Regular ArticleThe “normal” factor VIII concentration in plasma

IntroductionThe quantitation of factor (F)VIII by activity-based assays is influenced by the method, procedure, the quality and properties of reagents used and concentrations of other plasma proteins, including von Willebrand factor (VWF).ObjectiveTo compare FVIII concentrations measured by activity-based assays with those obtained by an immunoassay and to establish the influence of plasma dilution on the FVIII clotting activity (FVIIIc).MethodsThe APTT, a chromogenic assay (Coatest) and two in-house immunoassays were used. Albumin-free recombinant FVIII was used as the calibrator in all assays.ResultsFor a group of 44 healthy individuals (HI), the mean value observed for FVIII antigen (FVIIIag; 1.22 ± 0.56 nM; S.D.) is substantially higher than that for FVIIIc (0.65 ± 0.29 nM) and the chromogenic assay (FVIIIch; 0.50 ± 0.23 nM). A positive correlation between FVIIIag and VWFag with R2 = 0.20 was observed. Since plasma VWF has an inhibitory effect on FVIIIc, we evaluated the influence of plasma dilutions on FVIIIc in HI (n = 105). At a 4-fold dilution, estimates of FVIIIc by clotting assay were much lower than FVIIIag (0.77 ± 0.31 vs. 1.14 ± 0.48 nM). At 10- and 25-fold dilutions, the estimated FVIIIc increased to 0.87 ± 0.36 and 0.94 ± 0.44 nM, respectively.Conclusions1) In plasma, FVIIIag is higher than FVIIIc and FVIIIch; and 2) Real FVIII concentrations in plasma can be estimated by measuring FVIIIag.