Selecting an HIV Test: A Narrative Review for Clinicians and Researchers

Abstract

Given the many options available, selecting an HIV test for a particular clinical or research setting can be daunting. Making an informed decision requires an assessment of the likelihood of acute infection in the test population and an understanding of key aspects of the tests themselves. The ability of individual tests to reliably detect HIV infection depends on the target(s) being detected, when they can be expected to be present after infection, and the concentration of stable target in test specimens, all of which are explained by the virologic and serologic events after infection. The purpose of this article is to review the timeline of HIV infection, nomenclature, and characteristics of different tests; compare point-of-care and laboratory-based tests; discuss the impact of different specimens on test performance; and provide practical advice to help clinicians and researchers new to the field select a test that best suits their needs.

Figure 1. Timeline of virological and serological events following HIV infection

The length of time between an exposure event (X) and dissemination of HIV systemically depends on the mode of transmission. The eclipse period reflects time from exposure to the first detectable marker of infection: HIV RNA in the blood. Times to reactivity for each type of diagnostic test are depicted below the graph, from the earliest (nucleic acid amplification test, NAAT) to the latest (IgG sensitive assay). Adapted from Busch & Satten and Fiebig EW, et al. AIDS 2003;17(13):1871–9. Earliest time to reactivity estimates from Delaney, et al. IgM and IgG curves informed by Cooper DA, et al. J Inf Dis 1987;155(6):1113–8 and Gaines H, et al., AIDS 1988;2(1):11–5.

Figure 2. HIV serological tests

In IgG sensitive tests, anti-HIV IgG from a patient’s specimen binds to immobilized HIV antigens on the solid phase of the assay (gray line); detection most often uses protein A conjugated to colloidal gold. IgM/IgG sensitive tests permit binding of both IgM and IgG from specimens, with detection accomplished using labeled, synthetic HIV antigens (enzymatically-linked or conjugated to a chemiluminescent marker). The addition of simultaneous, monoclonal antibody-mediated p24 antigen detection distinguishes Ag/Ab combination assays from IgM/IgG sensitive tests.

Figure 3. HIV rapid tests

In lateral flow assays, specimen from a patient is added to one end of a nitrocellulose strip. As capillary action draws the specimen toward a wicking pad at the opposite end (open arrow), detection reagents are rehydrated and bind to patient antibodies. Anti-HIV antibodies present in the specimen bind immobilized HIV antigens in the test line, while anti-human antibodies on the control line non-specifically bind any immunoglobulin from the specimen. Accumulated colloidal gold produces a visible color change. “Dual path” tests combine two perpendicular lateral flow strips in a single test cassette. Patient specimen and buffer solution migrate through the first strip, depositing antibodies where the strips intersect and washing away colored markers that identify the second strip’s control and test lines. Once the markers have disappeared, additional reagents are applied to the second strip, rehydrating detection reagents to react with the patient specimen. Flow-through tests use a similar approach, except the detection reagent (e.g., protein A) is added separately rather than rehydrated from the test membrane. Generally, flow-through tests yield a result faster than lateral flow tests can.

Figure 4. Recommended laboratory HIV testing algorithm

Any assay capable of reliably detecting p24 antigen (Ag) and both IgM and IgG antibodies (Ab) is the recommended starting point for HIV screening in the CDC algorithm, updated in 2014. Reactive specimens from the initial test are subjected to an IgG-sensitive supplemental immunoassay capable of differentiating HIV-1 from HIV-2; this step replaces the HIV-1 Western blot. Indeterminate or negative results from the differentiation test indicate either acute HIV infection (positive for p24 antigen or IgM antibody) or a biological false positive test; the presence of detectable HIV RNA on a subsequent nucleic acid test is the arbitrator. Adapted from CDC.