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Blood Test Tracker

Hepatitis B Surface Antigen (HBsAg)

A protein from the hepatitis B virus. A positive result is linked to the virus being in your blood now, from a recent infection or a long-term one.

AI-verified against cited sources. Awaiting expert reviewAlso called HBsAg, HBs AgLOINC 5196-1

What it is

HBsAg stands for hepatitis B surface antigen. It's a protein on the outer coat of the hepatitis B virus. If the virus is in your blood, this test can usually pick up the protein. The result is a simple yes or no, printed as negative or positive (or non-reactive or reactive). It's one of the three tests in the standard hepatitis B panel, along with anti-HBs and total anti-HBc.

Why it matters

A positive HBsAg is the main sign that the hepatitis B virus is in the blood right now and could be passed to others. That guides follow-up tests, liver care, and protecting close contacts and newborns. A negative result means the protein wasn't found, but it is read together with the antibody tests. HBsAg can be missed very early in an infection, and it goes away once the body clears the virus.

If it’s high

A positive result means hepatitis B virus protein was found in your blood. This can be a new (acute) infection or a long-term (chronic) one, and your other results help tell which. Most healthy adults clear a new infection on their own, but infants and young children are much more likely to stay infected. A few positive results are false, which is why labs often run a confirmation test. A recent hepatitis B vaccine can also cause a brief positive.

Common causes

  • A new (acute) hepatitis B infection
  • A long-term (chronic) hepatitis B infection, including infection picked up at birth or in early childhood
  • A hepatitis B vaccine dose in the past few weeks (usually a brief positive)
  • A false-positive screening result that a confirmation test does not support

What can help

  • Ask your clinician about follow-up tests, such as a hepatitis B viral load (HBV DNA), liver blood tests and the e-antigen tests
  • Ask about testing and vaccination for the people you live with and for sexual partners
  • Ask whether limiting alcohol, and checking your medicines and supplements with your clinician, makes sense for your liver
  • If you are pregnant or planning a pregnancy, ask how your baby will be protected after birth

If this applies to you

HBsAg means a little more for some people. Open the one that fits you.

Fatty liver or liver condition

CDC recommends that all adults 18 and older be tested for hepatitis B at least once, using three tests together: HBsAg, anti-HBs and total anti-HBc. BSG also includes HBsAg in the first checks for abnormal liver tests.

Blood tests for liver health

Taking HIV PrEP

The pill forms of PrEP also treat hepatitis B. In someone with hepatitis B, stopping them can cause a flare of liver inflammation, so the CDC suggests hepatitis B testing before starting.

Blood tests on HIV PrEP

What can affect your result

  • Timing: the protein appears a few weeks after exposure, and it clears if the body clears the virus
  • A recent hepatitis B vaccine dose can cause a brief positive result
  • Window period: during a new infection, HBsAg can be gone while the surface antibody (anti-HBs) is not yet detectable, so your clinician may add an IgM anti-HBc test
  • Some assays are affected by high-dose biotin supplements
  • A hepatitis B vaccine dose in the past few weeks can cause a brief positive result (CDC: within 30 days of a dose).
  • Some immunoassays can be affected by high-dose biotin (vitamin B7) supplements. Mayo Clinic Laboratories asks patients to avoid biotin for 24 hours before its hepatitis profiles.
  • A first (screening) positive can be a false positive, which is why many labs confirm reactive results with a second test.

Becomes detectable a few weeks after exposure and disappears if the body clears the virus. A result that stays positive for about six months is called chronic.

When to talk to a clinician

Contact your clinician promptly about a positive result. Get urgent care for yellow skin or eyes, dark urine, severe belly pain, repeated vomiting or confusion. If you are pregnant and test positive, tell your pregnancy care team so your baby can be protected at birth.

Questions you could ask

  • Do my hepatitis B results together show that I am protected against hepatitis B?
  • What do my other hepatitis B results say about whether this is a new or a long-term infection?
  • Which follow-up tests should I have, and when?
  • Should the people I live with or have sex with be tested or vaccinated?
  • If I am pregnant or planning a pregnancy, how do we protect my baby?

Questions people ask

Does a positive HBsAg mean I have hepatitis B for life?

Not necessarily. A positive result can be an early infection that your body clears. The CDC says about 95% of infected adults recover completely, while most infected infants stay infected. If HBsAg is still positive after about six months, the infection is called chronic. Your clinician uses your other results to tell which applies.

Can I test positive right after a hepatitis B vaccine?

Yes, briefly. The vaccine is made from this protein, and the CDC says a short-lived positive can happen within 30 days of a dose. If you were vaccinated recently, tell whoever reviews your result.

Why did my lab run a second test after the first was positive?

Screening tests can give false positives, so many labs run a confirmation test on any reactive result before calling it positive. Mayo Clinic Laboratories, for example, confirms reactive screens automatically.

Show the science and sources

For clinicians and researchers

HBsAg is the envelope protein of HBV. It is made in large excess over complete virions and is found in serum during acute and chronic infection. It usually appears weeks after exposure. In self-limited infection it clears and anti-HBs follows. Persistence for about six months defines chronic infection. Loss of HBsAg with gain of anti-HBs is called functional cure (AASLD 2018), and spontaneous loss occurs at roughly 1% per year. Qualitative immunoassays report a signal-to-cutoff ratio. Many labs confirm reactive screens with a neutralization-type confirmation (Mayo runs confirmation when the cutoff index is above 1.00). Transient HBsAg positivity can follow vaccination (CDC: within 30 days; ACIP 2018: up to 18 days, or up to 52 days in hemodialysis patients). In the acute window period, between HBsAg clearance and anti-HBs appearance, IgM anti-HBc may be the only marker. Surface-gene mutants and occult infection can give HBsAg-negative results with detectable HBV DNA. Quantitative HBsAg (IU/mL) is a separate test used in treatment monitoring.

LOINC 5196-1 (primary), 5195-3, 7905-3. Specimen: serum-or-plasma.

Sources

  1. Clinical Testing and Diagnosis for Hepatitis B, CDC
  2. Clinical Overview of Hepatitis B, CDC
  3. Prevention of Hepatitis B Virus Infection in the United States: Recommendations of the Advisory Committee on Immunization Practices, MMWR Recommendations and Reports (CDC) (2018)
  4. Update on Prevention, Diagnosis, and Treatment of Chronic Hepatitis B: AASLD 2018 Hepatitis B Guidance, Hepatology (AASLD) (2018)
  5. Hepatitis B Virus Surface Antigen Screen, Serum (test HBGSN), Mayo Clinic Laboratories (Extended Catalog)
  6. Acute Viral Hepatitis Profile, Serum (test AHEP), Mayo Clinic Laboratories (Extended Catalog)
  7. Chronic Hepatitis B Screen, Serum (test CHBVS), Mayo Clinic Laboratories (Extended Catalog)
  8. Understanding Your Hepatitis B Test Results, Hepatitis B Foundation

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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