It is a scenario that keeps the worried well worried: the possibility of a false negative HIV antibody test. Since HIV screening began, it has been acknowledged that false negative results do occur with ELISA assays, albeit rarely, and in most cases are a consequence of testing during the so-called "window period" between infection and full-fledged antibody response.
With the advent of plasma HIV-RNA polymerase chain reaction (PCR) tests, the ability to diagnose individuals during this window period became possible, often within days to weeks of HIV acquisition. This ability to identify people with recent infection has led many clinicians to treat early HIV infection with antiretroviral treatment (ART) in an attempt to preserve HIV-specific immune function and reduce viral set-point.
However, according to a study conducted at the University of California San Francisco by Hare and colleagues and presented at the XV International AIDS Conference, when ART is administered this early in the course of HIV infection, the immune system's exposure to HIV antigens is reduced. This can blunt HIV antibody responses and lead to such HIV-infected individuals testing antibody negative on standard assays.
The investigators initiated their study when they observed an individual who tested negative on standard HIV antibody testing about 1 year after he had initiated ART during acute HIV infection. He had began HIV therapy with recent HIV infection and detectable HIV viral load. Soon after starting ART, his viral load fell below the level of assay detection and remained there.
A pilot study was performed to determine how frequent antibody negativity was among patients receiving ART during acute/early (within 6 months of infection) HIV infection and who maintained viral suppression for at least 24 weeks. Blood was collected from 87 such patients 48 weeks following therapy initiation and at a point representing the longest duration of viral load suppression below the limits of detection.
Four standard HIV antibody assays were used to analyze each patient's blood. Of the 87 subjects, 32% were found to have a negative or indeterminate antibody result by one or more assays at one of the time points. The sensitivity of the assays varied, with the Western blot best able to detect HIV antibodies and the Chiron HIV-1/HIV-2 RIBA assay the least sensitive.
Of the 87 subjects, 7 patients (8%) -- including the index case that prompted the investigations -- had standard EIA/Western blot results that, using current diagnostic algorithms, would be interpreted as negative or indeterminate. These subjects were further studied longitudinally to describe the dynamics of antibody response using additional antibody assays. This analysis found no difference between antibody negative and antibody positive patients in major characteristics, including CD4+ cell count (>500 cells/mm3), baseline viral load (~4.5 log cpm), age, proportion with acute rather than early HIV infection or time to viral rebound.
Again, assay differences were observed and there was no case of a single patient being HIV antibody negative to every assay tested at the same time point. Reversion of EIA preceded Western blot and the Bio-Rad HIV-1/HIV-2 Plus O EIA, the most sensitive assay in this population.
This study describes an important phenomenon that has implications for research and public health as more persons with early HIV infection are treated with ART. Certainly, patients treated during recent HIV infection need to be advised that their HIV antibody tests may be negative while their viral load levels are low. Clearly, they should not interpret this as a cure or a green light for transmission-risk behavior.
Subjects who discontinued ART in this study did indeed experience viral rebound. Equally important is the need to recognize that all these patients had detectable HIV in their plasma by PCR prior to ART initiation -- these were not cases of post-exposure prophylaxis. The report suggests that the use of more sensitive antibody assays and perhaps a combination of assays may be needed to detect HIV seropositivity in these unique patients.