Combined Factor V and Factor VIII Deficiency
Laboratory Evaluation
The combined performance of the following two assays is usually assessed to identify F5F8D:
- prothrombin time (PT) and
- activated partial thromboplastin time (APTT)
FVF8D is characterized by the prolongation of both first screening coagulation tests, with the APTT being disproportionately affected.
Specific assays of FV and FVIII coagulant activity are then necessary to evaluate the residual FV and FVIII coagulant activity. The tests depend upon the ability of a sample containing the factor under investigation to correct or shorten the delayed clotting of plasma that is completely deficient in that factor. Normal ranges can vary from laboratory to laboratory; however, the combined deficiency can be diagnosed when the coagulant factors activity levels are less than 45-50% for both FV and FVIII.
Factor antigen assays are not strictly necessary for diagnosis and treatment.
If continuous bleeding is observed after replacement therapy, the presence of an inhibitor should be investigated. The inhibitor is measured through the Bethesda assay, based on the neutralization of an external source of FV or FVIII.
Imaging techniques are not needed for minor bleeding. For major bleeding, which is extremely rare in F5F8D, the following procedures are necessary (as in all other rare coagulation disorders):
CT, computed tomography; MRI, magnetic resonance imaging.
