- 01Antibodies record history; antigen and RNA show the present
An antibody can come from a current infection, one your body cleared years ago, or a vaccine. An antigen or RNA test detects the virus itself, so it is the one that says whether an infection is active now.
- 02Reactive is a preliminary result
A reactive screening test means the sample triggered a sensitive first test. It becomes a diagnosis only when a second test using a different method confirms it, and in low-risk groups many reactive screens clear on that second test.
- 03A negative only covers exposures older than the window period
Every test has a window period between infection and the first day it can detect the virus. A negative result covers exposures older than that window; a more recent exposure can need a repeat test.
An antigen test (one that detects a piece of the virus) or an RNA test (one that detects its genetic material) tells you whether the virus is in your blood now. An antibody test (one that detects the immune system's response) tells you whether your body has ever met it, and for hepatitis B, whether you are protected. A "reactive" screening result is the first step of a fixed lab sequence, and it means infection only after a second, different test confirms it.
Antigen, antibody and RNA tests answer different questions
An antigen is a piece of the germ itself, such as the HIV p24 protein or hepatitis B surface antigen (HBsAg). A positive antigen test points to virus present in the blood now 1. An antibody is a protein your immune system makes after it meets a virus. A positive antibody test can mean a current infection, a past infection your body cleared, or a vaccine, depending on which antibody it is 1.
An RNA or DNA test, also called a nucleic acid test (NAT), detects the virus's genetic material. A NAT is the most direct "is it here now" test, which is why labs use HCV RNA to settle a reactive hepatitis C antibody and HIV-1 NAT to settle some HIV screens 2 3.
Which test tells you whether the virus is there now
IgM and IgG: recent versus at some point
Antibodies come in classes. IgM (immunoglobulin M) is usually the first antibody to appear after infection and fades within weeks to months. IgG (immunoglobulin G) appears later and often lasts for years or for life. Hepatitis B shows the split clearly: IgM antibody to the hepatitis B core marks a recent (acute) infection, while total anti-HBc stays positive long after 1. Epstein-Barr virus (EBV antibody) and cytomegalovirus (CMV antibody) panels report IgM and IgG separately for the same reason.
A positive CMV IgG on its own is common in healthy people. In US national survey data from 1999 to 2004, 50.4 percent (age-adjusted) of people aged 6 to 49 had CMV IgG antibodies, reflecting a past infection that the virus keeps in a dormant (latent) state for life, not an active illness 6. IgM usually means recent and IgG usually means at some point, and neither one, read alone, tells you an infection is still active.
| IgM | IgG | What it usually suggests | Caveat |
|---|---|---|---|
| Positive | Negative | A recent or very early infection | IgM can give false positives, and EBV and CMV IgM can cross-react (a test for one virus can light up because of the other). Ask your clinician whether and when to repeat the test; IgG appearing on a later sample supports a real recent infection |
| Positive | Positive | A recent infection that is maturing, or one that is resolving | Some labs add an IgG avidity test (it measures how tightly the antibody binds, which tightens as an infection ages) to estimate how recent |
| Negative | Positive | A past infection (the most common pattern in adults) | Often needs no action on its own; tell your clinician if you are pregnant, planning pregnancy, a transplant candidate or on immune-suppressing treatment, where CMV and EBV results can matter |
| Negative | Negative | No sign of past infection | Can be too early to show, inside the window period |
What reactive means on a screening test
A screening test is built to catch nearly every true infection, and it accepts some false alarms as the price of missing almost none. "Reactive" means the sample triggered that screen. The lab then runs a confirmatory (also called supplemental) test that uses a different method, and the confirmatory result is the one that decides 2 3.
How a reactive screen becomes an answer
The full HIV confirmation sequence is on the HIV Ag/Ab combo page, and the hepatitis C antibody-then-RNA sequence is on the anti-HCV page.
