhibitor.

The key idea is that an acquired factor VIII inhibitor acts as a neutralizing antibody against FVIII, leading to an abnormal coagulation pattern that is revealed by mixing and incubation tests.

Factor VIII inhibitors cause prolongation of the intrinsic pathway test (APTT) because they block FVIII activity. When you mix the patient's plasma with normal plasma, if the issue were a simple FVIII deficiency, the added FVIII from the normal plasma would correct the APTT. With an inhibitor present, however, the antibody continues to neutralize FVIII in the mixture, so the APTT remains prolonged—this is why mixing does not correct the abnormal APTT.

Moreover, many FVIII inhibitors are time- and temperature-dependent. If you incubate the mixed plasma, the inhibitor has more time to neutralize FVIII, and the APTT lengthens further.

The PT would usually be normal because the extrinsic pathway (which PT measures) is not involved. So the described pattern—prolonged APTT that fails to correct with mixing and that lengthens with incubation—is the best description of an FVIII:C inhibitor.

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