Hiv Post Exposure Prophylaxis Calculator
Determine if PEP is recommended based on exposure type, source status, and timing. Enter values for instant results with step-by-step formulas.
Formula
Adjusted Risk = Base Risk x Exposure Modifier x Source Status Modifier
Base transmission risk varies by exposure type (percutaneous 0.3%, mucous membrane 0.09%, receptive anal intercourse 1.38%). Modifiers adjust for exposure severity (hollow needle, deep injury) and source viral status (detectable, undetectable, unknown). PEP is recommended within 72 hours for significant exposures.
Worked Examples
Example 1: Healthcare Worker Needlestick from Known HIV+ Patient
Problem: A nurse sustains a hollow-needle needlestick injury from a known HIV-positive patient with a detectable viral load. The exposure occurred 1 hour ago.
Solution: Exposure: Percutaneous (needlestick)\nBase risk: 0.3%\nHollow needle modifier: x1.5\nSource: HIV-positive, detectable viral load (x1.0)\nAdjusted risk: 0.3% x 1.5 x 1.0 = 0.45%\nTime elapsed: 1 hour (within optimal 2-hour window)\nPEP recommendation: YES - Start immediately
Result: PEP RECOMMENDED | Risk: 0.450% | Start TDF/FTC + Dolutegravir immediately for 28 days
Example 2: Sexual Exposure with Unknown Source Status
Problem: A 28-year-old presents 18 hours after unprotected receptive anal intercourse with a partner of unknown HIV status.
Solution: Exposure: Sexual - Receptive Anal\nBase risk: 1.38%\nSource status: Unknown (x0.2)\nAdjusted risk: 1.38% x 0.2 = 0.276%\nTime elapsed: 18 hours (within urgent window)\nPEP recommendation: YES - High-risk exposure type warrants PEP
Result: PEP RECOMMENDED | Risk: 0.276% | Start TDF/FTC + Raltegravir within 72 hours for 28 days
Frequently Asked Questions
How soon after exposure must PEP be started to be effective?
PEP must be started as soon as possible after exposure, ideally within 2 hours, and no later than 72 hours (3 days) after the potential HIV exposure. Animal studies have shown that PEP effectiveness decreases significantly with each hour of delay. In primate models, PEP initiated within 24 hours of exposure reduced transmission by approximately 80 to 90 percent, while initiation at 48 hours provided substantially less protection. PEP started beyond 72 hours is generally not recommended because by this point, HIV has typically spread from the local infection site to regional lymph nodes and systemic circulation, making it much more difficult to prevent established infection. Emergency departments should have PEP starter packs available for immediate dispensing without waiting for specialist consultation.
What types of exposure carry the highest risk of HIV transmission?
HIV transmission risk varies significantly by exposure type. The highest-risk exposure is receptive anal intercourse with an HIV-positive partner, with an estimated per-act transmission probability of approximately 1.38 percent. Percutaneous exposure (needlestick injuries) carries a risk of about 0.3 percent, which increases substantially with deep injuries, large-bore hollow needles, visible blood on the device, and source patients with high viral loads. Receptive vaginal intercourse carries approximately 0.08 percent risk per act. Mucous membrane splashes and non-intact skin exposures carry approximately 0.09 percent risk. The source patient having a high viral load is the single most important factor increasing transmission risk, while an undetectable viral load reduces risk by approximately 96 percent or more.
What is the difference between PEP and PrEP for HIV prevention?
PEP (post-exposure prophylaxis) and PrEP (pre-exposure prophylaxis) are both antiretroviral-based HIV prevention strategies but differ in timing, duration, and target populations. PEP is an emergency intervention taken AFTER a specific exposure event, consisting of three antiretroviral drugs for exactly 28 days, and must be started within 72 hours. PrEP is a preventive measure taken BEFORE potential exposure on an ongoing basis, typically using two drugs (Truvada or Descovy), by individuals at ongoing high risk of HIV acquisition such as men who have sex with men, people with HIV-positive partners, or injection drug users. PrEP reduces HIV acquisition risk by approximately 99 percent when taken consistently. Patients requiring repeated PEP courses should be evaluated for PrEP candidacy to provide continuous protection.
What formula does Hiv Post Exposure Prophylaxis Calculator use?
The formula used is described in the Formula section on this page. It is based on widely accepted standards in the relevant field. If you need a specific reference or citation, the References section provides links to authoritative sources.
Can I share or bookmark my calculation?
You can bookmark the calculator page in your browser. Many calculators also display a shareable result summary you can copy. The page URL stays the same so returning to it will bring you back to the same tool.
How do I interpret the result?
Results are displayed with a label and unit to help you understand the output. Many calculators include a short explanation or classification below the result (for example, a BMI category or risk level). Refer to the worked examples section on this page for real-world context.