A comparison of FVII : C and FVIIa assays for the monitoring of recombinant factor VIIa treatment

The use of recombinant factor VIIa (rFVIIa) is on the increase, not only to treat haemophilic patients with inhibitors, but also patients with other c lotting disorders. However, the most appropriate method of monitoring this treatment remains a question that has yet to be resolved. We studied 24 pla sma samples from patients receiving rFVIIa treatment (three had haemophilia A with inhibitors, and three a congenital FVII deficiency) and compared th e results obtained from the FVII:C and FVIIa assays. Although a good correl ation between the two methods was obtained (r = 0.91), the values of the FV II:C method were 1.63 higher than those of the FVIIa method, with a relativ ely wide margin in the interval of the FVII:C/FVIIa ratios obtained [95% co nfidence interval (CI) 1.38-1.88, range 0.68-3.68]. This interval became wi der when we compared values of over 6 IU mL(-1), which led us to conclude t hat the two methods cannot be considered equivalent. As the FVIIa method sp ecifically measures FVIIa, and FVII:C assay is known to have a wide interla boratory variability, we believe that the FVIIa assay would be more suitabl e for the monitoring of rFVIIa treatment.