Fluorescent Treponemal Antibody Absorption Double-Staining Test Evaluation

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Abstract
The fluorescent treponemal antibody absorption (FTA-ABS) double-staining (DS) test was developed for microscopes equipped with incident illumination; the procedure offers many advantages over the FTA-ABS test. Fresh sera (346), including 35 from patients with syphilis, were evaluated by the FTA-ABS DS test. Parameters for investigation included 2 readers, each using a different microscope; a new FTA-ABS DS test reporting system; sera heated at 56.degree. C for 30 min vs. unheated sera; and sera retested after at least 2 wk of freezer storage. Agreement for FTA-ABS DS test readings between the 2 microscopes was 99%. Between-test agreement for the FTA-ABS test with the conventional reporting system and the FTA-ABS DS test with the new reporting system was 95%. Sensitivity calculations based on reactivity for the 35 syphilis sera were 94% for the FTA-ABS DS test and 91% for the FTA-ABS test. Specificity calculations based on non-reactivity of nonsyphilis sera were 98% for the FTA-ABS DS test and 93% for the FTA-ABS test. Differences in percentages appeared to be related to borderline readings in the FTA-ABS test; e.g., if the same reporting system was used for the reference FTA-ABS test, the specificity was 97%. When sera were examined with 48 h, no difference was observed in results obtained with heated and unheated sera. Sera frozen for 2 wk showed comparable results in the FTA-ABS DS test and the FTA-ABS test. These findings strongly suggest that the FTA-ABS DS test be accepted as a confirmatory test for syphilis. The new reporting system for the FTA-ABS DS test would be advantageous for the reference FTA-ABS procedure.