Dr. Wendy Stevens and colleagues from the University of Witwatersrand (Wits) conducted a cross sectional study assessing the prevalence of antibody-negative acute HIV infection among 1906 South African men and women visiting a primary health care clinic (the Esselen Street Clinic in Johannesburg) for treatment of their sexually transmitted infections or to learn their HIV status.
All the samples were first screened for HIV with the ELISA test - a total of 672 or 35.2% of tests came back positive.
The study then tested the antibody-negative or indeterminate samples with the HIV RNA test (the Roche MONITOR HIV-1 version 1.5 assay) that can detect HIV genetic material in a sample of blood a couple weeks before the ELISA tests can detect HIV antibodies. But rather than run separate HIV RNA tests on each of the patients - which would have been prohibitively expensive - the Wits team ran tests on pooled samples of blood.
In other words, samples were stored until there were at least one hundred specimens to pool. This pool was then amplified for evidence of HIV RNA. If the pool tested negative, then all the specimens were deemed to be negative. However, if the pool tested positive, progressively smaller pools (of 50, then of 10) were tested until each of the HIV RNA positive specimens were isolated.
In this case, 1200 HIV negative samples were pooled, three of the pools of 100 specimens were positive for HIV-1 RNA, then three were positive in the pools of 50, and seven pools of ten were positive. Individual sample testing revealed eight HIV RNA samples positive. Four of eleven indeterminate HIV ELISA samples were also tests and found to be positive for HIV RNA. Thus, the total number of acute HIV infections was twelve.
The prevalence of acute infections was 0.6% of all the patients in the study, and 0.99% in the antibody-negative patients screened.
If this figure is used to calculate the incidence rate per year (using a 28 day window between when PCR testing and the ELISA can detect HIV) the incidence rate per year of acute HIV was 12.9%. However, researchers have revised that calculation because the window between PCR and ELISA HIV detection is much shorter with the current generation of ELISA than with previous tests (around twelve days). If that shorter window period is used, the incidence rate per year in this cohort was 30.1%.
Dr. Stevens concluded that pooled HIV RNA amplification testing strategies was practical for high risk populations in the primary care setting in South Africa. “These results suggest that the impact of HIV testing programs incorporating acute HIV testing could be proportionately greater in the South African primary care setting than in most developed world settings. This maybe a result of acute infections in our population being driven by factors associated with [actual] HIV acquisition [rather than just] symptomatic sexually transmitted, or anxiety over risky sex.”
But while these methods may hold promise for surveillance purposes, they seem unlikely to improve individual patient management or to be of much use in prevention interventions targeting highly infectious patients with acute HIV- infection, because by the time enough samples are pooled to be tested, most patients experiencing acute HIV-infection will have seroconverted (become detectable by ELISA). Furthermore, even with pooled testing strategies, HIV RNA testing is inaccessible or still cost prohibitive in many sub-Saharan African settings.