Factor VIII/IX inhibitor testing practices in the United Kingdom: Results of a UKHCDO and UKNEQAS national survey.

Abstract

Inhibitor formation is the greatest challenge facing persons with haemophilia treated with factor concentrates. The gold standard testing methodologies are the Nijmegen-Bethesda assay (NBA) for FVIII and Bethesda assay (BA) for FIX inhibitors, which are affected by pre-analytical and inter-laboratory variability. To evaluate inhibitor testing methodology and assess correlation between self-reported and actual methodology. Methodology was evaluated using a survey distributed alongside a UK National External Quality Assessment Service Blood Coagulation external quality assurance (EQA) exercise for FVIII and FIX inhibitor testing. Seventy four survey and EQA exercise responses were received (response rate 63.2%), with 50 paired survey/EQA results. 47.1% (33/70) reported using the NBA and 42.9% (30/70) the BA for FVIII inhibitor testing. Review of FVIII inhibitor assay methodology demonstrated discrepancy (self-reported to actual) in 64.3% (BA reporting) and 27.6% (NBA reporting). Pre-analytical heat treatment was used by 32.4%, most commonly 56°C for 30minutes. Assay cut-offs of 0.1-1.0BU/mL were reported. EQA samples (acquired FVIII and congenital FIX) demonstrated titres and coefficients of variation (CV) of 3.1BU/mL (0.7-15.4BU/mL; CV=43%) and 18.0BU/mL (0-117BU/mL; CV=33%), respectively. No significant assay or laboratory factors were found to explain this variance, which could have resulted in change in management for 6 patients (5 misclassified high-titre FVIII inhibitors and 1 false negative for a FIX inhibitor). Heterogeneity was seen at each stage of assay methodology. No assay-related factors were found to explain variation in inhibitor titres. Further standardization is required to improve inhibitor quantification to guide patient care.