Study on the use of an enzyme-linked immunosorbent assay in determining human antibodies to diphtheria toxin as compared with a reference toxin neutralization assay

Serum samples from 156 Greek persons were assessed by an IgG-specific enzym e-linked immunosorbent assay (ELISA) and a reference tissue culture toxin-n eutralization (TN) assay for the quantitation of diphtheria toxin antibodie s. By the reference method, 7.7% of the persons were susceptible to diphthe ria (antitoxin < 0.01 IU/ml), 28.8% had basic protection (antitoxin 0.01-0. 09 IU/ml) and 63.5% were fully protective (antitoxin greater than or equal to 0.1 IU/ mi), while the corresponding figures were 17.9, 36.5 and 45.5% w hen they were tested by the immunoassay. None of the samples been susceptib le by the TN assay were found to have some protection when tested by ELISA. However, three (6.7%) of the 45 samples showing a basic protection with TN , were fully protective when titrated by the immunoassay. In addition, 31 ( 31.3%) of the 99 samples been fully protective by the bioassay, were found to be either basically protective or susceptible by means of the ELISA. Ove rall, validity features of the immunoassay were: sensitivity 68.7%, specifi city 94.7%, positive predictive value 95.8% and negative predictive value 6 3.5%. The ELISA tested in our study could be used to determine diphtheria a ntitoxin in individuals needed a booster immunization (susceptible or basic protective samples), although it might falsely include in the above catego ries samples that are within the fully protective levels of antibodies. (C) 1999 Elsevier Science Ltd. All rights reserved.