- 01A negative HIV test is dated by your exposure, not your draw
A nonreactive lab Ag/Ab test on blood from a vein covers most exposures more than about 45 days before the draw 4. Many infections show sooner, from about 18 days, but for anything under 45 days ask when to retest. The date of the exposure matters as much as the result.
The HIV Ag/Ab combo test answers one question: is there a sign of HIV in this blood sample? A nonreactive result means no, for most exposures older than the test's window period (usually 18 to 45 days for a lab test on blood from a vein). A reactive result means the screen found something, and the lab must confirm it with a second, different test before it counts as an HIV diagnosis 2.
What the HIV Ag/Ab combo test detects
The HIV Ag/Ab combo test looks for two things at once. The "Ag" part is an antigen: p24, a protein from the core of HIV itself. The "Ab" part is antibodies to HIV-1 and HIV-2, the proteins your immune system makes after it meets the virus 2. Because p24 shows up in blood before antibodies do, the combo test catches HIV earlier than older antibody-only tests. That is why labs call it fourth-generation.
The test reports one combined answer, so a reactive screen does not tell you whether the antigen, the antibodies, or both set it off. Working that out is the job of the follow-up tests. For the general difference between antigen, antibody and RNA tests across every infection on a lab report, see viral serology explained.
What nonreactive and reactive mean on your report
| Result on the report | What it means | Usual next step |
|---|---|---|
| Nonreactive (negative) | No p24 antigen or HIV antibodies found in this sample. | None for that exposure, if it was more than about 45 days before a lab draw. If it was more recent, or you take PrEP, ask when to retest. People with ongoing risk retest on a schedule (see below). |
| Reactive (preliminary positive) | The screen flagged the sample. HIV is possible but not confirmed. | The lab runs an HIV-1/HIV-2 antibody differentiation test, often on the same sample 2. |
| Reactive screen, differentiation test positive for HIV-1 or HIV-2 | HIV-1 or HIV-2 infection confirmed by two different tests. | Referral to HIV care and a viral load test. |
| Reactive screen, differentiation negative or indeterminate, HIV-1 RNA detected | Acute (very early) HIV-1 infection: virus present before antibodies have fully formed. | Referral to HIV care straight away. |
| Reactive screen, differentiation negative or indeterminate, HIV-1 RNA not detected | The screen was a false positive. No HIV-1 infection. | Often nothing more; a clinician may order HIV-2 testing if there was a specific HIV-2 risk 2. |
Many labs also print a number called the signal-to-cutoff ratio, sometimes written S/CO. The lab uses it internally to decide reactive or nonreactive. A higher number on a reactive screen does not confirm HIV on its own; every reactive screen still goes through the confirmation step 2.
Why a reactive screen is a preliminary result
Screening tests are built to miss as few infections as possible, so they flag some samples that turn out to be clean. Where HIV is uncommon, true infections are rare, so those occasional false flags make up a larger share of all reactive screens. That is the reason the CDC algorithm never ends at the screen 2. The general logic of screening tests and false positives is covered on the viral serology explained page.
How a reactive HIV Ag/Ab screen becomes a final answer
The window period: how long after exposure the test can see HIV
A window period is the gap between infection and the day a test can see it, so a negative result only covers exposures older than that gap. Each kind of HIV test has its own window period. The ranges below come from the CDC, based on a 2017 analysis of how soon each test type turns positive after infection 4. They describe most people, and a small number of people take longer.
| Test type | Sample | Usually detects HIV after exposure | Notes |
|---|---|---|---|
| Nucleic acid test (NAT, HIV RNA) | Blood from a vein | 10 to 33 days | Detects the virus directly; used to confirm and for suspected very early infection. |
| Lab Ag/Ab combo (fourth-generation) | Blood from a vein | 18 to 45 days | The standard screening test in most US labs. |
| Rapid Ag/Ab combo | Finger-stick blood | 18 to 90 days | Same targets, but less able to catch very early infection than the lab version. |
| Antibody-only test (including most rapid and home self-tests) | Finger-stick blood or oral fluid | 23 to 90 days | No p24 antigen, so a longer window. |
Example: 28 days since the last possible exposure, lab Ag/Ab testdays
Who should be screened, and how often
US screening advice comes from two bodies: the CDC and the US Preventive Services Task Force (USPSTF), the independent panel that grades screening tests.
