HIV Testing in High-Risk Settings
Blood-based laboratory testing is preferred whenever possible after any exposure event, with antigen/antibody tests strongly preferred over antibody-only tests.2 Unless NAT is performed, follow-up testing is required and should be undertaken 4 to 6 weeks and 3 months after the baseline test. Additional testing at 6 months should occur if the exposure event led to hepatitis C infection.2 Populations at high risk for HIV and with a high likelihood of recent infection, which may include individuals on PrEP, should receive a laboratory-based p24/IgM/IgG test that assesses serum or plasma. If laboratory-based testing is not possible, a blood-based POC test can be considered. In contrast, if the infection is unlikely to be recent, either a rapid antigen or a laboratory-based antigen/antibody test can be used. These test types have shown similar sensitivity in chronic HIV when blood-based specimens are used.<sup2
Laboratory Tests and Recommendations
Laboratory-based tests can evaluate serum, plasma, dried blood spots, oral fluid, or urine and may include NAT, antibody tests, and antigen/antibody tests. Serum- or plasma-based antigen/antibody tests are the most accurate and the Centers for Disease Control and Prevention’s preferred initial test for HIV.10 Nonreactive specimens on a serum- or plasma-based test require no further testing. Reactive specimens should be tested with an antibody immunoassay that differentiates HIV-1 antibodies from HIV-2 antibodies; this recommendation replaces use of the less specific, slower, and more labor-intensive Western blot.2,10 If the antibody test is also reactive, the specimen is considered HIV positive. If the results are indeterminate or nonreactive, a NAT should be undertaken.10 In the setting of a reactive rapid test, a serum- or plasma-based antigen/antibody test should be undertaken for confirmation. No further testing is required in the setting of a nonreactive serum- or plasma-based rapid test.10
Available Test Types
Three types of HIV tests are available: nucleic acid tests (NAT), antibody tests, and antigen/antibody tests. A NAT looks for viral RNA in the blood. It is a laboratory-based test that can detect HIV in the blood as early as 7 days after infection, but because the test is expensive, it is usually used following a high-risk exposure or for confirmatory testing.3 Most POC tests and self-tests are antibody tests, which assess for HIV antibodies in the blood or oral fluid. Antibody tests sensitive to IgM and IgG detect HIV a median of 23 days after infection, whereas those assessing IgG alone detect HIV at a median of 31 days after infection.2 Antigen/antibody tests assess for the p24 antigen in addition to IgG/IgM. These tests can detect HIV a median of 18 days after infection.2
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Most HIV tests assess blood (whole, serum, or plasma) or oral fluids. Although urine-based antibody tests are also available, they are less reliable.4 Blood serum or plasma specimens have the highest sensitivity, followed by whole blood and then oral fluid specimens.2 The reason for this is twofold. First, serum or plasma specimens contain the highest concentration of HIV antigens and antibodies per unit volume.2 For example, concentrations of HIV antibodies in plasma are 300-fold higher than concentrations in oral fluids.2 Second, HIV antibodies take longer to reach detectable levels in oral secretions than in blood. In a longitudinal study of a small Nigerian cohort of people who had undergone seroconversion, the median delay between plasma and oral fluid reactivity was 29 days, which occurred regardless of viral load or HIV subtype.5
Time From Infection to HIV Marker Detection
Understanding how long it takes for certain HIV markers to show reactivity following exposure is important to select the correct HIV test. The first reliable marker of HIV infection is HIV RNA, which becomes detectable in 50% of individuals within 12 days of becoming infected.2 The next marker to manifest is p24, a viral protein that makes up most of the HIV viral core. It usually becomes detectable around day 18, continues to rise until day 30, and is completely cleared by day 50; thus, it is only useful in detecting HIV before seroconversion. A serum antibody response first becomes detectable around day 20, which is when immunoglobulin class M (IgM) starts to rise. It then peaks around day 30 or 35, which is when the immunoglobulin class G (IgG) response begins.2
Other Specimen Considerations
Although sensitivity is an important consideration when deciding among HIV tests, there are other factors to consider, such as test invasiveness, available resources, need for additional studies, and patient adherence.6 Serum- and plasma-based HIV testing is the most invasive and requires the most resources, such as access to a skilled clinician or phlebotomist and access to special equipment (eg, needles, biohazard containers, laboratory). Whole blood testing can be performed by venipuncture or finger-stick. When blood-based tests yield a sufficient specimen sample using venipuncture, additional studies can be undertaken, such as HIV subtyping and assessment for coinfections, including hepatitis or syphilis. In contrast, oral fluid tests do not require special equipment and can be performed in any setting, including at home, providing results in minutes. These tests also eliminate safety and biohazard concerns, such as needlestick injuries and sharps disposal.
