Hepatitis B Surface Antigen Test (HBsAg)

Test for Active or Chronic Hepatitis B

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Hepatitis B surface antigen (HBsAg) is a blood test ordered to determine if someone is infected with the hepatitis B virus. If it is found, along with specific antibodies, it means the person has a hepatitis B infection. If your blood is positive for HBsAg, it means you are infectious for the virus and can pass it to other people through your blood or body fluids.

symptoms of hepatitis

Verywell / Brianna Gilmartin

Hepatitis B

Hepatitis B is a serious liver infection caused by the hepatitis B virus (HBV). For some people, hepatitis B infection becomes chronic, meaning it lasts more than six months. Having chronic hepatitis B increases your risk of developing liver failure, liver cancer or cirrhosis—a condition that causes permanent scarring of the liver.

Most people infected with hepatitis B as adults recover fully, even if their signs and symptoms are severe. Infants and children are more likely to develop a chronic hepatitis B infection.

A vaccine can prevent hepatitis B, but there's no cure if you have it. If you're infected, taking certain precautions can help prevent spreading HBV to others.

Hepatitis B Symptoms

Signs and symptoms of hepatitis B, ranging from mild to severe, usually appear about one to four months after you've been infected. They may include:

  • Abdominal pain
  • Dark urine
  • Fever
  • Joint pain
  • Loss of appetite
  • Nausea and vomiting
  • Weakness and fatigue
  • Yellowing of your skin and the whites of your eyes (jaundice)

Causes

The Hepatitis B virus is passed from person to person through blood, semen, or other body fluids.

Common ways HBV is transmitted include:

  • Sexual contact: You may become infected if you have unprotected sex with an infected partner whose blood, saliva, semen, or vaginal secretions enter your body.
  • Sharing of needles: HBV is easily transmitted through needles and syringes contaminated with infected blood. Sharing intravenous (IV) drug paraphernalia puts you at high risk of hepatitis B.
  • Accidental needle sticks: Hepatitis B is a concern for healthcare workers and anyone else who comes in contact with human blood.
  • Mother to child:  People who are pregnant and are infected with HBV can pass the virus to their babies during childbirth. However, the newborn can be vaccinated to avoid getting infected in almost all cases. Talk to your healthcare provider about being tested for hepatitis B if you are pregnant or want to become pregnant.

If you are positive for HBsAg, your blood and body fluids contain the virus and you can transmit it to others.

HBsAg

HBsAg is cleared within four to six months in self-limited infections (infections that resolve by themselves). It can be detected in the blood during both acute infections (infections that come on suddenly) and chronic infections (infections that last for longer than six months).

In addition to the signs and symptoms that a patient has, additional antibodies can be tested to distinguish between acute and chronic infections.

At the center of the hepatitis B virus is DNA, which contains the genes the virus uses to replicate itself. Surrounding the DNA is a protein called hepatitis B core antigen (HBcAG), which cannot be detected with blood tests.

Surrounding this is HBsAg, which is actually part of the "envelope" that protects the virus from attack by the body's immune system. However, the immune system is good at getting through this envelope in order to kill the virus. When it does, remnants of surface antigen protein are left in the blood like debris, which lab tests can detect.

Your body can produce antibodies to any of these antigens once you are exposed to the virus. These antibodies develop at different stages of the infection.

If you receive a vaccination for hepatitis B, you will develop anti-HBs antibodies, which will protect you from infection by hepatitis B. The HBsAg protein in the vaccine is manufactured by modified yeast cultures, so it is safe from transmission of hepatitis.

Screening Tests for Hepatitis B

Your blood may be screened for HBV for many different reasons. The three tests generally include HBsAg, antibody to HBsAg, and antibody to hepatitis B core antigen. This allows the healthcare provider to know whether you could benefit from vaccination, or if you have active or chronic hepatitis B and need counseling, care, or treatment.

You may be routinely screened if you are pregnant, are donating blood or tissue, need immunosuppressive therapy, or have end-stage renal disease. You will also be screened if you are in groups that are at higher risk for HBV.

Frequently Asked Questions

  • Why do you get an HBsAg test when you’re pregnant?

    The test can diagnose hepatitis B virus (HBV) infection in the mother so steps can be taken to avoid infecting the baby during delivery. Being infected during infancy raises the likelihood that a child will develop chronic HBV and increases the risk of long-term illness or death.

  • What’s the difference between positive HBsAg and HBsAb tests?

    An HBsAg test checks whether you currently have a hepatitis B infection. If it’s positive, you’re infected and can spread the virus to others through your blood. An HBsAb test looks for Hepatitis B surface antibodies. A positive or "reactive" test means you’re protected against the hepatitis B virus, usually due to a vaccination or having recovered from the virus.

  • Is hepatitis B curable?

    You can recover from acute hepatitis B infections, but there is no cure for a chronic hep B infection. With medication and lifestyle changes, you can manage hepatitis B, but you should continue to be monitored for liver function and cancer as well as overall health.

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8 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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