Detection of toxin production by Corynebacterium diphtheriae: results of a trial organised as part of the United Kingdom National External Microbiological Quality Assessment Scheme.

Four strains of Corynebacterium diphtheriae were sent to UK participants in the UK National External Microbiological Quality Assessment Scheme, who were asked to examine the strains for toxin production by in vitro methods. Laboratories achieved 162/176 (92%) and 160/175 (91%) correct results with two rapid toxin producers and 145/175 (82%) with a slow toxin producer. With a non-toxigenic strain 26/175 (15%) laboratories reported toxin production. Of the 173 laboratories reporting on all four strains, only 120 (69%) achieved the correct result for all. There was no significant association between the use of various methods and results, with the exception that laboratories using a full set of positive, weak positive, and negative controls made fewer errors than those not using controls. A number of unsatisfactory practices were revealed by the trial, however, and recommendations on preparation of inoculum, media, peptones, animal sera, and use of controls are made.