Two-Step Clostridioides difficile Testing in a Symptomatic Patient
A 74-year-old woman is admitted with a 3-day history of watery diarrhea, up to eight stools per day, and abdominal cramping. She completed a 10-day course of cefepime for a urinary tract infection 2 weeks ago. Her temperature is 38.3°C, heart rate 104/min, blood pressure 112/68 mmHg, and white blood cell count 18,400/µL with 86% neutrophils. Abdominal examination is diffusely tender without rebound; abdominal radiograph shows no megacolon. The laboratory uses a two-step stool algorithm: glutamate dehydrogenase antigen plus toxin A/B enzyme immunoassay, with reflex PCR for toxigenic Clostridioides difficile if GDH is positive and toxin is negative. The stool is liquid and takes the shape of the container. GDH is positive, toxin A/B enzyme immunoassay is negative, and PCR is positive. The hospitalist asks whether this represents colonization or infection and whether to start oral vancomycin. The infection preventionist also asks whether the patient needs isolation and whether the test should be repeated after treatment.
1. Interpret the three results and explain whether this is colonization or infection.
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GDH is a screening antigen found in both toxigenic and nontoxigenic C. difficile, so a positive GDH result indicates the organism is present but does not prove toxin production. The toxin A/B enzyme immunoassay detects free toxin and is negative. PCR detects the genes for toxin A and/or B, so a positive PCR confirms that a toxigenic strain is present. A positive GDH, negative toxin, and positive PCR pattern can represent colonization with a toxigenic strain, but it can also represent active infection when toxin is present below the assay detection limit or has been neutralized. The clinical picture matters: eight liquid stools per day, fever, leukocytosis, recent cefepime, and no alternative explanation make active C. difficile infection likely. The negative toxin does not exclude disease. In a symptomatic patient with this presentation, treatment for C. difficile infection is reasonable while confirming that no other cause explains the diarrhea.
2. What are the limitations of PCR, and why is repeat testing after treatment not indicated?
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PCR detects toxigenic genes but cannot distinguish colonization from active toxin-mediated disease. It remains positive after clinical resolution because the organism and its DNA can persist for weeks. It also does not quantify toxin production. For these reasons, PCR alone can lead to overdiagnosis if it is used in patients without compatible symptoms or in formed stool. Repeat testing after treatment is not indicated because a positive result does not prove treatment failure or ongoing infection, and a negative result does not reliably predict relapse. Test-of-cure testing is discouraged. The laboratory should reject formed stool and should not repeat testing within 7 days for the same episode unless there is a specific clinical reason. If symptoms persist or worsen, clinical reassessment is more useful than repeat testing.
3. What infection prevention and reporting steps are appropriate?
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Place the patient in contact precautions with gown and gloves, and perform hand hygiene with soap and water rather than alcohol-based hand rub when caring for a patient with known or suspected C. difficile infection, because alcohol does not reliably kill spores. Use dedicated equipment when possible and clean the environment with a sporicidal agent such as bleach. Notify infection prevention so that transmission can be monitored. The laboratory should report the positive result promptly and should not perform a test of cure. Public health reporting is not routinely required for a single case of C. difficile infection, but cluster or outbreak situations should be reported according to local requirements. The hospitalist should also review whether ongoing antibiotics can be narrowed or stopped, because antibiotic exposure is a key modifiable risk factor.