Health Encyclopedia

Hepatitis B Surface Antigen

Does this test have other names?

HBsAg

What is this test?

This test looks for hepatitis B surface antigens in your blood. The test is used to find out whether you have a recent or long-standing infection from the hepatitis B virus (HBV).

HBV has proteins called antigens on its surface that cause your immune system to make antibodies. It can take 30 to 150 days to develop symptoms of hepatitis B after you become infected. Hepatitis B surface antigens can be found in your blood within 12 weeks after the infection starts. They are one of the earliest signs of a hepatitis B infection.

HBV is one of five hepatitis viruses. The others are hepatitis A, C, D, and E. Ninety-five percent of hepatitis infections are caused by these five viruses. HBV is spread through blood, seminal fluid, and vaginal secretions. The virus causes an infection in the liver. In most cases, this virus clears up on its own within six months. Bit in 6 to 10 percent of adults and 25 to 50 percent of children, the virus does not go away. This is called having a chronic infection. It may lead to liver cell damage; scarring, or cirrhosis; or liver cancer.

Hepatitis B surface antigens are an early sign of an acute infection, and they are also present during chronic, or long-term, infection.

Why do I need this test?

You may need this test if your doctor suspects you have a liver infection caused by HBV. You may need this test if you have symptoms of hepatitis B. Symptoms usually start slowly. Many people have no symptoms or only feel like they have a mild case of the flu. If you have symptoms, they may include:

  • Extreme tiredness

  • Nausea

  • Loss of appetite

  • Muscle aches

  • Fever

  • Jaundice, or yellowed skin and eyes

  • Dark-colored urine

You may also have this test if you have a history that puts you at risk for being in contact with the virus. Risk factors for hepatitis B infection include:

  • Having sex with someone infected with the virus

  • Living in close contact with someone who has the virus

  • Being a man who has sex with men

  • Being a child born to a mother who has the virus

  • Sharing needles for intravenous, or IV, drug use

  • Working in a health care center where you are exposed to blood

You may also have this test several times if you've already been diagnosed with hepatitis B, to see whether your infection is getting better.

What other tests might I have along with this test?

Your doctor may order other blood tests to look for HBV. These tests can look for antigens on the surface, envelope, and core of the virus, as well as the antibodies to these antigens. Because the symptoms of all five hepatitis infections are much the same, this blood test is often done along with other hepatitis blood tests to tell your doctor which type of virus and what stage of infection you may have.

Your doctor may also order a series of blood tests is called a hepatitis B monitoring panel to see if your infection is getting better.

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.

Normal results are negative or nonreactive, meaning that no hepatitis B surface antigen was found.

If your test is positive or reactive, it may mean you are actively infected with HBV. In most cases this means that you will recover within six months. If you recover, you will have immunity from the virus and will not be able to transmit the virus to others. A positive test may also mean you have chronic hepatitis B infection. If you do not recover in six months, the virus may stay in your blood, cause liver problems, and can infect others. Your doctor may give you medications if you don't recover after six months.

How is this test done?

The test requires a blood sample, which is drawn through a needle from a vein in your arm.

Does this test pose any risks?

Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.

What might affect my test results?

Other factors aren't likely to affect your results.

How do I get ready for this test?

You don't need to prepare for this test.