Background: The PrEPX trial provided HIV pre-exposure prophylaxis (PrEP) to 2600 people in Victoria, Australia. To determine participants' HIV risk, PrEPX used six eligibility criteria derived from the Australian PrEP guidelines, based on the CDC PrEP guidelines. Participants who fulfilled no eligibility criteria could be enrolled if clinically assessed to need PrEP. This study evaluated whether PREPX eligibility criteria correlated with biological HIV risk markers, namely syphilis, anorectal chlamydia or anorectal gonorrhoea (STIs). Methods: Adjusted odds ratios (aOR) were calculated to assess whether each eligibility criterion predicted a diagnosis of the aforementioned STIs at enrolment. Results: 1774 participants were included, 10.2% tested positive for STIs. Eligibility criteria predicted STI diagnoses as follows: (1) aOR 2.5 (95%CI 1.4-4.4) for condomless anal intercourse (CLAI) with a HIV-positive regular sexual partner (RSP) with detectable viral load; (2) aOR 1.8 (95%CI 1.3-2.5) for receptive CLAI with casual sexual partners; (3) aOR 1.8 (95%CI 1.3-2.5) for previous STIs; (4) aOR 2.1 (95%CI 1.4-3.0) for methamphetamine use; (5) aOR 0.8 (95%CI 0.6-1.1) for unsuccessful condom use; and (6) aOR 1.0 (95%CI 0.7-1.4) for insertive CLAI when uncircumcised. Participants who enrolled outside the eligibility criteria had an STI frequency of 7.1%. Conclusions: Eligibility criteria 1-4 predicted diagnoses of STIs, eligibility criteria 5-6 did not. Our findings support the use of PrEP eligibility criteria recommended in current guidelines. Participants enrolled outside the eligibility criteria had substantial prevalence of STIs, suggesting that people who request PrEP but do not fulfil eligibility criteria may nonetheless benefit from PrEP.
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