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.
Calculator
Adjust values & calculatePEP should be started as soon as possible. Initiation within 2 hours provides maximum effectiveness.
Tenofovir DF 300 mg + Emtricitabine 200 mg (Truvada) PLUS Raltegravir 400 mg BID or Dolutegravir 50 mg daily
Duration: 28 days
Follow-Up Schedule
Formula
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.
Last reviewed: January 2026
Worked Examples
Example 1: Healthcare Worker Needlestick from Known HIV+ Patient
Example 2: Sexual Exposure with Unknown Source Status
Background & Theory
The Hiv Post-Exposure Prophylaxis Calculator applies the following established principles and formulas. Health and medicine calculators are grounded in validated physiological measurement methods established through decades of clinical research. Body Mass Index, or BMI, is calculated by dividing weight in kilograms by height in meters squared (kg/m²), a formula originating from Adolphe Quetelet's 19th-century statistical work and later codified by the WHO into standard classifications: underweight below 18.5, normal weight 18.5 to 24.9, overweight 25 to 29.9, and obese at 30 and above. Basal Metabolic Rate quantifies the minimum energy required to sustain life at rest. The Mifflin-St Jeor equation, published in 1990 and widely regarded as the most accurate for most adults, calculates BMR as (10 × weight in kg) + (6.25 × height in cm) − (5 × age) ± sex adjustment. The older Harris-Benedict equations, revised in 1984 by Roza and Shizgal, remain in common use. Total Daily Energy Expenditure is derived by multiplying BMR by a physical activity factor ranging from 1.2 for sedentary individuals to 1.9 for extremely active ones, following the methodology validated by doubly labeled water studies. Body fat percentage can be estimated without laboratory equipment using the U.S. Navy circumference method, which uses neck, waist, and hip measurements, or via BMI-derived equations adjusted for age and sex. The Jackson-Pollock skinfold method offers higher precision with calipers. Blood pressure classification, according to the American College of Cardiology and the 2017 ACC/AHA guidelines, defines normal as below 120/80 mmHg, elevated as 120 to 129 systolic, and hypertension stage 1 as 130 to 139 systolic or 80 to 89 diastolic. Target heart rate zones for aerobic exercise are derived from maximum heart rate estimates, most commonly using the formula 220 minus age in years, with moderate-intensity training typically defined as 50 to 70 percent of maximum heart rate and vigorous intensity at 70 to 85 percent, consistent with CDC and American Heart Association guidelines. These thresholds guide safe and effective cardiovascular conditioning.
History
The history behind the Hiv Post-Exposure Prophylaxis Calculator traces back through the following developments. The history of health measurement stretches back to ancient Greece, where Hippocrates around 400 BCE laid the foundation for observational medicine by systematically recording patient symptoms, diet, and environment. His humoral theory, though scientifically superseded, established the principle that the body operates as an interconnected system subject to measurable imbalance. The transformation toward modern medicine accelerated in the 19th century. Louis Pasteur and Robert Koch developed germ theory in the 1860s and 1870s, identifying microorganisms as disease agents and enabling targeted interventions. Florence Nightingale, working during the Crimean War in the 1850s, introduced statistical analysis to nursing practice, demonstrating through data visualization that sanitation reduced mortality. Her work is foundational to evidence-based health measurement. The discovery of vitamins in the early 20th century, beginning with Casimir Funk's coinage of the term in 1912 and culminating in the isolation of vitamins A through K, created the field of nutritional science and gave rise to dietary reference intake frameworks. The World Health Organization, founded in 1948, subsequently established global standards for health metrics, disease classification through the International Classification of Diseases, and recommended daily allowances. The BMI as a clinical screening tool gained traction in the 1970s through Ancel Keys' large-scale epidemiological work, which validated Quetelet's index as a population-level obesity indicator. Through the 1980s and 1990s, the Framingham Heart Study produced landmark data linking cholesterol, blood pressure, and lifestyle factors to cardiovascular disease risk, directly shaping the numeric thresholds still used in health calculators. The evidence-based medicine movement, formalized by Gordon Guyatt and colleagues at McMaster University in the early 1990s, demanded that all health recommendations derive from systematically graded clinical evidence. The digital health era beginning in the 2000s brought these formulas to consumer devices, wearable sensors, and smartphone applications, expanding access to health self-monitoring on a global scale and enabling population-level data collection that continues to refine clinical reference ranges.
Frequently Asked Questions
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.
Why might my result differ from another tool or reference?
Differences typically arise from rounding conventions, the specific version of a formula (for example, simple vs compound interest), or unit inconsistencies between inputs. Check that both tools are using the same formula variant and the same units. The References section links to the authoritative source behind the formula used here.
How do I verify Hiv Post Exposure Prophylaxis Calculator's result independently?
The Formula section on this page shows the equation used. You can reproduce the calculation manually or in a spreadsheet using those steps. Compare your answer against the worked examples in the Examples section, which use known reference values so you can confirm the calculator is behaving as expected.
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.
References
Reviewed by Rahul Singh, Health & Wellness Specialist · Editorial policy