Significance of Factor VIII Inhibitors by ELISA in Patients with Hemophilia A.

Background: The Bethesda assay traditionally has been used to detect Factor VIII inhibitors in patients with Hemophilia A, but recent evidence suggests that it is not sensitive to all inhibitors, particularly non-inhibitory or low-titer antibodies.

Methods: Patients with Hemophilia A without prior history of inhibitor were recruited. Study questionnaire collected demographic and clinical information, bleeding history and factor usage over the preceding 6 months. Functional status was assessed by the Hemophilia Activities List (HAL). Factor VIII inhibitor was assessed by both the Bethesda assay and Factor VIII inhibitor ELISA (GTI Diagnostics). T-test was performed to assess statistical significance.

Results: Data is available for 26 patients, 19 severe, 2 moderate and 7 mild. All subjects had a negative Bethesda assay, but 10 (39%) had a detectable inhibitor by ELISA. 9/10 inhibitor patients had severe hemophilia, while one had mild hemophilia. In severe hemophiliacs, there were no differences in age, HIV status, CD4 count, Hepatitis C positivity or viral load between those with and those without inhibitors. Inhibitors were more frequent in those using plasma-derived concentrates 4/5 (80%), than in those using recombinant products 6/14 (43%), p=0.15. There was no difference in bleeding frequency or functional status in patients with or without inhibitors, although those with inhibitors had more frequent infusions.(Table). In patients on prophylaxis, those with inhibitors had a higher bleeding frequency compared to those without an inhibitor, (p =0.2). 11 patients were not on prophylaxis and had a higher bleeding frequency (p = 0.02) than patients on prophylaxis irrespective of inhibitor presence. However those with inhibitors required more factor doses per bleed compared to those without an inhibitor (4.4 vs. 1.5, p=0.16) even though the mean factor dose was the same (25.3 units/kg vs 25.2 units/kg).

Conclusions: The Factor VIII ELISA assay detected inhibitors in 39 % of Hemophilia A patients who had undetectable inhibitors by standard Bethesda assay. This data suggests that these inhibitors may be clinically relevant, given that inhibitor patients who are not on prophylaxis require more doses of factor per bleeding event. Further study is necessary to determine mechanism and clinical significance of these Factor VIII inhibitors.

Table. Characteristics of severe hemophilia patients with and without ELISA Factor VIII inhibitor