HAEMOSTASIS
laboratories to be able to measure plasma factor VIII levels accurately and precisely in order to assess the treatment response. Most laboratories use one-stage assays for this purpose, while a few use chromogenic assays, two stage assays or a combination of assay type. UK NEQAS BC obtained plasma samples following infusion of one of three different recombinant factor VIII concentrates and distributed these to 57 UK haemophilia centre laboratories for determination of factor VIII. Results showed that the measured levels of factor VIII varied significantly (up to 40%) depending on the type of assay used and according to the reagents and calibration material employed in local assay systems. This variation, if translated into patient samples, could be clinically significant and suggests that improved standardisation in the post-infusion assay of recombinant factor VIII is desirable.
Inhibitor development The haemophilia theme continued in the final formal presentation of the meeting, Inhibitor Development in Patients Treated with Plasma-Derivedor Recombinant Clotting Factor, given by Dr David Lillicrap (Kingston, Canada). The production of antibodies (inhibitors) directed against infused clotting factor
concentrates is a significant adverse and costly side effect of haemophilia treatment. The rate of incidence of inhibitor development is about 25% for factor VIII (haemophilia A) and 2% for factor IX (haemophilia B). It is thought that this difference may be related to the fact that many haemophilia B patients have a central tolerance to factor IX through the synthesis of a dysfunctional factor IX protein and because of the structural similarity of factor IX to other vitamin K-dependent proteins. The susceptibility of an individual to develop inhibitors is multifactorial in aetiology, with genetic and environmental factors playing a part. In haemophilia A, the specific mutant factor VIII genotype is the strongest predictor of inhibitor formation; patients with multidomain deletions have about a 70% likelihood of developing an inhibitor. Of the acquired risk factors, high-intensity treatment and the presence of a co- existing inflammatory condition convey increased risk. The immunogenic potential of different factor concentrates has been studied, particularly since the introduction of recombinant factor VIII concentrates some 20 years ago. A systematic review has suggested that recombinant products may be associated with greater immunogenicity; however, several independent studies have shown
inconsistent results and a meta-analysis has failed to show any significant product- related difference in immunogenicity. An ongoing prospective study comparing inhibitor development rates in patients treated with plasma-derived and recombinant factor VIII concentrates should give further information.
Concluding updates The meeting concluded with two brief updates on the point-of-care testing programmes and the results of a participants’ questionnaire, given by Mrs Dianne Kitchen and Dr Ian Jennings of the UK NEQAS BC team in Sheffield. ProfessorWalker brought the
proceedings of another very informative and enjoyable meeting to a close by thanking the speakers and UK NEQAS BC staff for their contributions, companies for their sponsorship and delegates for their attendance.
About the author
PeterCotton is a member of the UK NEQAS for Blood Coagulation steering committee. The next UK NEQAS BC annual scientific meeting is scheduled for 4–5 September.
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