Studies on the correlation of a range of immunoassays for diphtheria antitoxin with the guinea-pig intradermal test.

The diphtheria antitoxin titres determined in human sera by the intradermal titration method have been compared with the titres obtained by a range of ELISA assays. A direct ELISA based on highly purified diphtheria toxoid correlated poorly with the in vivo method but an assay based on capture of toxoid by a non-neutralizing monoclonal antibody produced results that correlated very well. The performance of assays involving partial blockade of antigen and non-neutralizing monoclonal antibodies was intermediate.