Stool PCR may not be a substitute for enrichment culture for the detection of salmonella

Purpose. Polymerase chain reaction (PCR) is increasingly being used to detect enteric pathogens and is currently NICE’s recommended practice. We wished to evaluate the performance characteristics of PCR for the detection of salmonella in consecutive stool samples in a real-world setting, compared to the gold standard of enrichment culture.

Methodology. We performed a prospective study over 9 months in which the PCR and culture results for salmonella were scrutinized for all stool samples sent to the laboratory. All stool samples underwent selenite enrichment culture for salmonella with confirmation being obtained using the API 10S and serotyping. Samples also underwent PCR using the BD MAX Enteric Bacterial Panel. The sensitivity and specificity of PCR in detecting salmonella were compared to those of enrichment culture.

Results. Six thousand three hundred and seventy-two stool culture and PCR pairs from 5619 patients were analysed. The prevalence of salmonella was found to be 1.2 %. The sensitivity, specificity, positive predictive value and negative predictive value of PCR versus culture were 89 % (67/75), 99.8 % (6286/6297), 86 % (67/78) and 99.9 % (6286/6294), respectively.

Conclusion. Enrichment culture is significantly more sensitive than PCR using the BD MAX Enteric Bacterial Panel for detecting salmonella in stool. Where PCR testing is used for the detection of enteric pathogens, we recommend that enrichment culture for salmonella be continued in parallel, unless the PCR method is shown to be at least as sensitive as culture.