The between-Laboratory Variation of Factor VIII Inhibitor Testing: The Experience of the External Quality Assessment Program of the ECAT Foundation

The detection and quantification of factor VIII (FVIII) inhibitors is clinically important both for the identification of hemophilia A patients with inhibitors and for the management of immune tolerance treatment. Only limited data are available on the between-laboratory variation of FVIII inhibitor testing. This report describes the evaluation of the results of the large-scale external quality assessment program of the European Concerted Action on Thrombosis Foundation. This study includes the results of six different surveys for the period 2006 to 2008 with 100 to 170 participating laboratories. The overall between-laboratory variation ranged from 28% to 52% with a slightly lower variation for the Nijmegen assay (similar to 39%) on average than for the Bethesda assay (similar to 45%). The use of buffered normal pooled plasma as FVIII source showed better performance compared with the use of nonbuffered pooled plasma; likewise the use of FVIII-deficient plasma compared with the use of imidazole buffer. However, the combination of both was essential for lowest between-laboratory variation. The Nijmegen assay also showed better performance with respect to specificity and sensitivity than the Bethesda assay, although the results for neither were entirely satisfactory. In general, it can be concluded that the measurement of FVIII inhibitory antibodies with the Nijmegen assay should be favored over the use of the Bethesda assay. However, further improvement of the laboratory test for FVIII inhibitors is urgently needed.