Determination of the Factor VIII Plasma Activity of Hemophilia A Patients Treated with a New Recombinant Factor VIII Concentrate

Abstract

There are two assays suitable for the determination of the factor VIII activity in the patient’s plasma, both of which are commercially available and standardized: the onestage assay and the chromogenic assay. In clinical practice the one-stage assay has gained widespread acceptance due to its simplicity and level of automation. It is based on aPTT and assesses fibrin formation. The two-stage assay, though principally identical to the chromogenic assay, has been almost completely replaced by the latter in clinical practice. The rather more complex chromogenic assay utilizes the photometric detection of a dye which is released from a chromogenic substrate and in its formation speed correlates with factor VIII activity. This assay yields reproducible measuring results and is less sensitive to interference factors than the one-stage assay. It is recommended as a reference test by the ISTH and the European Pharmacopoeia [1]. The one-stage assay determines lower activities for recombinant factor VIII concentrates than the chromogenic assay. This difference is all the greater the more the concentration and composition of the phospholipids used in the aPTT reagent deviate from the physiological state [2]. Interlaboratory experiments showed that if the standardized test protocol of ISTH/SSC is complied with, the discrepancies between one-stage and chromogenic assay are limited to a maximum of 10 % with recombinant concentrates [1]. This difference should not be of any relevance in daily clinical practice. The study presented here is intended to investigate whether those results also apply to ADVATE, a new recombinant factor VIII concentrate. ADVATE only differs from its precursor Recombinate in the elimination of human or animal derived additives during manufacturing, and the introduction of the S/D procedure. For that reason, one-stage assay should be equally suitable for treatment control as with Recombinate. Therefore, we have compared four different aPTT reagents using the one-stage assay. As a reference we used the measuring results obtained by means of a chromogenic assay.