What Is the Hepatitis B Surface Antibody (HBsAb) Test?

Are You Immune to the Hepatitis B Virus?

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The HBsAb test, or hepatitis B surface antibody test, looks for antibodies that your immune system makes in response to the surface protein of the hepatitis B virus. The hepatitis B surface antibody is also referred to as anti-HBs and should not be confused with HBsAg, which stands for hepatitis B surface antigen. 

What is the Hepatitis B Surface Antibody?

When you are exposed to Hepatitis B, your body mounts an immune reaction against it as an invader.

This happens whether you are exposed due to blood or sexual contact or if you are vaccinated with the Hepatitis B vaccine. The virus has proteins on its surface (antigens) that cause your immune reaction. With the vaccine, the sample contains the protein only and not the virus itself.

The first response is with Hepatitis B IgM antibodies, which are produced to fight against several parts of the virus including its core. These antibodies are seen in the initial response, but they eventually fade away. Your immune system then begins to produce IgG antibodies. It continues to produce these antibodies for the rest of your life. In this way, your immune system is always ready to attack Hepatitis B virus when it is exposed to it.

What Does a Positive or Negative HBsAb Test Mean?

When HBsAb is positive (antibodies are present), it usually means that you have recovered from hepatitis B infection and have some immunity, or that you once received  the hepatitis B vaccination and are immune.

If your HBsAb test is negative, it can mean many different things—but, in general, it means you are not immune to the Hepatitis B virus. If your other Hepatitis B tests are negative, it means you are either very early in the incubation stage of infection or that you are not infected, in which case your doctor may recommend getting the vaccine.

However, it can also be negative when other Hepatitis B tests are positive, showing active or chronic infection. This is why further testing is necessary, especially for the hepatitis B surface antigen (HBsAg), which shows that the virus itself is circulating in your bloodstream and that you have an active or chronic infection.

When is the HBsAb Test Done?

This test may be done to look for prior exposure to Hepatitis B or whether your vaccination was successful. It may also be done if you have Hepatitis B to see if you are recovering. While it is standard as of 1991 to vaccinate babies and children for Hepatitis B, many adults were not vaccinated as children and they may be at risk. It is also possible for your antibody levels to drop over the years and if the test is negative, you may need a booster.

How is the HBsAb Test Done?

The HBsAb test is done by drawing a blood sample. It is sent to the lab for analysis. Your doctor will receive the results and evaluate them in light of your vaccination history, exposure risk, symptoms, and the results of other hepatitis tests.

A Word From Verywell

Ask your doctor if you have further questions about the test and why it is being performed.

Sources:

American Association for Clinical Chemistry. (February 2015). Hepatitis B Testing.

Centers for Disease Control and Prevention. (August 2016). Hepatitis B FAQs for Health Professionals

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