Serologic response to treatment of infectious syphilis

To evaluate the serologic response to treatment of patients with infectious syphilis.

Historical cohort study of all cases of infectious syphilis in Alberta from 1981 to 1987.

A total of 1090 patients were entered; 857 with primary syphilis, 183 with secondary syphilis, and 50 with early latent disease. Two hundred and eight patients were excluded who either were pregnant, had negative serologic results before treatment, had clinical relapse, were treatment failures, or were lost to follow-up.

All 882 evaluable patients were treated with a recommended antibiotic regimen for infectious syphilis and returned for re-assessment including repeat serologic testing.

Seventy-two percent (95% CI, 66% to 77%) and 56% (CI, 43% to 70%) of patients with initial episodes of primary or secondary syphilis had seroreverted according to rapid plasma reagin (RPR) test results by 36 months. A 2- and 3-tube decline was seen by 6 and 12 months in primary and secondary syphilis. Early latent syphilis resulted in only a 2-tube decrease at 12 months. Serologic response was not affected by sex, age, race, or sexual orientation. Patients with their first infection were more likely to experience RPR seroreversal than those with repeat infections. The RPR reversal rates also depended on the pretreatment titer and stage of disease. At 36 months, 24% (CI, 20% to 28%) of patients had nonreactive fluorescent treponemal antibody absorption tests (FTA-Abs), and 13% (CI, 11% to 15%) had nonreactive microhemoglutination tests for Treponema pallidum (MHA-TP).

Adequate therapeutic response for syphilis must be based on illness episode and the pretreatment RPR titer. Treponemal tests can demonstrate seroreversion after 36 months, and a negative treponemal test does not rule out a past history of syphilis.