A New Eu³⁺‐Labeled Method for Anticardiolipin Antibody IgM

BACKGROUND: The anticardiolipin antibodies (aCL) test has become a laboratory standard for the clinical diagnosis of antiphospholipid syndrome (APS). To better the quantitative detection of aCL-IgM so as to classify patients correctly and timely as APS positive, we established herein a new immunoassay based on a time-resolved fluoroimmunoassay (TRFIA). METHODS: The complex of cardiolipin plus bovine anti-β2 glycoprotein-I was used as antigen fixed on microtiter plates to detect serum aCL-IgM, and Eu(3+) -labeled rabbit antihuman IgM was used as conjugate. The precision, sensitivity, specificity, coefficient of recovery, and stability of the assay were evaluated, and comparison with the traditional, classical enzyme-linked immunosorbent assay (ELISA) was also made. RESULTS: The detection limit of the aCL-IgM TRFIA kit we established was 0.1 MPL U/ml, with a wider detectable range than commercial ELISA ones when a strong-positive specimen was diluted from 2,630.9 to 0.08 MPL U/ml. There was a good liner range within 0.16 to 2,630.9 MPL U/ml, whereas it was within 5.14 to 328.86 MPL U/ml when using three commercial ELISA ones. The average intra- and interassay variability was 3.19 and 3.70%, respectively. The mean recovery rate was 101.95%. The clinical diagnostic specificity was 98%. Additionally, the established assay kit presented good characteristics of stability and correlated well with the ELISA, and the correlation coefficient was 0.955. CONCLUSION: The aCL-IgM TRFIA provides an approach to a more sensitive and reliable diagnosis of APS. Further validation of its use is required.