Background: In 2013, guidelines for reducing the risk of HIV, hepatitis B and C disease transmission through organ transplants were released. These include criteria which result in a donor being designated as increased risk (IR) for infection if certain behaviors occurred within the previous 12 months. HIV transmission risk from IR donors despite negative nucleic acid testing (NAT) varies based on the type and timing of the behavior. A more precise risk estimate may result in improved organ utilization. We developed a risk model to quantify probability of HIV infection among IR donors. Methods: We evaluated published reports describing per-act HIV transmission risk for the following IR behaviors: unprotected, receptive anal male-to-male intercourse with partner of unknown HIV status (MSM), non-medical injection drug use (IDU) with needle sharing, and sex with a commercial sex worker (CSW) of unknown HIV status. The limits of viral load detection for commercially available NAT assays were gathered. Data from the following studies were analyzed: MSM seroconversion- California, Colorado and Illinois (1998), serodiscordant couples – Uganda (2005); IDU cohort-Thailand (2002), female sex worker cohort-Kenya (2008). Data were integrated into a Monte Carlo model to estimate the upper-end probability of undetected HIV infection by day following IR exposure. Risk estimates assume a single exposure, transmission risk per-act at the reported 95% CI, and NAT screening. Risks were computed based on log-normal distributed per act viral inoculum, log-normal distributed NAT detection threshold, and a normally distributed viral exponential growth rate. Viral growth from initial inoculum to NAT detection was simulated 1000x per behavior; mean initial viral inoculum was assumed to be proportional to the per-act infection risk. Results: Percent risk probability of HIV infection undetected by screening, expressed by percentage by days since exposure for different increased risk activities.

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