Comparison of the fluorescent treponemal antibody absorption (FTA-ABS) test with the FTA-ABS double staining test for detection of antitreponemal immunoglobulin M in the 19S fraction of human serum.

A widely used immunoglobulin M (IgM) detection assay for the diagnosis of neonatal congenital syphilis is the fluorescent treponemal antibody absorption test used with fractionated serum (FTA-ABS 19S IgM test). Reading the results of the FTA-ABS test is more cumbersome than reading those of the FTA-ABS double staining (FTA-ABS-DS) test, a confirmatory test for specific IgG. To verify that the FTA-ABS-DS test used with an anti-human IgM conjugate could detect specific IgM in fractionated serum samples (FTA-ABS-DS 19S IgM test), 164 fractionated (QUIK-SEP IgM Isolation System; ISOLAB, Inc., Akron, Ohio) serum specimens from infected neonates or adults or from IgG-seronegative subjects were tested by both techniques. The sensitivity limits of the two tests were assessed with reactive serum samples diluted to an endpoint titer. Samples nonreactive by the FTA-ABS 19S IgM test (n = 74) were either nonreactive (n = 65), minimally reactive (n = 5), or reactive (n = 4) by the FTA-ABS-DS 19S IgM test. Samples minimally reactive by the FTA-ABS 19S IgM test (n = 32) were minimally reactive (n = 1) or reactive (n = 31) by the double staining test. All samples reactive by the FTA-ABS 19S IgM test (n = 58) were also reactive by the FTA-ABS-DS 19S IgM test. There was a directly proportional linear relationship (r = 0.9794) between titers obtained by both tests. FTA-ABS-DS 19S IgM titers were constantly equal to or higher than FTA-ABS 19S IgM titers. Fluorescence intensity reading repeatability was 91.4% for the FTA-ABS-DS 19S IgM test and 81.7% for the FTA-ABS 19S IgM test (P = 0.015). Because the more easily read FTA-ABS-DS 19S IgM test is at least as sensitive as, if not more sensitive than, the FTA-ABS 19S IgM test, it is a good alternative to the latter test for the detection of specific IgM in human fractionated sera for those using fluorescence microscopes with incident light.