Assessing the Use of Polymerase Chain Reaction to Screen for shedding of Salmonella enterica in infected humans.

Recovery from enteric bacterial illness often includes a phase of organismal shedding over a period of days to months. The monitoring of this process through laboratory testing forms the foundation of public health action to prevent further transmission. Regulations in most jurisdictions in the United States exclude individuals who continue to shed certain organisms from sensitive occupations and situations such as food handling, providing direct patient care, or attending daycare. The burden that this creates for recovering patients and their families/co-workers are great, so any effort to provide efficiency to the testing process would be of significant benefit. We sought to assess the ability of polymerase chain reaction (PCR) for the detection of Salmonella enterica shedding, and to compare that ability to culture-based testing. PCR would be faster than culture, and would allow results to be generated more quickly. Herein, we show data that indicates that while PCR and culture testing agree in the majority of cases, there are incidents of discordance between the two tests, whereupon PCR shows positive results when culture indicates lack of detectable viable organisms. Using culture-based testing as the standard, the negative predictive value of PCR is found to be 100%, while the positive predictive value was 79%. The nature of this discordance is briefly investigated. We found that it is possible that PCR may not only detect non-viable organisms in stool, but also viable organisms that remain undetectable by standard culture methods.