Factor IX Metal Ion-Dependent Antigen Assays for Measurement of Warfarin Effect

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Abstract
Factor IX metal ion-dependent antigen was assayed using monoclonal antibodies in 521 samples obtained after prothrombin time testing in patients treated with warfarin. Factor IX metal ion-dependent antigen was less than measured Factor IX clotting activity and less than total Factor IX antigen adsorbable to aluminum hydroxide, suggesting that the metal ion-dependent antigen assay measures a subpopulation of circulating Factor IX in patients treated with oral anticoagulants. There was a graded decrease of Factor IX metal ion-dependent antigen as prothrombin times increased in patient samples. In two hospitals, the median prothrombin times and Factor IX antigen levels were 19 and 20 seconds and 0.10 and 0.11 U/mL (kU/L), respectively. This study shows that immunoassays measure the biologic effect of warfarin and provide information that may supplement the prothrombin time test for patient monitoring. Factor IX metal ion-dependent antigen assays may be useful in efforts to standardize laboratory tests for warfarin effect.