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Available Point-of Care Tests
POC HIV tests, also known as rapid antibody tests, are portable, self-contained, easy-to-use tests that yield results in ≤20 min (range, 1-20 min).2,7 In the United States, 8 rapid tests are approved by the US Food and Drug Administration: Alere Determine HIV-1/2 Ag/Ab Combo Test; Clearview HIV 1/2 Stat-Pak; DPP HIV 1/2 Assay; INSTI HIV-1/HIV-2 Rapid Antibody Test; OraQuick Advance Rapid HIV-1/2 Antibody Test; Reveal Rapid HIV-1 Antibody Test; SURE CHECK HIV 1/2 Assay; and Uni-Gold Recombigen HIV-1/2 Test.2,7 These tests have comparable sensitivity and specificity but differ in time to test result, approved specimen type and volume required, shelf-life (range, 12-24 mo), antibodies assessed for, Clinical Laboratory Improvement Amendments waiver status (ie, Clinical Laboratory Improvement Amendments-waived tests can be used in a wider range of settings), and median time to detection following infection.2,7 A drawback of POC tests is a higher risk for false-negative results than with laboratory tests, particularly early after infection or when oral fluid is used.2 Photo Credit: Medical Images/BSIP/Alice S.
Available HIV Self-Tests
Two HIV self-tests are approved by the US Food and Drug Administration: the Home Access HIV-1 Test System and the OraQuick In-Home HIV test.8 The former only assesses for HIV-1 antibodies and requires use of a blood sample obtained by finger prick, which is sent to a licensed laboratory for evaluation. The latter assesses for HIV-1/2 antibodies using oral fluid collected by swabbing the gums with a test stick, which is then placed into a test tube for processing. Results are available in 20 minutes. Although this test is convenient, up to 1 in 12 people may receive a false-negative result because of the lower HIV antibody concentrations in oral fluid.8</sup Despite concerns with self-tests, the literature indicates that most people perform and interpret these tests properly and that the results are usually accurate.9 Test users also generally understand the need to confirm a positive test result but might not recognize the need for more sensitive testing to confirm a negative result.9 Photo Credit: Medical Images/Scott Camazine
Cellphone Technology to Detect Acute HIV
Detecting acute HIV infections has remained a major challenge, particularly in developing countries and other resource-restricted areas, where use of less sensitive oral-fluid-based rapid antibody tests can miss the diagnosis and follow-up testing might prove challenging. Recently, investigators from Brigham and Women’s Hospital in Boston reported on a new mobile HIV diagnostic tool they developed that combines cellphone-based optical sensing, loop-mediated isothermal DNA amplification, and micromotor motion to detect HIV-1 RNA molecularly.11,12 The tool had 99.1% specificity and 94.6% sensitivity at a clinically relevant threshold value of 1000 virus particles/mL, with results available in 1 hour. The cost of each test, which includes a microchip, 3D-printed phone attachment, and reagents, was less than $5. The investigators note that the same technology could be used to develop low-cost diagnostic tools to detect other pathogens rapidly. Photo Credit: IBM Research/Science Source
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HIV Testing in the PrEP Setting
Individuals on PrEP who become infected with HIV, such as those with poor adherence to treatment, may have delayed seroconversion on blood-based and oral-fluid-based rapid antibody tests.2 In one study, oral antibody reactivity was first observed a median of 125 days (range, 14-547 days) after HIV RNA or antibodies were observed in the blood.13 In another study, individuals who received PrEP were 3.49 times more likely than those on placebo to experience a delay of >100 days in detecting the infection.14 Based on these and other findings in the literature, it has been suggested that laboratory-based serum or plasma HIV tests should be used whenever possible to monitor individuals on PrEP.2 Additionally, in the setting of known or suspected poor PrEP adherence, the threshold for augmenting p24/IgM/IgG assays with NAT should be lowered.2
Selecting an HIV Test
Accurate and early detection of HIV is essential to prevent HIV transmission and optimize outcomes. The results of 2 pivotal randomized trials, START and TEMPRANO, showed that antiretroviral therapy is most beneficial when started early, when pretreatment CD4 T lymphocyte counts are >500 cells/mm.3,1 Knowledge about HIV status can also help at-risk individuals take measures to reduce their risk for becoming infected, such as by using preexposure prophylaxis (PrEP). Many different types of HIV tests are currently available, including laboratory-based tests, point-of-care (POC) tests, and self-tests. Knowing the advantages and drawbacks of these tests and how best to use them can make HIV test selection a less intimidating process.
