Evaluation of Real Time Polymerase Chain Reaction for Salmonella Invasion Gene A and Salmonella Tetrathionate Respiration Gene as a Diagnostic Test for Typhoid Fever

Abstract

Background: Enteric (typhoid and paratyphoid) fever is a serious systemic disease caused by Salmonella enterica (S. enterica) that needs appropriate microbiological diagnosis mainly with atypical clinical signs due to antibiotics misuse. In Egypt, there are insufficient data about molecular detection of enteric fever. Objectives: To evaluate the quantitative real- time polymerase chain reaction (qPCR) for Salmonella invasion gene A (Inv A) and Salmonella tetrathionate respiration gene (ttr) of S. enterica in typhoid fever patients. Methodology: This was a case-control study that included 100 patients with typhoid fever and 100 control subjects. Ten ml blood were obtained for blood culture, Widal testing and enrichment on tryptic soya broth (TSB) with bile. Positive blood culture was sub cultured, identified and serotyped for isolation of S. enterica. Finally, qPCR was applied for detection of Inv A and ttr genes from blood samples and after enrichment. Results: Blood culture was positive in 51% and Widal test was positive in 70% of patients. Combination of Widal and/or blood culture increased their sensitivity, specificity and accuracy to 82%, 89.2% and 85.5% respectively. qPCR was positive in 95% of patients (95 for ttr and 89 for Inv A) while, pre-enriched blood qPCR was positive in 99%. The highest sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV) and accuracy was for qPCR for pre-enriched blood (99%, 100%, 100%, 99.01%, 99% respectively). Conclusion: The study highlighted the high accuracy of qPCR for genes Inv A and ttr in diagnosis of S. typhi from blood.