Frequency of factor VIII (FVIII) inhibitor in haemophilia A.

OBJECTIVE To determine the frequency of factor VIII specific inhibitors in haemophilia A. STUDY DESIGN Cross-sectional study. PLACE AND DURATION OF STUDY National Institute of Blood Disease and Bone Marrow Transplantation, Karachi, from August 2007 to March 2009. METHODOLOGY Venous blood samples of diagnosed haemophilia A patients were collected in tubes containing 0.109 M (3.2%) trisodium citrate, centrifuged without delay at 1200 G for 15 minutes. Factor VIII inhibitors were screened by APTT based method using 50:50 patients' plasma mixed with normal plasma incubated together for 2 hours at 37°C. Quantitative assay was carried out to measure Bethesda units (BU). Samples were labelled as low titre inhibitor when less than 5 BU detected, while high titre inhibitor when more than 5 BU were detected. RESULTS A total of 140 Haemophilia A patients were evaluated for allo-antibodies who received treatment with FVIII concentrates / FFP/ cryoprecipitate. Among them 21 patients (15%) were found to have positive screening test results for inhibitors. The mean age of patients with inhibitors was 11.9 + 8.81 years. Thirteen were high responders (62%) while 8 were low responders (38%). The mean inhibitor level in low (titre) responders was 2.46 + 1.31 BU while in high (titre) responders it was 29.15 + 12.81 BU. According to severity of the disease 12/21 with severe haemophilia A (57.2%) developed inhibitors, whereas 8/21 with moderate (38%) and 1/21 with mild haemophilia A (4.7%) showed positive results for inhibitors. CONCLUSION Fifteen percent haemophilia A patients developed inhibitors in this cohort, majority with severe and moderate haemophilia A. Age and severity of disease were found to be main contributing factors in patients who developed inhibitors.