Comparison of the factor VII:C clot analysis and a modified activated factor VII analysis for monitoring factor VII activity in patients treated with recombinant activated factor VII (NovoSeven®)

Bleeding episodes in haemophilia A and B inhibitor patients are now frequently treated with recombinant activated factor VII (NovoSeven, Novo Nordisk A/S, Bagsvaerd, Denmark). Until now, the FVII:C coagulation assay has been used to monitor NovoSeven-mediated coagulation. However, a new assay (Staclot VIIa-rTF, Diagnostica Stago, France) has been designed to specifically detect activated factor (F)VII. Replacement of the buffer supplied by the manufacturer with a PIPES buffer containing BSA (modified FVIIa assay), resulted in a linear standard curve, greater sample stability and a reduced coefficient of variation. The FVII:C assay and the modified FVIIa assay were compared in a recovery experiment using the International FVIIa standard No 89/688(IS). Recovery of FVIIa was 93-97% for the modified FVIIa assay and 91-115% for the FVII:C assay. However, because samples in the FVII:C assay were not parallel to the standard curve, confidence limits for recovery were as wide as 67-130% compared with 92-106% for the FVIIa assay. In conclusion, a modified version of the Staclot VIIa-rTF assay, suitable for monitoring treatment with NovoSeven, even at low concentrations, has been developed. It provides an alternative to the FVII:C assay, which is not suitable for monitoring FVIIa at low concentrations.