C. difficile Testing
Clostridioides (Clostridium) difficile Toolkit for Long-term Care Facilities

Clostridioides (Clostridium) difficile infection (CDI) may be suspected in residents with new onset of diarrhea who have 3 or more unformed stools (conform to the shape of the container) in a 24-hour period. To determine if a patient has CDI, consider both clinical symptoms and lab test results.

Facilities can use multiple tests to confirm or to assist in confirming CDI in symptomatic patients. However, it is important to know what test is being used because they have widely varying turn-around times, sensitivities, specificities, positive predictive values, and negative predictive values.

Lab tests

• Culture
  • Most sensitive test available, but often associated with false-positive results due to presence of non-toxigenic strains
  • Labor intensive and requires careful laboratory quality control
  • Results are available within 48-96 hours
  • Detects the presence of C. difficile antigen (GDH) by latex agglutination or immunochromatographicassays
  • Rapid test results (
  • Should be combined with toxin testing to verify diagnosis
  • Enzyme immunoassay (EIA) detects toxin A, toxin B, or both A and B
  • Same-day results
  • Easy to use
  • Low labor costs
  • Detects toxigenic C. difficile in stool
  • A potential advantage is ability to determine strains, for instance, whether they make toxin and to which toxin type they belong
  • A more sensitive and more specific approach

  Stool samples

  A C. diff stool culture is a specific kind of culture that is different than the typically requested bacterial stool culture. The culture has to be grown in an anaerobic environment, making it more labor intensive with a relatively slower turn-around time.

  Before submitting a sample for C. diff testing, consider other causes of loose stools. This could include medications (stool softeners or proton pump inhibitors) or underlying conditions such as Crohn’s or bowel surgery.

  Obtaining stool samples

  • Only unformed stools should be tested (Bristol stool chart types 5-7). Testing asymptomatic residents (those with formed stools) is not recommended. Testing formed stools can lead to detecting colonized, not infected, cases. Since all positives must be reported to NHSN, this can lead to over-reporting CDI.
  • Fresh stool is required.
  • C. difficile toxin is very unstable and degrades at room temperature in as short as two hours. False negative results occur when specimens are not kept refrigerated until testing can be done.
  • Collect specimen in clean, watertight container.
    • Transport media is not necessary and may increase false positive results.
    • Transport specimens as soon as possible and store at 2 - 8 °C until tested.
    • Storage at room temperature may decrease the sensitivity of some tests, possibly due to toxin inactivation.

    Testing recommendations for stool collection and submission

    DO’s

    DON’Ts

    Submit fresh stool samples for CDI testing from resident with suspected CDI: 3 or more unformed stools per 24 hours.

    Test asymptomatic residents for CDI.

    Avoid repeat testing; submit one specimen per resident.

    Perform tests-of-cure on any resident post-treatment.

    Refrigerate (store at 2 - 8 degrees Celsius; 36-46 degrees Fahrenheit) until testing can be done.

    Transport specimen in media; this may increase false positive test results.

    Collect specimen in clean, watertight container. Transport specimens as soon as possible after collection.

    Wait to transport specimens

    Specimen rejection criteria for C. difficile toxin in stool samples

    Rejection Criteria

    Report Comment

    Formed Stool

    “Formed stool received. Test cancelled.”

    Rectal Swab

    “Rectal swab received; unsuitable for testing. Please resubmit a stool sample.”

    >3 negative specimens processed.

    “Three negative specimens from this patient have already been processed in the last week. Test cancelled.”

    Positive within 7 days

    “Please refer to previous positive C. difficile toxin result.”

    Submitted in enteric pathogen transport medium or SAF container.

    "Specimen received in enteric pathogen transport medium. Unsuitable for C. difficile testing. Please repeat."
    -OR-
    "Specimen received in SAF (fixative for parasite). Unsuitable for C. difficile toxin testing. If you have any questions, please call the Microbiology Lab."

    Colon examination

    In some cases, examination of the colon can be used to help confirm a diagnosis of CDI. If C. difficile colitis is not accompanied by pseudomembrane formation, endoscopic findings are relatively nonspecific, but a biopsy specimen may reveal changes typical of pseudomembranous colitis.
    

    Endoscopy

    • Pseudomembranous colitis can only be diagnosed by direct visualization of pseudomembranes on lower gastrointestinal endoscopy
    • At least 90% of patients with pseudomembranous colitis demonstrate either C. difficile or its toxin in stool samples
    • Because of its cost, risk to the patient, and the availability of other diagnostic tests, endoscopy is usually reserved for special situations
    • The American College of Gastroenterology Guidelines recommend endoscopy for situations such as the following:
      • a rapid diagnosis is needed and test results are delayed or insensitive tests are used
      • the patient has an ileus and stool is not available
      • other colonic diseases that can be diagnosed with endoscopy are being considered

      Imaging tests

      • Computerized tomography (CT) scan
        • Provides detailed images of the colon
        • This scan can show a thickening of the wall of the colon, which is common in pseudomembranous colitis

        Sensitivity and specificity of diagnostic laboratory tests for CDI

        C. difficile Laboratory Tests

        Substance detected

        Time required

        Sensitivity* %

        Specificity** %