Predicting culture positivity rates for Shiga toxin-producing Escherichia coli and Shigella species based on real time PCR cycle threshold values

PCR assays for the detection of Shiga toxin-producing Escherichia coli (STEC) and Shigella species/Enteroinvasive E. coli (EIEC) provide a robust, specific and sensitive diagnostic test. Real time PCR cycle threshold (ct) values provide an indication of bacterial load within a faecal specimen. We reviewed real time PCR ct values to determine whether there was a cut off after which obtaining a positive culture would be unlikely, enabling resources to be focused on specimens most likely to be culture positive. There were 685 faecal specimens included in the study, divided into three groups with ct values < 30 (n = 251), 30-35 (n = 352) and > 35 (n = 82). Of these, 330/685 (48.2%) were culture positive for either STEC (n = 316) and/or Shigella/EIEC (n = 14). The highest proportion of PCR-culture positive specimens had ct values < 30 (<30 n = 175/251, 69.7%; 30-35 n = 137/352, 38.9%; >35 n = 18/82, 22.0%). Culture was three times more likely to be successful in specimens where the ct value was < 30 compared to > 35. However, in some instances culture was achieved where the ct was > 35. Although ct values can be used to predict successful culture, specimens testing positive for STEC by real time PCR from vulnerable patients and those reporting clinically severe symptoms, should be referred to the reference laboratory regardless of the ct value.