Compiled by Christina T. Loguidice
- U.S. Department of Health and Human Services. AIDSinfo. Statement by the HHS Panel on Antiretroviral Guidelines for Adults and Adolescents Regarding Results from the START and TEMPRANO Trials [press release]. July 28, 2015. Accessed January 15, 2019.
- Hurt CB, Nelson JAE, Hightow-Weidman LB, Miller WC. Selecting an HIV test: a narrative review for clinicians and researchers. Sex Transm Dis. 2017;44(12):739-746.
- HIV.gov. HIV Testing Overview. Updated May 14, 2018. Accessed January 15, 2019.
- Fearon M. The laboratory diagnosis of HIV infections. Can J Infect Dis Med Microbiol. 2005;16(1):26-30.
- Luo W, Masciotra S, Delaney KP, et al. Comparison of HIV oral fluid and plasma antibody results during early infection in a longitudinal Nigerian cohort. J Clin Virol. 2013;58(Suppl 1):e113-e118.
- World Health Organization. Specimens used for HIV testing. In: Guidelines for Using HIV Testing Technologies in Surveillance: Selection, Evaluation, and Implementation: 2009 Update. 2009. Accessed January 15, 2019.
- Centers for Disease Control and Prevention. Rapid HIV tests suitable for use in clinical settings (CLIA-moderate complexity) [For use with blood, serum or plasma]. Updated August 10, 2016. Accessed January 8, 2019.
- Centers for Disease Control and Prevention. Home Tests. Updated October 16, 2015. Accessed January 8, 2019.
- Ibitoye M, Frasca T, Giguere R, Carballo-Diéguez A. Home testing past, present and future: lessons learned and implications for HIV home tests (a review). AIDS Behav. 2014;18(5):933-949.
- Centers for Disease Control and Prevention. 2018 Quick Reference Guide: Recommended Laboratory HIV Testing Algorithm for Serum of Plasma Specimens. CDC Stacks. Updated January 2018. Accessed January 9, 2019.
- Draz MS, Kochehbyoki KM, Vasan A, et al. DNA engineered micromotors powered by metal nanoparticles for motion based cellphone diagnostics. Nat Commun. 2018;9:4282.
- Brigham and Women’s Hospital. Research Briefs. Cellphone Technology Developed to Detect HIV [press release]. October 16, 2018. Accessed January 15, 2019.
- Curlin ME, Martin MT, Leelawiwat W, et al. Analysis of false negative HIV tests based on oral fluid in 3 clinical trials. Presented at: Conference on Retroviruses and Opportunistic Infections (CROI) 2015; February 23-26, 2015; Seattle, WA. Abstract 635.
- Donnell D, Ramos E, Celum C, et al. The effect of oral preexposure prophylaxis on the progression of HIV-1 seroconversion. AIDS. 2017;31(14):2007-2016.