How often did syphilis tests have corresponding HIV tests in Ontario, Canada? A retrospective analysis of comprehensive laboratory data

OBJECTIVES: Canadian guidelines recommend HIV testing for individuals being evaluated for syphilis. Our objective was to examine three aspects of HIV testing (ie, if an HIV test occurred, the timing of the HIV test in relation to the syphilis test and the proportion with a positive HIV test result) among syphilis tests between 2017 and 2022 from individuals with no evidence of a previous HIV diagnosis. DESIGN AND SETTING: This study is a retrospective analysis of comprehensive laboratory testing data from Ontario's provincial public health laboratory. PARTICIPANTS: Direct fluorescent antibody (DFA) and serological non-prenatal syphilis tests were conducted from 1 January 2017 to 31 December 2022, from individuals aged ≥15 years with no evidence of a previous HIV diagnosis (n=3 001 058 total tests). Positive syphilis tests were categorised using the rapid plasma reagin (RPR) titre as 'current' (DFA+/RPR≥1:8) or 'historical' (RPR<1:8). Exposure categories were assigned using individually linked HIV exposure category data retrieved from the laboratory's HIV DataMart. PRIMARY AND SECONDARY OUTCOME MEASURES: The number and proportion of syphilis tests with a corresponding HIV test on the same day or within 7, 28, 90 or 180 days, and, among those with an HIV test within 28 days, the number and proportion with an HIV-positive test result. RESULTS: From 2017 to 2022, 1 516 726 and 1 484 332 syphilis tests among males and females, respectively, were included in the analysis. Individuals with a positive syphilis result were less likely to be tested for HIV within 28 days of their syphilis test compared with those with a negative syphilis test result (74.7% vs 91.1% in males, 97.5% CI (-0.17 to -0.16); 65.2% vs 92.4% in females, 97.5% CI (-0.28 to -0.26)). Males with 'current' positive syphilis test results were less likely than males with 'historical' positive syphilis results to be tested for HIV within 28 days (69.1% vs 76.6%, 97.5% CI (-0.084 to -0.066)); this was not true in females (67.1% vs 64.4%, 97.5% CI (0.0062 to 0.049)). Males overall and males with 'current' syphilis were more likely to be diagnosed as HIV-positive (p<0.025). CONCLUSIONS: Most individuals who tested for syphilis at Public Health Ontario were also tested for HIV; however, those who tested positive for syphilis were less likely to be tested, representing an opportunity for enhanced HIV testing. Ensuring that individuals with syphilis are tested for HIV may help identify previously undiagnosed individuals living with HIV.

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