Which serologic marker indicates acute Hepatitis B infection?

Anti-HBc IgM (IgM antibody to hepatitis B core antigen) is the most reliable marker of acute hepatitis B infection. It appears early in acute infection and remains detectable for 6 months. HBsAg indicates active infection (acute or chronic) but does not distinguish between the two. HBsAb (anti-HBs) indicates immunity — either from resolved infection or vaccination. Anti-HBc IgG indicates past (resolved) infection and confers no immunity by itself. The IgM class switch to IgG over time distinguishes acute from past infection.