Discrepancy in factor VIII 1-stage/2-stage activity in a child with Arg(531)--> His mutation.

Routine screening of infants born to known hemophilia carriers includes a factor VIII (FVIII) level. In routine practice, mild FVIII deficiency variants may be missed by laboratories that exclusively use a one-stage activated partial thromboplastin time-based activity assay. This case illustrates such a possibility with a discrepancy between the one-stage and two-stage assays performed on a child who carries the Arg(531) --> His mutation.