Staining intensities in the fluorescent treponemal antibody-absorption (FTA-Abs) test: association with the diagnosis of syphilis.

In 1984 the reporting system for the fluorescent treponemal antibody-absorption (FTA-Abs) test was changed by the Centers for Disease Control (CDC; Atlanta, GA) to eliminate the borderline report. Factors influencing the reliability of the FTA-Abs test results, i.e., sensitivity, specificity, prevalence of syphilis, prescreening of sera with nontreponemal tests, and reproducibility, were considered before the change in the reporting system was recommended and are reported here. The borderline report, when associated with syphilis, was most frequently also associated with the diagnosis of early primary, dark-field-positive, nontreponemal test-nonreactive syphilis. Whereas elimination of the borderline report decreased the sensitivity of the FTA-Abs test as a confirmatory test from 100% to 99.5%, the specificity increased from 82.5% to 88.7%. The 1+ staining intensity had an association of approximately 5% with the diagnosis of syphilis. The changes in the reporting system were designed to assist the clinician in interpreting the results of the FTA-Abs test in those cases that present diagnostic dilemmas.