SENSITIVITY AND SPECIFICITY OF THE APC RESISTANCE ASSAY IN DETECTION OF INDIVIDUALS WITH FACTOR-V LEIDEN

Resistance to activated protein C (APC) is the most common cause of fa milial thrombophilia, The partial thromboplastin time (PTT)-based test for resistance to APC has been widely employed as a screening test fo r this disorder. However, the utility of this test for screening is no t well characterized. More than 90% of patients with resistance to APC have the G1691A mutation in factor V (factor V Leiden), The authors s tudied the ability of a commercial APC resistance assay to correctly i dentify the factor V Leiden genotype in 130 individuals, At the recomm ended assay cut-off value of 2, the sensitivity of the APC resistance assay was 50%, with a specificity of 98%. Increasing the cut-off value increased the sensitivity but decreased the specificity of the test, Receiver operating characteristic (ROC) curve analysis indicated that the test was of intermediate utility. There was considerable overlap i n APC ratios in the range of 2 to 3 between subjects with a normal fac tor V genotype and heterozygotes for factor V Leiden, The authors conc lude that the APC resistance assay in its present form is not a useful screening test for factor V Leiden heterozygotes. Until the performan ce of this assay is improved, patients should have molecular diagnosti c testing performed to determine their factor V Leiden status.