Immunoenzymatic assay of anti-diphtheric toxin antibodies in human serum

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Abstract
An enzyme-linked immunosorbent assay (ELISA) was developed for measuring IgG anti-diphtheric toxin antibodies in human serum. The assay was done in plastic plates coated with purified diphtheric toxoid. Since a straight-line relationship was found between logs of extinction values and of antibody concentrations, with a very constant slope, serum titers could be expressed as log10 of the serum dilution corresponding to a definite optical density, such as 0.5. The assay furnished highly reproducible titers on a continuous range, with coefficients of variation < 10%. Only 1 or 2 serum dilutions were usually sufficient for serum titration. To establish correspondence of the ELISA titers with biologically determined antitoxin IU, a regression equation was fitted between the respective values for 112 serum samples titrated in both tests. The ELISA titer of 2.38 corresponded to an antitoxin titer of 0.01 IU, which is considered as the minimal protective level. Simple to perform, economical and precise, the ELISA seems to be a very practical procedure for seroepidemiological purposes.