Hiv Post Exposure Prophylaxis Calculator

Post-exposure prophylaxis exploits a narrow biological window. After a breach, HIV first infects dendritic cells and CD4 lymphocytes at the exposure site, and only over roughly the next 24 to 72 hours do those cells carry the virus to regional lymph nodes and then into systemic circulation. Antiretroviral drugs given before that dissemination completes can prevent the infection from ever becoming established, which is why the 72-hour cut-off is a biological boundary rather than an administrative one and why every hour of delay costs efficacy. The standard course is 28 days of three drugs, typically tenofovir with emtricitabine plus an integrase inhibitor such as dolutegravir or raltegravir. Risk stratification starts from published per-act transmission estimates: about 1.38 percent for receptive anal intercourse, 0.3 percent for a percutaneous needlestick, roughly 0.09 percent for mucous membrane or non-intact skin exposure, and 0.08 percent for receptive vaginal intercourse. Occupational modifiers derived from case-control work raise that baseline for deep injury, large-bore hollow needles and visible blood on the device, while an undetectable source viral load reduces transmission risk by 96 percent or more, the finding underlying the Undetectable equals Untransmittable position. These figures are point estimates from observational cohorts with wide confidence intervals, and multiplying modifiers together is a clinical heuristic rather than a validated model. A calculated percentage should never delay treatment: guidance is to start immediately on assessment, test the source in parallel, and stop the course if the source proves definitively negative.

Prophylaxis after occupational exposure began informally in the late 1980s with zidovudine, used without trial evidence because health workers exposed to HIV had no alternative. The decisive evidence arrived in 1997, when Denise Cardo and colleagues published a retrospective case-control study in the New England Journal of Medicine drawing on surveillance data from the United States, France and the United Kingdom. It identified the exposure characteristics that raise risk, deep injury, visible blood on the device, a needle previously placed in an artery or vein, and terminal source illness, and found that zidovudine prophylaxis was associated with roughly an 81 percent reduction in the odds of seroconversion. Animal work in macaques through the 1990s independently established that efficacy fell sharply as initiation was delayed, which is where the 72-hour limit originates. The United States Public Health Service issued occupational PEP guidelines in 1996 and revised them repeatedly, with the 2013 update led by Kuhar and colleagues simplifying regimen choice and abandoning the older exposure-severity tables that had governed whether two or three drugs were given. Non-occupational PEP for sexual and injection exposures was formally addressed by CDC guidance in 2005 and updated in 2016. The World Health Organization consolidated its own recommendations in 2014, extending PEP access in resource-limited settings and endorsing simplified regimens.