Hepatitis B Surface Antigen

The hepatitis B surface antigen test, often written as HBsAg, looks for a protein on the surface of the hepatitis B virus in your blood. This protein is the first marker to appear after infection, usually within a few weeks, and its presence means you have an active hepatitis B infection right now. A positive HBsAg result does not by itself tell whether the infection is acute or chronic, so follow-up testing is needed. If the antigen stays positive for more than six months, the infection is considered chronic. If it clears, you have recovered. The test is also used to screen blood donors and to check whether treatment is working.

What HBsAg Is

The surface antigen is a protein on the outer shell of the hepatitis B virus. When the virus is replicating, large amounts of this protein circulate in the blood. Detecting HBsAg means the virus is present and active, which is why it is the primary screening test for hepatitis B infection.

The test is also used to screen donated blood, since infected blood can transmit the virus to recipients. Its presence is the earliest reliable sign of an acute infection, often before symptoms appear.

What a Positive Result Means

A positive HBsAg result means you currently have hepatitis B and can pass the virus to others. It does not by itself distinguish acute from chronic infection. If you have been positive for less than six months, the infection is likely acute, and many adults clear it on their own.

A positive result also carries the same meaning in pregnancy, where routine testing is standard because the virus can pass to the baby at birth. Prompt medical follow up is important in every case.

What a Negative Result Means

A negative HBsAg result means no hepatitis B surface antigen was found, so there is no active infection at the time of testing. It does not necessarily mean you are unexposed, since other markers like the core antibody can reveal a past infection.

If surface antibody is also present, you are immune, either from the vaccine or from recovering from a past infection. The full panel of markers tells the complete story.

Follow-Up Testing After a Positive Result

If HBsAg is positive, follow up testing includes the hepatitis B core antibody, the surface antibody, and liver function tests. These help determine whether the infection is acute or chronic and whether the liver is affected. The surface antigen is repeated at later visits to track progress.

If the antigen clears within six months, the infection has resolved. If it persists, chronic hepatitis B is diagnosed, and monitoring, vaccination for household contacts, and possibly treatment begin.

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