Diagnosis of hepatitis A infection: comparative specificity of IgM capture assays using antigens derived from tissue cultures and marmoset faeces.

Hepatitis A virus (HAV) antigens from two tissue culture sources were compared with that from the faeces of infected marmosets to determine whether the former were satisfactory substitutes. Sera from 313 healthy blood donors and 417 patients with various clinical conditions were tested for IgM class antibody to HAV (anti-HAV IgM) using an IgM antibody capture assay (MACRIA) with each of the 3 antigens. Forty-eight specimens, all from cases of acute hepatitis, were positive in MACRIA with all 3 antigens. Only 2 of the 313 blood donors' sera reacted at all. These reactions were weak and did not arise with all antigens. Weakly reactive specimens were also found in 3 out of the 13 clinical categories. Overall 12 weak reactions arose with the faecal antigen and 8 and 7 with the two tissue culture antigens. Rheumatoid factor (RhF) was detected in 8 of the weakly reactive specimens and these had significantly higher titres of anti-HAV than sera known to contain RhF that were unreactive in MACRIA. It is concluded that tissue culture derived HAV antigen should replace that from primates on the grounds of quality, economy and convenience: also that non-specific activity in HAV MACRIA is usually due to a combination of RhF and high anti-HAV titres, but is infrequently strong enough to cause reactions interpreted as positive.