Evaluation of the equivocal test results of Treponema pallidum haemagglutination assay.

Two hundred and eighty Rapid Plasma Reagin (RPR) positive sera with an emphasis on cases with negative and borderline positive Treponema pallidum haemagglutination assay (TPHA) results were selected. Modified TPHA (M-TPHA) and fluorescent treponemal antibody absorption (FTA-abs) tests were used for comparison. One hundred and twenty five samples were TPHA negative, of which 78 and 69 cases were also negative by M-TPHA and FTA-abs, respectively. Eighty one sera negative by TPHA at a titre of 1/80 and positive at 1/40, considered to be negative according to the manufacturer's instructions, were also negative by M-TPHA (n = 11) and by FTA-abs (n = 1). Fifty borderline positive TPHA specimens gave one negative result by both M-TPHA and FTA-abs. The remaining 24 sera were positive by all three tests. Because of the high percentage of TPHA negative results among the positive RPR sera which became reactive when rechecked by the FTA-abs, it is concluded that as a confirmatory test the TPHA should be used not instead of but in addition to the FTA-abs.