Pigment visibility on rectal swabs used to detect enteropathogens: A prospective cohort study

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Data are lacking regarding the impact of visible pigment on rectal swab diagnostic accuracy. We describe the test characteristics of rectal swabs with and without pigment in children with gastroenteritis. Between December 2014 and September 2017, children (age, 18 years) with 3 episodes of vomiting and/or diarrhea in a 24-h period and symptoms for 7 days were enrolled through two pediatric emergency departments and from a province-wide nursing telephone advice line in Alberta, Canada. Specimens were analyzed by employing nucleic acid amplification panels. The primary outcomes were the sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) for the rectal swabs, with stool specimen results being used as the reference standard. An enteropathogen was detected in 76.0% (1,399/1,841) of the paired specimens. A total of 54.4% (1,001/1841) of the swabs had visible pigment. The respective enteropathogen detection characteristics of swabs with and without visible pigment were as follows: 92.2% (95% confidence interval [CI], 90.0%, 94.0%) versus 83.7% (95% CI, 80.5%, 86.4%) for sensitivity, 94.3% (95% CI, 90.5%, 96.6%) versus 91.2% (95% CI, 86.3%, 94.5%) for specificity, 97.9% (95% CI, 96.4%, 98.8%) versus 96.5% (95% CI, 94.5%, 97.8%) for PPV, and 80.9% (95% CI, 76.0%, 85.1%) versus 65.8% (95% CI, 60.0%, 71.1%) for NPV. Processing of swabs without visible pigment would increase the rate of identification of positive swabs from 50.0% (682/1,365) to 88.3% (1,205/1,365). There is a modest decrease in the reliability of a negative test on swabs without evidence of pigment, but the overall yield is significantly greater when they are not excluded from testing. Hence, rectal swabs without visible feces should not be routinely rejected from testing.

Original languageEnglish (US)
Article numbere00213-19
JournalJournal of clinical microbiology
Issue number6
StatePublished - Jun 2019

Bibliographical note

Funding Information:
by grant number NIH P30DK052574 (ARAC, Digestive Diseases Research Core Center). The Pediatric Emergency Medicine Research Associate Program (PEMRAP) is supported by a grant from the Alberta Children’s Hospital Foundation.

Funding Information:
This research was supported by APPETITE, which is funded by a grant from the Alberta Innovates Team Collaborative Research Innovation Opportunity. APPETITE is also supported by the Alberta Children’s Hospital Research Institute (Calgary, AB, Canada) and the Women and Children’s Health Research Institute (Edmonton, AB, Canada) through a partnership award. S.B.F. is supported by the Alberta Children’s Hospital Foundation Professorship in Child Health and Wellness. P.I.T. is also supported

Publisher Copyright:
Copyright © 2019 American Society for Microbiology. All Rights Reserved.


  • Diagnostics
  • Diarrhea
  • Rectal swab
  • Transmissible gastroenteritis virus
  • Visible pigment


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