What is the best indicator of an acute infection with hepatitis A virus?

In the context of an acute infection with hepatitis A virus, the presence of IgM antibodies is the most reliable indicator. When a person is infected with hepatitis A, the immune system responds by producing IgM antibodies specifically against the virus. These antibodies typically appear within a couple of weeks after infection and may persist for several months, making them a key marker for diagnosing recent or acute hepatitis A virus infections.

The detection of IgM is particularly significant because it signals a recent immune response to the infection, distinguishing it from past infections, which would be indicated by the presence of IgG antibodies. In cases where IgG antibodies are detected, it typically indicates past infection or vaccination rather than an active, acute infection.

Moreover, HBsAg and anti-HBc are specific to hepatitis B virus infections, not hepatitis A, so they are not relevant in this case. Thus, the presence of IgM antibodies stands out as the best indicator of an acute hepatitis A virus infection.