OBJECTIVETo determine the cut-off value for coagulation factor Ⅷ activity (FⅧ:C) to von Willebrand factor antigen (vWFAg) ratio which can classify obligatory carriers of hemophilia A and normal females, and assess its feasibility to diagnose suspected carriers in affected families through comparison with the method of gene diagnosis.

METHODSFⅧ:C assay was carried out by a one-stage method in both obligatory carriers and normal females. vWF antigen was measured with ELISA assay. The FⅧ:C/vWF ratio was calculated. Statistic analysis was carried out to determine the cut-off value which can classify the two groups. The ratio was then used to diagnose suspected carriers from families affected with hemophilia A. The results were compared with that by long distance polymerase chain reaction, genetic linkage analysis and/or direct sequencing.

RESULTSThe FⅧ:C/vWFAg value for 90.6% of obligatory carriers was under 0.82. Should 0.82 be selected as the cut-off value to diagnose the 42 suspected carriers, most of them can be readily diagnosed. The results were all in agreement with that of genetic analysis.

CONCLUSIONCut-off value of FⅧ:C/vWFAg may be used for initial diagnose of the suspected carriers from families affected with hemophilia A. The method is quite convenient and reliable.