**Anti-Hepatitis A Virus (Anti-HAV) IgM** is a crucial diagnostic antibody produced by the body during an acute or recent infection with the Hepatitis A virus. Its presence in a patient's blood sample serves as the primary marker for diagnosing sudden-onset, active Hepatitis A.
Unlike the IgG antibody, which represents long-term immunity, IgM antibodies appear early after viral exposure—typically coinciding with the onset of symptoms like jaundice, fatigue, and abdominal pain. Anti-HAV IgM levels remain elevated for roughly three to six months, making this test essential for differentiating a current, active illness from past exposure.