Specific immunoglobulin M response to hepatitis A virus determined by solid-phase radioimmunoassay.

Immunoglobulin M antibody to hepatitis A virus (IgM anti-HAV) is found in most patients with acute type A hepatitis. To determine the duration of this IgM response as well as to confirm that IgM anti-HAV is a specific marker for acute infection, we developed a solid-phase radioimmunoassay for IgM anti-HAV. This new assay is 25-fold more sensitive than a conventional blocking radioimmunoassay for anti-HAV, and interference due to rheumatoid factor was eliminated by simultaneously testing sera against virus-free control antigen. Maximum IgM anti-HAV titers (1:6,400 to greater than or equal to 1:51,200) were detected during the first 30 days after the onset of illness. Although the IgM anti-HAV titer subsequently declined 64-fold over the ensuing 90 days, low-titer IgM anti-HAV (1:100 to 1:400) persisted in many sera for 90 to 150 days. Acute sera having an IgM anti-HAV titer of greater than or equal to 1:25,600 possessed a significantly higher mean IgM concentration (492 mg/dl) than acute sera with an IgM anti-HAV titer of less than or equal to 1:12,800 (344 mg/dl; P < 0.05). IgM anti-HAV titers did not correlate with other clinical or laboratory measures of disease severity. Detection of IgM anti-HAV proved to be both a highly specific (>99%) and a sensitive (>99%) method for the diagnosis of type A hepatitis.