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Undetectable or Absent? Resolving Discordant HIV Test Results in ImPrEP CAB Brasil

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Released: August 27, 2026

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HIV infection while on long-acting (LA) cabotegravir (CAB) pre-exposure prophylaxis (PrEP) is very rare, but it can still occur. The challenge is that LA CAB suppresses viral replication early after infection, making these incident infections difficult to diagnose. Learn how the ImPrEP CAB Brasil study, presented at the AIDS 2026 conference, sought to determine if commercial clinic assays or specialized high-sensitivity research tests provide better evidence of HIV infection.

Brenda Crabtree Ramírez
Brenda Crabtree Ramírez, MD
ImPrEP CAB Brasil

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Long-acting (LA) cabotegravir (CAB) is highly effective for HIV pre-exposure prophylaxis (PrEP), but rare HIV infections can still occur during use. These incident infections can be challenging to diagnose, particularly when serologic test results are inconclusive, as LA CAB suppresses viral replication early after infection. The ImPrEP CAB Brasil study, presented at the AIDS 2026 conference, provides the first analysis of ultrasensitive HIV-1 RNA testing for individuals receiving LA CAB PrEP with inconclusive HIV test results in routine public health settings.

ImPrEP CAB Brasil
ImPrEP CAB Brasil is a real-world, prospective implementation study evaluating same-day initiation of LA CAB PrEP in public health clinics across 6 Brazilian cities. Participants aged 18-30 years initiated LA CAB after a negative third-generation rapid HIV antibody test and an undetectable HIV-1 RNA test. At each follow-up visit, LA CAB was administered only after another negative rapid antibody test. For research purposes, additional HIV diagnostic tests—including HIV-1 RNA, rapid fourth-generation antigen/antibody assays, and confirmatory HIV-1/HIV-2 antibody differentiation assays—were performed retrospectively. Individuals with discrepant results underwent further evaluation using highly sensitive research assays, including a cell-associated HIV DNA assay and an ultrasensitive HIV-1 RNA single-copy assay, to determine whether the discrepant results represented true HIV infection or false-positive serologic results.

Study Results
Among 1220 participants receiving LA CAB PrEP, 4 acquired HIV. Among these individuals, 1 had undiagnosed acute HIV at enrollment, 1 acquired HIV over a year following their last LA CAB dose, and 2 acquired HIV with on-time LA CAB injections.

What is interesting is the analysis of 14 cases where retrospective HIV testing detected at least 1 positive or inconclusive serologic HIV test, but with no detectable HIV-1 RNA via commercial assays. When these cases were further evaluated using highly sensitive research assays, none had any detectable HIV by either single-copy RNA assay or cell-associated HIV DNA testing. Most participants remained on LA CAB PrEP without evidence of HIV infection, suggesting that the inconclusive serologic results were most likely false-positive tests or an “abortive” HIV infection (when the virus enters a cell but does not complete replication), rather than missed HIV infections.

Clinical Implications
This study provides important evidence for real-world implementation of LA CAB from Latin America, a region where LA injectable PrEP is not yet widely available. As countries prepare for future implementation of LA CAB, these findings are highly relevant because they demonstrate that routine third-generation rapid HIV and point-of-care HIV-1 RNA tests—the assays already available in virtually all HIV testing programs, including resource-limited settings across Latin America and the Caribbean—appear to be adequate for HIV screening in LA CAB programs.

Of importance, the study suggests that adding expensive, highly specialized, and ultrasensitive diagnostic tests does not provide additional clinical value for resolving most inconclusive HIV test results. These findings support the use of existing HIV testing infrastructure for LA CAB implementation, which could facilitate broader and more equitable access to injectable PrEP in low-income and middle-income countries while avoiding unnecessary diagnostic complexity and costs.

Your Thoughts
How would you resolve a discrepant or inconclusive HIV test result for a patient on LA CAB PrEP? Does this study convince you that routine HIV diagnostic tests are sufficient for detection of incident HIV infection on LA CAB PrEP? Leave a comment to join the discussion!

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