Why screening a low-risk group produces some false positives
When an infection is rare in the group being tested, even a very accurate screen produces more false alarms than true positives. CDC's hepatitis C guidance notes that false-positive antibody results are more common in populations where the infection is uncommon 3. The arithmetic below uses a made-up test that catches every true case and wrongly flags 1 in 200 uninfected people (99.5 percent specificity, meaning 99.5 percent of uninfected people correctly test negative).
| Illustrative group of 10,000 people | Low-prevalence group (0.1% infected) | Higher-prevalence group (5% infected) |
|---|---|---|
| People with the infection | 10 | 500 |
| False positives (0.5% of uninfected) | About 50 | About 48 |
| Total reactive screens | About 60 | About 548 |
| Share of reactive screens that are true infections | About 1 in 6 | About 9 in 10 |
The window period: why a negative can need a repeat
A window period is the gap between infection and the first day a test can see it, so a negative result only covers exposures older than that gap. Tests that detect the virus directly have shorter window periods than antibody tests, because the virus multiplies before the immune system produces enough antibody to measure. The fourth-generation HIV test adds the p24 antigen to the antibody test for exactly this reason: p24 appears before antibodies, which shortens the window 2. CDC says a lab-based Ag/Ab test can usually detect HIV 18 to 45 days after exposure; rapid finger-stick tests can take longer 8.
An antigen/antibody lab test using blood from a vein can usually detect HIV 18 to 45 days after exposure.
Hepatitis C follows the same pattern. CDC guidance says most infected people have detectable HCV RNA within 1 to 2 weeks of exposure, while the antibody appears on average 8 to 11 weeks after exposure, so a negative antibody test soon after an exposure does not rule infection out 9. Window-period figures are population ranges, and an individual can fall at either end.
Hepatitis B: the triple panel shows all three questions at once
CDC recommends hepatitis B screening at least once for all adults 18 and older using three tests together 1. HBsAg asks whether the virus is there now. Anti-HBs, the surface antibody, asks whether you are protected; a level of 10 mIU/mL (milli-international units per millilitre, the standard unit for this antibody; see what lab units mean) or higher counts as protective 1. Total anti-HBc, the core antibody, asks whether you have ever had a real hepatitis B infection. Vaccination produces anti-HBs only, never anti-HBc, which is what lets the panel separate a vaccinated person from someone who recovered from infection 1.
| HBsAg | Anti-HBc (total) | Anti-HBs | What it usually means | Usual next step |
|---|---|---|---|---|
| Negative | Negative | Negative | Never infected and not immune (or a very recent infection still inside the window period) | Ask your clinician about the hepatitis B vaccine |
| Negative | Negative | Positive (10 mIU/mL or more) | Immune from vaccination | Usually none |
| Negative | Positive | Positive | Past infection that cleared; immune | Usually none, but tell clinicians before chemotherapy or immune-suppressing drugs, which can reawaken the virus |
| Positive | Positive | Negative | Current infection (IgM anti-HBc helps separate recent from long-standing) | Clinician follow-up with further hepatitis B tests |
| Negative | Positive | Negative | Isolated core antibody: a cleared infection whose anti-HBs faded, a false positive, or a low-level infection | Clinician decides which further test sorts it out |
Interpretation patterns follow CDC's 2023 hepatitis B screening recommendations 1. If HBsAg is positive during pregnancy, make sure the delivery team knows: the baby needs hepatitis B immune globulin and the first vaccine dose within 12 hours of birth 10. Yellowing skin or eyes, dark urine, or new confusion need prompt care whatever a hepatitis test showed. The marker pages for HBsAg, anti-HBs and anti-HBc cover each test in detail.
Hepatitis C: antibody first, then RNA
CDC recommends hepatitis C screening at least once for every adult 18 and older and during every pregnancy, except in settings where fewer than 0.1 percent of people have the infection 4. The screen is the anti-HCV antibody test. A reactive anti-HCV result goes on to an HCV RNA test, and the RNA result tells you whether the virus is present now 3.
No current HCV infection.CDC, MMWR 2013;62(18), interpretation for a reactive HCV antibody with HCV RNA not detected
That CDC line covers a large group of people: those whose bodies cleared hepatitis C on their own and those treated and cured, plus some false-positive antibody results 3. CDC adds one exception: if the exposure was in the last 6 months, or risk is ongoing, a repeat HCV RNA test may be needed, because the virus can be at undetectable levels early on 3. The anti-HCV page covers clearance, cure and reinfection. For the liver enzymes that often appear on the same report, such as ALT and AST, see liver enzymes explained or the liver topic.
