Performance of recalibrated ReFacto laboratory standard in the measurement of FVIII plasma concentration via the chromogenic and one-stage assays after infusion of recalibrated ReFacto (B-domain deleted recombinant factor VIII).

The use of ReFacto Laboratory Standard (RLS) in the one-stage clotting assay was proposed to reduce the underestimation of factor VIII (FVIII) plasma concentration after the infusion of 'ReFacto' (B-domain deleted recombinant FVIII) in haemophilia A patients. Both ReFacto and RLS were recently recalibrated, with the resulting materials containing approximately 20% more protein than the previous products. The aim of this study was to evaluate the performance of recalibrated RLS in the measurement of FVIII plasma concentration after the infusion of recalibrated ReFacto. In 13 severe haemophilia A patients, 25 IU kg(-1) of ReFacto were injected intravenously. Venous blood samples were collected at 0.25, 0.5, 1, 3, 6, 9, 24, 28 and 32 h after the end of the infusion. Pharmacokinetic parameters were measured for the chromogenic and one-stage assays using International Plasma Standard (IPS) and RLS for both assays and assuming a non-compartmental drug disposition. Comparisons among assays and standards were performed using anova. Pharmacokinetic estimates obtained with the chromogenic method were in agreement with those published in the literature. The one-stage method was confirmed to be more sensitive to lower plasma concentrations of FVIII. The measured maximum plasma concentration (C(max)) was slightly higher than theoretical values and independent of the assay used. C(max), area under the curve (AUC) and volume of distribution at steady state (V(ss)) presented non-significant differences among the methods and standards used. The clinical utility of RLS in the evaluation of FVIII concentration after the infusion of ReFacto seems to be reduced since recalibration of the product.