Evaluation of APTT reagent sensitivity to factor IX and factor IX assay performance. Results from the College of...

Hereditary factor IX deficiency (hemophilia B) is among the more common hereditary bleeding disorders and factor IX assays are among the more common specific factor assays performed by coagulation laboratories. To assess the sensitivity of various reagents used for performance of activated partial thromboplastin times and factor IX assays, a series of samples with varying levels of factor IX were included in the 1988 College of American Pathologists Survey Program. We found significant differences in the sensitivity of reagents to factor IX deficiency. Surprisingly, the least sensitive reagents were among the most commonly used reagents. Significant differences in the classification of activated partial thromboplastin times as abnormal were noted between H1 and H2 survey participants. As with factor VIII assays, significant differences in the dose-response curves for factor IX deficiency were noted between reagents, with more responsive reagents giving more precise results for factor IX assays. Comparison of factor IX assay performance in 1988 with 1980 performance indicates a substantial improvement in assay precision. However, a further improvement in assay performance could be expected if current recommendations were followed.