Rectal Exam for Suspected C. difficile Infection
A rectal exam is not necessary for diagnosing C. difficile infection in patients who can produce diarrheal stool specimens, as diagnosis relies on laboratory testing of unformed stool samples. 1
When Rectal Sampling May Be Indicated
For patients with suspected severe CDI complicated by ileus who cannot produce stool specimens, perirectal swabs provide an acceptable alternative to stool specimen analysis. 1
- A prospective study of 139 patients demonstrated that perirectal swabs tested by PCR had sensitivity of 95.7%, specificity of 100%, positive predictive value of 100%, and negative predictive value of 99.1% compared to stool specimens 1
- This approach is specifically reserved for patients unable to produce specimens due to ileus or severely disturbed bowel function 1
Standard Diagnostic Approach
Diagnostic testing should only be performed on diarrheal stool (≥3 unformed stools in 24 hours) from symptomatic patients, not on formed stool. 1
The diagnostic algorithm prioritizes:
- Laboratory testing of diarrheal stool specimens as the primary diagnostic method, using nucleic acid amplification tests (NAAT) or two-step algorithms starting with toxin-EIA 1
- Clinical context including recent antibiotic use, hospitalization, advanced age, fever, abdominal pain, and leukocytosis to guide testing decisions 1
- Imaging (CT) showing colonic wall thickening >4mm, accordion sign, pericolonic stranding, or unexplained ascites when stool testing is not feasible 2, 3
Role of Endoscopy (Not Rectal Exam)
Flexible sigmoidoscopy is appropriate only for patients with high clinical suspicion and negative stool tests, or when urgent differentiation from other causes of acute colitis is needed. 2
- Endoscopic visualization of pseudomembranes can suffice for diagnosis in the absence of another obvious cause 2
- Full colonoscopy should be avoided in severe or fulminant colitis due to increased perforation risk 2
- Laboratory testing and imaging should be prioritized over endoscopy for initial diagnosis 2
Critical Pitfalls to Avoid
- Do not test formed stool as this results in false positives and unnecessary antibiotic therapy 1
- Do not perform routine rectal exams for C. difficile diagnosis when patients can produce diarrheal stool 1
- Ensure testing criteria are met: ≥3 unformed stools in 24 hours, no laxative use, and no alternative explanation for diarrhea 1
- Reserve perirectal swabs exclusively for patients with ileus who cannot produce stool specimens 1