title: Differences in outcome according to Clostridium difficile testing method: a prospective multicentre diagnostic validation study of C difficile infection.
abstract: BACKGROUND
  Diagnosis of Clostridium difficile infection is controversial because of many laboratory methods, compounded by two reference methods. Cytotoxigenic culture detects toxigenic C difficile and gives a positive result more frequently (eg, because of colonisation, which means that individuals can have the bacterium but no free toxin) than does the cytotoxin assay, which detects preformed toxin in faeces. We aimed to validate the reference methods according to clinical outcomes and to derive an optimum laboratory diagnostic algorithm for C difficile infection.
  
  
  METHODS
  In this prospective, multicentre study, we did cytotoxigenic culture and cytotoxin assays on 12,420 faecal samples in four UK laboratories. We also performed tests that represent the three main targets for C difficile detection: bacterium (glutamate dehydrogenase), toxins, or toxin genes. We used routine blood test results, length of hospital stay, and 30-day mortality to clinically validate the reference methods. Data were categorised by reference method result: group 1, cytotoxin assay positive; group 2, cytotoxigenic culture positive and cytotoxin assay negative; and group 3, both reference methods negative.
  
  
  RESULTS
  Clinical and reference assay data were available for 6522 inpatient episodes. On univariate analysis, mortality was significantly higher in group 1 than in group 2 (72/435 [16·6%] vs 20/207 [9·7%], p=0·044) and in group 3 (503/5880 [8·6%], p<0·001), but not in group 2 compared with group 3 (p=0·4). A multivariate analysis accounting for potential confounders confirmed the mortality differences between groups 1 and 3 (OR 1·61, 95% CI 1·12-2·31). Multistage algorithms performed better than did standalone assays.
  
  
  CONCLUSIONS
  We noted no increase in mortality when toxigenic C difficile alone was present. Toxin (cytotoxin assay) positivity correlated with clinical outcome, and so this reference method best defines true cases of C difficile infection. A new diagnostic category of potential C difficile excretor (cytotoxigenic culture positive but cytotoxin assay negative) could be used to characterise patients with diarrhoea that is probably not due to C difficile infection, but who can cause cross-infection.
authors.affiliation: Centre for Infection and Immunity, Division of Clinical Medicine, St George's, University of London, London, UK; Department of Medical Microbiology, St George's Healthcare NHS Trust, London, UK.
authors.forename: Timothy D
authors.initials: TD
authors.lastname: Planche
authors.forename: Kerrie A
authors.initials: KA
authors.lastname: Davies
authors.forename: Pietro G
authors.initials: PG
authors.lastname: Coen
authors.forename: John M
authors.initials: JM
authors.lastname: Finney
authors.forename: Irene M
authors.initials: IM
authors.lastname: Monahan
authors.forename: Kirsti A
authors.initials: KA
authors.lastname: Morris
authors.forename: Lily
authors.initials: L
authors.lastname: O'Connor
authors.forename: Sarah J
authors.initials: SJ
authors.lastname: Oakley
authors.forename: Cassie F
authors.initials: CF
authors.lastname: Pope
authors.forename: Mike W
authors.initials: MW
authors.lastname: Wren
authors.forename: Nandini P
authors.initials: NP
authors.lastname: Shetty
authors.forename: Derrick W
authors.initials: DW
authors.lastname: Crook
authors.forename: Mark H
authors.initials: MH
authors.lastname: Wilcox
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D019540:Area Under Curve; D002648:Child; D002675:Child, Preschool; D016360:Clostridioides difficile / Q000235:genetics / Q000302:isolation & purification*; D004269:DNA, Bacterial / Q000737:chemistry / Q000235:genetics; D003967:Diarrhea / Q000175:diagnosis* / Q000382:microbiology*; D004761:Enterocolitis, Pseudomembranous / Q000175:diagnosis* / Q000382:microbiology*; D004768:Enterotoxins / Q000032:analysis; D005243:Feces / Q000382:microbiology; D005260:Female; D005969:Glutamate Dehydrogenase / Q000032:analysis; D006801:Humans; D016015:Logistic Models; D008297:Male; D008875:Middle Aged; D016133:Polymerase Chain Reaction; D011237:Predictive Value of Tests; D011446:Prospective Studies; D012372:ROC Curve; D012680:Sensitivity and Specificity; D055815:Young Adult
chemical_list: D004269:DNA, Bacterial; D004768:Enterotoxins; D005969:Glutamate Dehydrogenase