In order to "detect and quantify current risk factors for HIV seroconversion among gay men seeking repeat tests at sexual health clinics," the authors conducted an unmatched case control study in London, Brighton, and Manchester.

The authors identified 75 case subjects (who had recently tested HIV-positive after having received a negative result within the past two years) and 157 control subjects (who had recently tested HIV-negative after having received a negative result within the past two years). The subjects all completed a computer-assisted self-interview that focused on their lifestyle and sexual behavior in the period between the tests.

The men in both groups were similar socio-demographically, in years since initiating sex with men, in lifetime number of HIV tests, in reasons for seeking previous HIV tests, and in the time since their last HIV test (mean=10.5 months). Risk factors noted in the period between tests included unprotected receptive anal intercourse (URAI) with partners not believed to be HIV-negative (adjusted odds ratio (AOR) and 95 percent confidence interval 4.1, 1.8 to 9.3), where increased risk was associated with concomitant use of nitrite inhalants, receiving ejaculate, and increasing numbers of partners. The researchers also noted independent risk for unprotected insertive anal intercourse (UIAI) with more than one man (AOR 2.7, 1.3 to 5.5) and use of nitrite inhalants (AOR 2.4, 1.1 to 5.2).

"HIV serodiscordant unprotected anal intercourse remains the primary context for HIV transmission among gay men, with increased risk associated with being the receptive partner, receiving ejaculate, and use of nitrite inhalants," the authors concluded. "Although the HIV transmission risk of URAI is widely acknowledged, this study highlights the risk of UIAI and that nitrite inhalants may be an important facilitator of transmission when HIV exposure occurs."

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