Appearance and persistence of hepatitis A IgM antibody in acute clinical hepatitis A observed in an outbreak.

143 patients with acute hepatitis A observed during a common source outbreak were tested for IgM class antibody to hepatitis A virus (IgM anti-HAV) by commercially available solid phase radioimmunoassays. All patients were IgM anti-HAV positive though six of them were initially negative when tested within three days after the onset of symptoms. The levels of IgM anti-HAV peaked two to four weeks after onset, and declined gradually thereafter. The probability of IgM anti-HAV positivity was 25% at the end of the sixth month, and 3.4% at the end of the ninth month after onset. No significant sex or age difference in the persistence of IgM anti-HAV was observed. It was suggested that the timing of the assay and the level of IgM anti-HAV should be considered in the etiological diagnosis of acute hepatitis.