Comparison between the standard anticardiolipin antibody test and a new phospholipid test in patients with connective tissue diseases

Objective. Antiphospholipid (aPL) antibodies are present in patients with s ystemic lupus erythematosus (SLE) and/or antiphospholipid antibody syndrome (APS) and are associated with recurrent thromboses, thrombocytopenia, and pregnancy losses. The presence of aPL antibodies is routinely tested using a standardized ELISA that utilizes cardiolipin as antigen (aCL ELISA). This test, although sensitive: is frequently positive inpatients with nonrelate d autoimmune disorders and some infectious diseases, making the test less s pecific. Thus there is a need for more specific tests for aCL with equivale nt sensitivity to the standard assay. We evaluated the diagnostic utility o f a new aPL antibody test kit with a unique phospholipid mixture designed t o be more specific than the standard anticardiolipin ELISA. Methods, aPL antibodies (IgG, ISM) were measured by both a standard ELISA a nd a new ELISA kit (APhL(R) (TM)ELISA Kit, Louisville APL Diagnostics, Inc. , Louisville, KY, USA) in the baseline serum from patients enrolled in a 5 year inception cohort, prospective study of early rheumatoid diseases: rheu matoid arthritis (N = 70), SLE (70), scleroderma (45), inflammatory myositi s (36), and early undifferentiated connective tissue disease (CTD) (165). D iagnosis was based on standardized criteria and determined at the last stud y visit. A nested group of patients with Sjogren's syndrome (44) was also d efined. Serum from 200 blood donors (BD) served as controls. Patients with known APS (33) and antinuclear cytoplasmic antibody positive renal vasculit is (52) were also studied. Laboratory personnel were blinded to sample diag nostic group. Results, The kit was 90.9% sensitive for detecting APS. Seven patients miss ed by the kit all had standard aCL values < 40 PL units. Assuming controls do not have APS, the kit was 99.5% specific vs 96.0% for the standard assay . For the patients with CTD, the kit never detected a patient that was not also detected by the standard aCL assay. Conclusion. The APhL (TM)ELISA Kit appears to be more specific than the sta ndard aCL ELISA without adding potential false positive results. The new te st may be useful for followup study for patients found to be aCL positive b y standard assays to increase specificity for aCL screening.