Background Many individuals with HIV in the USA are unaware of their diagnosis, and therefore cannot be engaged in treatment services, have worse clinical outcomes and are more likely to transmit HIV to others. Mobile van testing may increase HIV testing and diagnosis. Our objective was to characterise risk factors for HIV seroconversion among individuals using mobile van testing.

Methods A case cohort study (n=543) was conducted within an HIV surveillance dataset of mobile van testing users with at least two HIV tests between September 2004 and August 2009 in Baltimore, Maryland. A subcohort (n=423) was randomly selected; all additional cases were added from the parent cohort. Cases (n=122 total, two from random subcohort) had documented seroconversion at the follow-up visit. A unique aspect of the analysis was use of Department of Corrections data to document incarceration between the times of initial and subsequent testing. Multivariate Cox proportional hazards models were used to compare HIV transmission risk factors between individuals who seroconverted and those who did not.

Results One hundred and twenty-two HIV seroconversions occurred among 8756 individuals (1.4%), a rate higher than that in Baltimore City Health Department’s STD Clinic clients (1%). Increased HIV seroconversion risk was associated with men who have sex with men (MSM) (HR 32.76, 95% CI 5.62 to 191.12), sex with an HIV positive partner (HR 70.2, 95% CI 9.58 to 514.89), and intravenous drug use (IDU) (HR 5.65, 95% CI 2.41 to 13.23).

Conclusions HIV testing is a crucial first step in the HIV care continuum and an important HIV prevention tool. This study confirmed the need to reach high-risk populations (MSM, sex with HIV-positive individuals, individuals with IDU) and to increase comprehensive prevention services so that high-risk individuals stay HIV uninfected. HIV testing in mobile vans may be an effective outreach strategy for identifying infection in certain populations at high risk for HIV.