HIV diagnostic testing enters a new era of clinical and laboratory precision

More than four decades after the first cases of AIDS were reported in the United States, HIV remains one of the world's most persistent public health challenges. According to the latest figures from UNAIDS, 40.9 million people are living with HIV worldwide and 1.2 million new infections occurred last year alone. Advances in treatment and prevention, from daily antiretroviral pills to long-acting injectable options, have transformed HIV from a fatal diagnosis into a manageable chronic condition for millions. Yet all of that progress depends on one critical first step: an accurate and timely diagnosis.

That was the focus of a recent SelectScience^® webinar, HIV diagnostic testing in practice: Clinical insights, laboratory challenges, and emerging solutions, which brought together two experts from the University of California, San Francisco. Dr. Monica Gandhi, a professor of medicine in the Division of HIV, Infectious Diseases and Global Medicine at UCSF, opened the session with a sweeping look at the epidemiology, treatment, and prevention of HIV. She was joined by Dr. Colette Match, director of microbiology at the UCSF Clinical Lab at Zuckerberg San Francisco General Hospital and a board-certified clinical microbiologist, who walked attendees through the laboratory side of diagnosis, from screening algorithms to the real-world cases that keep testing complicated even in 2026.

A global epidemic that's still evolving

Asked how HIV entered the human population, Gandhi pointed to "the bush meat trade," which she described as "an illegal practice of hunting and eating primates" in West Africa in the early 20th century. From there, she said, "it really was all the social disruption that occurred in West Africa that led to the spread of the epidemic," which later moved into East and Southern Africa, still the hardest-hit regions today.

By contrast, Gandhi said, the United States has what she called "a concentrated epidemic," explaining that "concentrated epidemic means in men who have sex with men, transgender women, those who have a lot of sexually transmitted infections, people who inject drugs, commercial sex workers."

Asked which populations should be prioritized over the next decade, Gandhi said, "The populations that are most falling behind are what we call key populations, and that is men who have sex with men, transgender women, sex workers, and those who inject drugs," adding that these groups require "the prevention tools they need, which are really long-acting or oral PrEP options."

The screening algorithm: from antigen to antibody

Match walked attendees through the diagnostic process step by step. "In the United States in particular, we start with an HIV-1/2 antigen antibody immunoassay," she said. "If that's negative, you can be pretty confident that that patient is negative and you should report that."

A positive result, she explained, moves to "your antibody differentiation assay, and that will tell you if they have HIV-1 or HIV-2 antibodies or both." If those results are negative or indeterminate, Match said, "it's recommended to go onto an HIV nucleic acid test," which can be specific to HIV-1 or cover both HIV-1 and HIV-2 "depending on the patient population."

"You want to start with your most sensitive assay," Match said, describing the logic behind the algorithm's design.

Closing the diagnostic window

Match traced the evolution of HIV immunoassays back to the first FDA-approved test in 1985. "Your first generation, you're only detecting about 50 days after that transmission event," she said. Sensitivity has improved with each successive generation, she explained, "and now with the fourth generation, we can detect the HIV p24 antigen, and you're down to less than 20 days from the time of transmission that you can detect an acute HIV infection."

Even so, Match cautioned, "none of these immunoassays are perfect." Citing a study of confirmed acute infections tested on three different antigen/antibody assays, she said, "50% had false negatives. So, 12 out of 24 were only detected by RNA," reinforcing why laboratories rely on the multi-step algorithm rather than a single test.

Balancing sensitivity and specificity

According to Match, high sensitivity comes with a tradeoff. "The goal of our screening algorithm is to cast a wide net," she said. "We are trying to detect as many people as possible and then ensure correct diagnosis with our additional layers of testing. And so, false positives are going to be expected with these screening assays, and this has been shown to be true for basically all of the antigen-antibody immunoassays that are on the market today."

Citing data from a U.S. national reference lab covering more than 21 million tests, Match said, "they had over 30,000 false positives. So, with a false positive rate of 0.14%." She added that the false positive rate "is actually much higher" in older adults and women, while the positive predictive value is highest "for younger men who are of sort of more sexually active age."

She illustrated the issue with a case from her own lab involving a 32-year-old woman whose antigen/antibody test was reactive but whose differentiation and nucleic acid tests were both negative, calling it "a case where the HIV antigen/antibody was a false positive."

PrEP adds a new layer of complexity

Gandhi described what she called a genuine "conundrum" involving patients on long-acting PrEP. In one case, she said, a 48-year-old man on long-acting cabotegravir "had routine HIV antigen antibody testing, and it was positive, and his HIV RNA test was negative," after several late injections.

Referencing newly presented findings from the M-PrEP study in Brazil, Gandhi said, "what they found is that actually, HIV RNA was all you needed," concluding, "we're not going to need those deep sort of research tools. We really need what we have clinically available to diagnose HIV and PrEP."

Match said her own lab has seen similar discordant results, describing a PrEP patient who tested positive on "this low-level nucleic acid test" that later proved to be a false positive. Her recommendation, she said, was "automated reflex testing," explaining, "this also helps prevent additional nucleic acid testing ordering because if the doctor knows that it will automatically reflex to that test, they won't separately order it."

Asked what would move the needle most over the next five years, Gandhi pointed to "a once-a-month pill that's being studied," called MK-8527, saying, "I do think the future is long-acting, whether in a pill form or injectable."

Match, for her part, said there is already progress being made in point-of-care testing and getting answers to patients more quickly. Looking ahead, she emphasized the importance of rapid diagnosis and treatment initiation, noting that timely testing and access to therapy will continue to play a critical role in reducing transmission and improving outcomes in the ongoing fight against HIV.