Relevance and Criticality in an External Quality Assessment for the Determination of Diphtheria Antitoxin

Accurate determination of diphtheria antitoxin is of value in determining the rates of immunity within broad populations or the immune status of individuals who may be at risk of infection, assessing responses to vaccination and immunization schedule efficacy. An External Quality Assessment (EQA) study for diphtheria serology was carried out within the European Diphtheria Surveillance Network (EDSN). Sixteen national laboratories from 15 European countries participated by testing a standard panel of 150 sera using their current routine method: VERO cell Toxin Neutralization Test (TNT), Dual Double Antigen timeresolved fluorescence immunoassay (dDA-DELFIA), fluorescent bead-based multiplex assay (MIA), in-house or commercial Enzyme-Linked Immunosorbent Assays (ELISA). The objective of the study was not to identify the best assay but to verify whether laboratories using their current method would categorize (negative, equivocal or positive) a serum sample in the same way. Performance of each laboratory was determined by comparison of results on a quantitative and qualitative basis versus TNT results from a single reference laboratory, as this test is considered the in vitro “gold standard”. Quantitative data were analyzed using Pearson’s correlation coefficient as well as Lin’s concordance-correlation coefficient. Qualitative diagnostic concordance was established by Cohen’s kappa (k) coefficient. Performance of laboratories using TNT was very good, whilst the performance using other in vitro methods was variable. Laboratories that used ELISA performed less well than those using DELFIA or MIA. EQA is important both for laboratories that use in vitro non standardized methods or commercial ELISA kits.