Multicenter evaluation of molecular and culture-dependent diagnostics for Shigella species and Entero-invasive Escherichia coli in the Netherlands

- Used diagnostics for Shigella and Entero-invasive E. coli varies among laboratories.
- The diversity of diagnostics has no effect on qualitative detection of S. flexneri.
- The diversity of diagnostics has effect on semi quantitative detection of S. flexneri.
- Defining a cut-off in Ct-value for notification of shigellosis is not advisable.

An inter-laboratory collaborative trial for the evaluation of diagnostics for detection and identification of Shigella species and Entero-invasive Escherichia coli (EIEC) was performed. Sixteen Medical Microbiological Laboratories (MMLs) participated. MMLs were interviewed about their diagnostic methods and a sample panel, consisting of DNA-extracts and spiked stool samples with different concentrations of Shigella flexneri, was provided to each MML. The results of the trial showed an enormous variety in culture-dependent and molecular diagnostic techniques currently used among MMLs. Despite the various molecular procedures, 15 out of 16 MMLs were able to detect Shigella species or EIEC in all the samples provided, showing that the diversity of methods has no effect on the qualitative detection of Shigella flexneri. In contrast to semi quantitative analysis, the minimum and maximum values per sample differed by approximately five threshold cycles (Ct-value) between the MMLs included in the study. This indicates that defining a uniform Ct-value cut-off for notification to health authorities is not advisable.