Hepatitis B Core Antibody Test

The hepatitis B core antibody test, written as anti-HBc, looks for antibodies your immune system makes against the core of the hepatitis B virus. It is one of several hepatitis B markers, and it tells a different part of the story than the surface antigen and surface antibody tests. Unlike the surface antibody, the core antibody does not form in response to the vaccine, so a positive core antibody indicates exposure to the virus itself at some point, either a past infection or a current one. When measured as IgM and IgG, the test can help show whether an infection is recent or longstanding. A positive core antibody with a negative surface antigen usually means a resolved past infection.

What the Core Antibody Is

The core antibody is produced in response to the hepatitis B core antigen, which is the inner protein of the virus. Because the vaccine uses only the surface protein, vaccinated people produce surface antibodies but not core antibodies. A positive core antibody, therefore, signals actual exposure to the virus.

This distinction is useful: it helps tell vaccination from natural infection. The test is part of the full hepatitis B serology panel that your clinician interprets together to map your status.

IgM and IgG Core Antibodies

The core antibody is measured in two forms. Total core antibody, including both IgM and IgG, shows any past exposure. A positive IgM core antibody indicates a recent infection, usually within the past few months, and points to an acute case.

A positive IgG core antibody, without recent IgM, indicates a past or resolved infection. When the core antibody is read alongside the surface antigen and surface antibody, the combination reveals whether you have a current infection, a resolved one, or no exposure at all.

What a Positive Result Means in Context

A positive core antibody with a negative surface antigen typically means you had a hepatitis B infection in the past and have cleared it, leaving immunity. If the surface antibody is also present, you are protected. A positive core antibody with a positive surface antigen means an active infection.

In rare cases, a person tests positive for the core antibody alone, which can indicate a past infection, a resolving infection, or a low-level chronic infection. This pattern requires careful interpretation by a clinician, often with additional testing.

Why Testing the Core Antibody Matters

The core antibody test is essential for a complete hepatitis B picture because it distinguishes an infection from vaccination. It helps detect past infections, supports a diagnosis of acute hepatitis B when IgM is positive, and contributes to decisions about monitoring and vaccination.

For people who have never been vaccinated, knowing their core antibody status clarifies their exposure and immunity. When results are uncertain, your clinician combines the markers and, if needed, additional tests to give you a clear and accurate answer.

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