A simple method for detection of human factor IX inhibitor using ELISA.

We developed a simple ELISA for FIX inhibitors (FIXinh). The relationship between absorbance and various dilutions of plasma containing known FIXinh (28 Bethesda units/ml) was linear, ranging from 200 to 25,600-fold dilution on semi-log paper. Converted into the value of Bethesda Inhibitor Assay, ELISA can detect as low as 0.219 Bethesda units/ml. Absorbance for 30 normal individuals was 0.213 +/- 0.008, which corresponds to < 0.44 Bethesda units/ml. With ELISA, only the value for hemophilia B with FIXinh, which was revealed as positive by Bethesda Inhibitor Assay, was significantly higher than that of normal controls, hemophilia B without FIXinh, hemophilia A with FVIIIinh and systematic lupus erythematosus (SLE) with lupus anticoagulant (LA). The value in the patient with SLE, who had a high level of FVIIIinh (1000 Bethesda units/ml), was 3 Bethesda units/ml of FIXinh with Bethesda Inhibitor Assay, which was not significantly higher than that of normal subjects with ELISA. FIXinh in hemophilia B patients by ELISA for FIXinh was not affected by the presence of other anticoagulants.