New bacterial absorption method for determination of hepatitis A IgM and IgA antibodies.

Antibodies against hepatitis A virus (anti-HAV) can be determined by a commercially available radioimmunoassay (RIA) (HavabTM, Abbott). To discriminate between recent and past hepatitis A infection this RIA was used in combination with absorption with protein A-containing staphylococci. However, nonabsorbable anti-HAV was repeatedly detected in late-convalescent sera using this methods. The nature of these antibodies was studied in serum samples from 12 such patients. In all patients, the late-convalescent sera contained no IgM class anti-HAV as judged by sucrose density gradient centrifugation. The restricted specificity of staphylococcal protein A explains the lack of absorption. Some recently described streptococcal strains capable of binding all IgG subclasses (including IgG3) as well as both IgA subclasses were, therefore, added to the staphylococci. Absorption studies using these strains indicated that the previously nonabsorbable anti-HAV in these 12 patients was mainly of the IgA class. A bacterial mixture including IgA-binding streptococci seems preferable to routine determination of IgM anti-HAV in acute hepatitis A diagnosis. The results also indicate that IgA anti-HAV in serum can persist for more than two years after a hepatitis A infection.