The year 2021 marked four decades since the first report of acquired immunodeficiency syndrome or AIDS (now called stage 3 human immunodeficiency virus [HIV]) in 1981. Since that time, significant scientific, technological, and societal progress toward controlling the HIV epidemic and reducing the number of new infections has resulted in a dramatic decrease in the mortality rate of HIV. According to the WHO, death from HIV peaked in 2004, and the global estimate of people dying from HIV in 2021 was 68 percent fewer than in 2004. This reduction in mortality is also seen in the WHO African Region, where HIV mortality has decreased by nearly 55 percent since 2010. Nevertheless, HIV continues to be one of the major causes of mortality due to a sexually transmitted infection. Here, we review the impact of HIV diagnostics and current testing guidelines, and discuss the future of HIV testing in light of the latest clinical advancements in point-of-care (POC) diagnostics.
“HIV continues to be one of the major causes of mortality due to a sexually transmitted infection.”
The importance of HIV testing and routine screening
Data published in 2020 indicates that globally, there are approximately 37.7 million people living with HIV. In the US alone, the latest reports by the CDC indicate that approximately 1.2 million people live with HIV. Some 40 percent of new infections are transmitted by individuals that are not aware of their HIV status, thus regular HIV testing is key to combating and preventing HIV transmission. According to CDC guidelines, people between the ages of 13 and 64 should be screened for HIV at least once. However, for people with certain risk factors, such as high-risk sexual behavior, intravenous drug abuse, or previous hepatitis or tuberculosis infections, once a year screening is advised.
Current HIV testing and diagnostics
Standard methods for HIV testing include serologic assays: enzyme linked immunosorbent assay (ELISA), or screening via antibody test followed by confirmatory testing using p24 antigen detection and/or viral nucleic acid amplification/detection tests (NAAT).
Detecting recent or early exposure to HIV via NAATs
NAAT techniques are used in the case of recent exposure and for early detection (before people develop anti-HIV antibodies). NAAT techniques are also used for HIV patients that are on antiretroviral therapy (ART). In addition, for people on ART, the WHO recommends monitoring treatment adherence by regularly assessing patients’ viral loads.
All of these standard tests take several days to receive the final results. And while these tests are robust and highly accurate, they require significant operational infrastructure, making them ideal for resource-rich but not resource-poor settings. Hence, for the past decade, clinical professionals have focused on POC rapid tests and self-tests for HIV that follow the ASSURED (affordable, sensitive, specific, user-friendly, rapid and robust, equipment-free, and deliverable) criteria.
Diagnostic HIV tests
Diagnostic HIV tests are broadly categorized as POC or laboratory tests, and four generations of these tests have been launched since the first FDA-approved HIV diagnostic test in 1985.
The so-called test generations are based on the analyte being targeted:
First and second generation HIV tests include immunoglobulin G (IgG) sensitive assays that detect IgG against HIV.
Third generation tests, i.e., IgM/IgG sensitive assays, shorten the detection window as they detect IgM antibodies, which manifest earlier than IgG antibodies.
Fourth generation tests include antigen/antibody combination assays that, in addition to IgG/IgM antibodies, detect the p24 antigen, which is part of the HIV capsid protein. Levels of p24 antigen are detectable in plasma by 15 days post exposure, making it an ideal target to detect infection before the onset of a detectable antibody response.
HIV testing at the point of care
Rapid POC tests are an important aspect of HIV diagnostics, especially in resource-limited environments with limited access to centralized laboratories that run the latest fourth generation HIV diagnostics. Almost all rapid tests only detect HIV antibodies (i.e., IgM, IgG, or combination of both) with the exception of Determine HIV Early Detect and Determine HIV-1/2, which both also detect the p24 antigen.
The nature of the sample type and the analyte being detected affects the performance of the rapid test. IgM antibodies appear sooner in the blood compared to IgG antibodies, which are longer lasting. Most rapid tests use whole blood derived from a finger prick, which has a lower concentration of immunoglobulins when compared to plasma. Oral fluid has a further lower concentration of antibodies. Sensitivity (the percentage of true positives when disease is present) and specificity (the percentage of true negatives when disease is not present) are the two main accuracy criteria of a test. While rapid tests have high specificity, the sensitivity of these tests may be lower in case of acute HIV infection compared to an established infection. Immune response may be delayed in an individual taking PrEP (pre-exposure prophylaxis) or HIV treatments, further delaying detection when using antibody tests versus laboratory tests.
POC tests like the Abbott PIMATM Analyser and the BD FACSPrestoTM System are a popular choice in developing parts of the world. These systems enumerate CD4+ cells, which are targeted by HIV. However, these tests are generally limited to detecting established HIV infections or fully manifested AIDS.
Recent advances in HIV testing
“There has been great progress toward developing NAAT-based technology to detect viral load.”
Over the past few years, there has been great progress toward developing NAAT-based technology to detect viral load, further shortening the time to detection for HIV infections. Microfluidic technologies with the efficiency and accuracy of “sample to result” in a few minutes have emerged as the main focus of POC approaches. For example, the Cepheid Xpert HIV-1 Viral Load POC test has been shown to be comparable to the lab-based COBAS TaqMan HIV-1 test. Both microfluidic tests use plasma as a sample type, requiring laboratory technicians to separate the plasma from whole blood.
Can we eradicate HIV?
The COVID-19 pandemic has further underscored the need for inexpensive, rapid, portable, and easy-to-use diagnostics, which will hopefully lead to further innovations for HIV screening in low-resource, as well as emergency, settings. Though accurate HIV POC devices are being manufactured and marketed, their availability and widespread use remains low.
Created in 1996, the United Nations Programme on HIV/AIDS (UNAIDS) has since launched the “95-95-95” testing and treatment strategy to combat and eradicate HIV by 2030. The program’s goal is to diagnose 95 percent of all HIV-positive people, provide ART for 95 percent of the HIV-positive population, and achieve viral suppression in 95 percent of people seeking treatment.
“The tremendous achievements made thus far in the effort to control the devastating impact of the HIV epidemic are a testament to human resolve and collaboration.”
As Drs. Anthony S. Fauci and Cliff Lane wrote in a recent paper published in The New England Journal of Medicine, “the challenge before us is to deploy¾for everyone who needs them¾the innovative strategies for HIV prevention and treatment that have resulted from the unprecedented basic scientific and clinical advances made over the past four decades.” Accordingly, UNAIDS plans to achieve its goal by implementing more and better public awareness and prevention initiatives, decentralizing testing, and improving treatment plans. Still, the tremendous achievements made thus far in the effort to control the devastating impact of the HIV epidemic are a testament to human resolve and collaboration.