H. pylori: an everyday antibody that outlives the infection
H. pylori is a stomach bacterium rather than a virus, but its tests follow the same rules. The H. pylori IgG antibody can stay positive after the infection is cured, so the American College of Gastroenterology advises against using serology to confirm the infection is gone; a urea breath test or the stool antigen test shows active infection 5.
Who gets screened, and the practical details
| Infection | Who CDC recommends screening | Screening test | Follow-up after a reactive screen |
|---|---|---|---|
| HIV | Everyone 13 to 64 at least once; at least yearly for people with certain risk factors 8 (details on the HIV Ag/Ab page) | HIV Ag/Ab combo | Antibody differentiation assay, then HIV-1 NAT if needed 2 |
| Hepatitis C | All adults 18 and older at least once; every pregnancy 4 | Anti-HCV | HCV RNA 3 |
| Hepatitis B | All adults 18 and older at least once 1 | Triple panel: HBsAg, anti-HBs, anti-HBc | Clinician-directed further hepatitis B tests |
- Fasting: none of these blood tests needs fasting, and they can be drawn with other bloodwork.
- Ordering: each is a separate order. A complete blood count or metabolic panel at a routine physical does not include HIV, hepatitis B or hepatitis C testing unless someone adds it.
- Cost: under the Affordable Care Act, most non-grandfathered private plans cover screenings that the US Preventive Services Task Force (an independent federal panel) rates A or B, its two highest grades, without a copay. The Task Force's age ranges and risk groups differ from CDC's in places; for example, it recommends hepatitis B screening for people at increased risk rather than every adult. Coverage and confirmatory-test billing vary, so check with your plan.
- Confidentiality: results are part of your medical record. By law, labs and clinicians report confirmed HIV, hepatitis B and hepatitis C results to public health departments, which keep them confidential. Copies you share with an app or other third party may not carry the same legal protection. Many health departments and community clinics offer free or low-cost confidential testing, and some offer anonymous HIV testing.
CDC recommends testing every adult for hepatitis B and hepatitis C, and everyone aged 13 to 64 for HIV, and all three infections have effective treatment. In a low-risk group, a reactive screen that clears on the confirmatory test is common. For reading any other line on your report, start with how to read any lab result and reference ranges explained, or browse the infections topic.
This page is general education about how infection tests work. It is not medical advice and cannot interpret your own result. Your clinician, who knows your history, exposures and full results, decides what a result means for you and what testing or care comes next.
Link · CDC, MMWR Recommendations and ReportsScreening and testing for hepatitis B virus infection: CDC recommendations, 2023The source for the triple-panel approach and the full table of hepatitis B result patterns.cdc.gov
Citations
- Conners EE et al. Screening and Testing for Hepatitis B Virus Infection: CDC Recommendations, United States, 2023. MMWR Recomm Rep 2023;72(RR-1):1-25
- CDC and APHL. Laboratory Testing for the Diagnosis of HIV Infection: Updated Recommendations (2014)
- CDC. Testing for HCV Infection: An Update of Guidance for Clinicians and Laboratorians. MMWR 2013;62(18):362-365
- Schillie S et al. CDC Recommendations for Hepatitis C Screening Among Adults, United States, 2020. MMWR Recomm Rep 2020;69(RR-2):1-17
- Chey WD et al. ACG Clinical Guideline: Treatment of Helicobacter pylori Infection. Am J Gastroenterol 2024;119(9):1730-1753
- Bate SL, Dollard SC, Cannon MJ. Cytomegalovirus seroprevalence in the United States: the National Health and Nutrition Examination Surveys, 1988-2004. Clin Infect Dis 2010;50(11):1439-1447
- 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):1-56
- CDC. Getting Tested for HIV (window periods and who should get tested)
- Moorman AC et al. Testing and Clinical Management of Health Care Personnel Potentially Exposed to Hepatitis C Virus: CDC Guidance, United States, 2020. MMWR Recomm Rep 2020;69(RR-6):1-8
- Schillie S et al. Prevention of Hepatitis B Virus Infection in the United States: Recommendations of the Advisory Committee on Immunization Practices. MMWR Recomm Rep 2018;67(RR-1):1-31
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.