| Who | Recommendation | Source |
|---|---|---|
| Everyone aged 13 to 64 in any health-care setting | At least once, offered routinely unless you decline (opt-out) | CDC 1 |
| Adolescents and adults aged 15 to 65 | Screen (grade A, the strongest USPSTF rating) | USPSTF 3 |
| Younger or older people at increased risk | Screen (grade A) | USPSTF 3 |
| All pregnant people | Screen in every pregnancy (grade A) | USPSTF 3, CDC 1 |
| People at higher risk (new or multiple partners, a partner with HIV, shared injection equipment, a recent sexually transmitted infection) | At least once a year 1; CDC says some sexually active gay and bisexual men may benefit from every 3 to 6 months 4 | CDC 1 4 |
How the test is done
- It is a standard blood draw from a vein. No fasting is needed, and it can be drawn with any other labs.
- It is a separate order: a routine checkup panel does not include HIV unless your clinician adds it or the clinic offers it under opt-out screening.
- Because USPSTF grades HIV screening A 3, most US private insurance plans cover it as preventive care without a copay; check with your plan. Health departments and many clinics test for free.
- Home self-tests sold in pharmacies are mostly antibody-only, so their window is longer (23 to 90 days) and a reactive self-test still needs a lab test to confirm it 4.
HIV screening is often paired with hepatitis C and hepatitis B screening. Hepatitis C uses the same two-step logic of an antibody screen followed by an RNA test (see anti-HCV); hepatitis B is checked with HBsAg and anti-HBs. For a general method that works on any lab report, see how to read any lab result.
If the result is confirmed: what HIV means today
A confirmed HIV result leads to a viral load test, which counts copies of the virus in the blood, and to daily antiretroviral treatment. Treatment holds the virus down so the immune system can recover, and people who stay on it can expect a long life with HIV as a chronic, managed condition.
Next step: ask for a referral to an HIV clinic or specialist, ideally within days. The first visit usually includes a viral load, a CD4 count (a type of immune cell that shows how strong your immune system is), and routine baseline labs such as a full blood count, kidney function and liver enzymes.
Treatment also stops sexual transmission. In the PARTNER2 study of 782 male couples where one partner had HIV with a suppressed viral load, researchers counted about 76,000 acts of condomless sex and found zero HIV transmissions linked to the partner on treatment 5. This is the evidence behind U=U, short for "undetectable equals untransmittable". It applies while treatment continues and the viral load stays undetectable, confirmed by regular viral load tests, and it does not prevent other sexually transmitted infections.
...the risk of HIV transmission in gay couples through condomless sex when HIV viral load is suppressed is effectively zero.Rodger AJ et al., PARTNER2, The Lancet, 2019 5
Many people living with HIV do not know they have it, which is why the CDC recommends offering the test to everyone rather than only to people who ask 1. More infection tests are collected under infections.
Citations
- Branson BM et al. Revised Recommendations for HIV Testing of Adults, Adolescents, and Pregnant Women in Health-Care Settings. MMWR Recomm Rep 2006;55(RR-14):1-17
- CDC and APHL. Laboratory Testing for the Diagnosis of HIV Infection: Updated Recommendations. 2014
- US Preventive Services Task Force. Screening for HIV Infection: Recommendation Statement. JAMA 2019;321(23):2326-2336
- CDC. HIV testing guidance: who should get tested, how often, and window periods by test type, based on Delaney KP et al. Time until emergence of HIV test reactivity following infection with HIV-1. Clin Infect Dis 2017;64(1):53-59
- Rodger AJ et al. Risk of HIV transmission through condomless sex in serodifferent gay couples with the HIV-positive partner taking suppressive antiretroviral therapy (PARTNER): final results of a multicentre, prospective, observational study. Lancet 2019;393(10189):2428-2438
- Tanner MR et al. Antiretroviral Postexposure Prophylaxis After Sexual, Injection Drug Use, or Other Nonoccupational Exposure to HIV: CDC Recommendations, United States, 2025. MMWR Recomm Rep 2025;74(1)
- CDC. Preexposure Prophylaxis for the Prevention of HIV Infection in the United States, 2021 Update: A Clinical Practice Guideline
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.