A re-appraisal of the normal cut-off assignment for anticardiolipin IgM tests

Background: Recent reports show an apparent large number of individuals with low to moderate titers of anticardiolipin antibodies (ACA), particularly of the IgM isotype with no clinical signs of antiphospholipid syndrome (APS). The significance of these results is unknown. This study examined the prevalence of low positive titers of IgM ACA antibodies in a large number (n = 982) of normal blood donors (Group 1) and in a group of 159 individuals > 60 years of age (Group 2). The effect of re-defining the currently used cut-off values for the IgM ACA tests was also examined. Methods: IgM ACA antibodies were tested in three ELISA assays: the Bindazyme(TM) Anti-IgM Cardiolipin EIA kit (assay A), an 'in-house' ACA test (assay B), and the APhL((R)) ELISA kit (assay C). Results: The normal range cut-offs were re-calculated using the 95th percentile of the data for Group 1 (12.4 MPL U mL(-1) for assay A, 5.4 MPL U mL(-1) for assay B and 9.5 MPL U mL(-1) for assay C) and Group 2 (9.9 MPL U mL(-1) for assay A, 5.5 MPL U mL(-1) for assay B and 13.2 MPL U mL(-1) for assay C). These values were not significantly different from the current cut-off values for each assay. The prevalence of low positive results in Group 1 relative to the re-defined cut-off for that group were: 1.0%, 1.1% and 0.9% in assay A, B and C; and in Group 2: 0.6%, 0.6% and 0.6%, respectively. An indeterminate zone (between the 95th and 99th percentile) was then established for the two groups. The prevalence in Group 1 was 3.8%, 3.9% and 3.9% for assays A, B and C, respectively, and for Group 2: 4.4% in all three assays. Conclusions: The data confirm that the current cut-off point for each of the three assays is correct. We suggest based on this study that the low positive range is re-assigned 'indeterminate' and recommend that samples falling in this category should be retested to confirm positivity at a later date.