The “normal” factor VIII concentration in plasma

Abstract: Introduction: The 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). Objective: To 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). Methods: The APTT, a chromogenic assay (Coatest) and two in-house immunoassays were used. Albumin-free recombinant FVIII was used as the calibrator in all assays. Results: For a group of 44 healthy individuals (HI), the mean value observed for FVIII antigen (FVIIIag; 1.22±0.56nM; S.D.) is substantially higher than that for FVIIIc (0.65±0.29nM) and the chromogenic assay (FVIIIch; 0.50±0.23nM). 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±0.48nM). At 10- and 25-fold dilutions, the estimated FVIIIc increased to 0.87±0.36 and 0.94±0.44nM, respectively. Conclusions: 1) In plasma, FVIIIag is higher than FVIIIc and FVIIIch; and 2) Real FVIII concentrations in plasma can be estimated by measuring FVIIIag. [Copyright &y& Elsevier]