Molecular test to determine toxigenic capabilities in GDH-positive, toxin-negative samples: evaluation of the Portrait toxigenic C. difficile assay

Br J Biomed Sci. 2013;70(2):62-6. doi: 10.1080/09674845.2013.11669937.


New recommendations for testing and reporting of Clostridium difficile were introduced in the NHS in 2012. These guidelines have improved identification of potential C. difficile infection (CDI) cases, but questions remain around the management of glutamate dehydrogenase (GDH)-positive, toxin-negative patients. This study aims to assess the introduction of the Portrait C. difficile assay as the third step to identify the presence of the toxigenic C. difficile B (tcdB) gene and thus determine toxigenic capability. Stool samples with a GDH-positive, toxin-negative result were tested using the Portrait analyser to detect the presence of tcdB. A retrospective evaluation was performed, assessing the clinical course of patients who were isolated as a result of the current algorithm using GDH enzyme immunoassay (EIA) and toxin EIA. Of the stool samples tested, 40% carried the tcdB gene. Four tcdB-positive stool samples initially toxin A/B-negative subsequently became positive. Thirteen patients were isolated, four of which did not have the tcdB gene. The total time to 'process' a positive CDI case was 102 hours and cost pounds 592. The additional time and cost of incorporating the Portrait toxigenic C. difficile assay was 105-115 minutes and pounds 46.48 to pounds 51.88. This study confirms that toxigenic capabilities in GDH-positive, toxin-negative specimens can facilitate effective treatment and infection prevention, and results show there is potential value in repeat toxin testing.

Publication types

  • Evaluation Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Bacterial Proteins / genetics*
  • Bacterial Toxins / genetics*
  • Clostridioides difficile / genetics*
  • Clostridioides difficile / isolation & purification
  • Clostridioides difficile / pathogenicity
  • Enterocolitis, Pseudomembranous / diagnosis*
  • Enterocolitis, Pseudomembranous / microbiology*
  • Enterotoxins / genetics*
  • Feces / microbiology
  • Glutamate Dehydrogenase / metabolism
  • Humans
  • Microbiological Techniques
  • Retrospective Studies
  • United Kingdom
  • Virulence


  • Bacterial Proteins
  • Bacterial Toxins
  • Enterotoxins
  • tcdA protein, Clostridium difficile
  • toxB protein, Clostridium difficile
  • Glutamate Dehydrogenase