Diagnostic accuracy of antigen-based immunochromatographic rapid diagnostic tests for the detection of Salmonella in blood culture broth

BACKGROUND: In low resource settings, Salmonella serovars frequently cause bloodstream infections. This study investigated the diagnostic performance of immunochromatographic rapid diagnostic tests (RDTs), which detect Salmonella antigens, when applied to stored grown blood culture broth.

MATERIAL/METHODS: The SD Bioline One Step Salmonella Typhi Ag Rapid Detection Kit (Standard Diagnostics, Republic of Korea), marketed for the detection of Salmonella enterica serovar Typhi (Salmonella Typhi) in stool and the Salmonella Ag Rapid Test (Creative Diagnostics, USA), marketed for the detection of all Salmonella serotypes in stool, were selected for evaluation based on a pre-test evaluation of six RDT products. The limits of detection (LOD) for culture suspensions were established and the selected RDT products were assessed on 19 freshly grown spiked blood culture broth samples and 413 stored clinical blood culture broth samples, collected in Cambodia and the Democratic Republic of the Congo.

RESULTS: The LOD of both products was established as 107-108 CFU/ml. When applied to clinical blood culture broth samples, the diagnostic sensitivity and specificity of the SD Bioline RDT were respectively 100% and 79.7% for the detection of Salmonella Typhi; 94.4% (65/69) of false-positive results were caused by Salmonella Enteritidis. When considering the combined detection of Salmonella Typhi and Enteritidis (both group D Salmonella), sensitivity and specificity were 97.9% and 98.5% respectively. For Creative Diagnostics, diagnostic sensitivity was 78.3% and specificity 91.0% for all Salmonella serotypes combined; 88.3% (53/60) of false negative results were caused by Salmonella Paratyphi A.

CONCLUSIONS: When applied to grown blood culture broths, the SD Bioline RDT had a good sensitivity and specificity for the detection of Salmonella Typhi and Salmonella Enteritidis. The Creative Diagnostics product had a moderate sensitivity and acceptable specificity for the detection of all Salmonella serovars combined and needs further optimization. A RDT that reliably detects Salmonella Paratyphi A is needed.