Your PrEP Lab Results Explained: HIV, Kidney and Hep B

The standard PrEP panel at a glance

HIV tests

This is the test that matters most. PrEP's labels require it because PrEP drugs alone aren't enough to treat HIV. Taking them with an undiagnosed infection can let drug-resistant virus develop.

Antigen/antibody test

The standard lab HIV test looks for HIV antibodies and the p24 antigen, a piece of the virus. CDC lists a window period of 18 to 45 days for a lab antigen/antibody test using blood from a vein. Results read nonreactive (negative) or reactive (needs confirmation).

HIV RNA (viral load) test

A nucleic acid test looks for the virus's genetic material and can find infection earlier. CDC lists 10 to 33 days. On oral PrEP, CDC's guidance pairs antigen/antibody and RNA testing at least every 3 months. For Yeztugo, CDC recommends an RNA test at the start when available, but says it shouldn't delay the first injection.

If your HIV result isn't clearly negative

A single reactive result isn't a diagnosis. PrEP can also blur early test signals. CDC's guidance is to confirm with more lab testing, and to repeat both tests in about 5 to 7 days if results are mixed. Call your prescriber the same day and don't change your dosing on your own. Our sister site explains how HIV test types differ.

Kidney tests: creatinine and eCrCl

Tenofovir disoproxil fumarate (TDF), the drug in generic Truvada, can reduce kidney function slightly in some people. So oral PrEP includes kidney monitoring.

• Serum creatinine is a waste product your kidneys filter. Higher creatinine can mean lower filtering.
• Estimated creatinine clearance (eCrCl) is calculated from creatinine, age, sex and weight. CDC's guidance uses the Cockcroft-Gault formula. Some lab reports show eGFR instead, which is a related estimate.
• Thresholds: Truvada's label says it isn't recommended for PrEP when creatinine clearance is below 60 mL/min. Descovy's label allows standard dosing at 30 mL/min or higher. CDC recommends injectable cabotegravir for people with serious kidney disease (eCrCl below 30).

Creatinine also rises with dehydration, a hard workout the day before, high muscle mass, high-protein diets and creatine supplements. A mildly high result often gets rechecked before anything changes. The labels also warn about combining TDF with high-dose or multiple NSAIDs like ibuprofen or naproxen, so mention regular painkiller use. Some providers also check urine for protein or glucose at the start. Our long-term safety guide covers what the kidney data show.

Hepatitis B tests

CDC says everyone considered for oral PrEP must be screened for hepatitis B. Tenofovir and emtricitabine are also active against hepatitis B, which changes how PrEP is started and stopped. The common markers:

Hepatitis B also matters for the on-demand "2-1-1" schedule, which CDC says isn't appropriate for people with active hepatitis B. It's one reason your prescriber may steer you toward a specific option (see oral vs injectable PrEP). Some panels include hepatitis C screening as well, especially for people with injection-related risk.

STI tests

PrEP doesn't prevent bacterial STIs, so PrEP care screens for them. Most have no symptoms. CDC's 2021 guideline recommends:

• Syphilis: blood test at the start and at least every 6 months.
• Gonorrhea and chlamydia: for sexually active people, at the start and then at intervals based on exposure. That's often every 3 months for men who have sex with men, and less often for others.

Site matters. A urine sample won't catch a throat or rectal infection, so ask for swabs at the sites you use. A "reactive" syphilis result often needs a second, confirmatory test. Our sister site explains syphilis blood test results, and our free STI testing guide covers where to test.

Lipids (Descovy only)

CDC recommends checking cholesterol and triglycerides before starting F/TAF (Descovy) and every 12 months after. If your numbers rise, that's a conversation about diet, exercise, other health factors or a different PrEP option. It's not an emergency. See Descovy vs. Truvada.

When a result is flagged: what usually happens

1. Your prescriber reviews it against your earlier results and your health history.
2. Many results get repeated before anything changes, especially a slightly high creatinine or a mixed HIV result.
3. Treatable findings get treated. A positive gonorrhea, chlamydia or syphilis test leads to treatment, and often to partner notification help. You usually keep taking PrEP.
4. Some findings change the option, not the goal. A kidney or hepatitis B result may mean switching drugs or adding monitoring, not stopping prevention.

The one result that changes everything is a confirmed HIV diagnosis. If that happens, PrEP is replaced by full HIV treatment, and you'll be connected to care.

How to get the most from your results

• Save copies of every report in one place. Trends matter more than single numbers.
• Drink water normally and skip an unusually hard workout the day before kidney labs.
• Tell your provider about supplements, especially creatine and protein powders, and any regular painkillers.
• If a test was missed or a kit was rejected, redo it. Many services won't refill PrEP without a current HIV result.

Want the logistics side, like how kits ship and how long results take? Our MISTR labs guide walks through one service's process.

This guide is general health information, not medical advice, and it doesn't replace a conversation with a clinician who knows your history. Guidance and program rules change; we last reviewed this page on October 12, 2026. Spot something out of date? Tell us.