Clostridioides difficile Colitis
The most likely diagnosis is Clostridioides difficile colitis (option D). This 60-year-old woman presents with the classic triad: recent antibiotic exposure for a urinary tract infection, frequent watery diarrhea (six episodes daily), and systemic inflammatory markers (leukocytosis with WBC 13,000, elevated CRP). 1
Clinical Reasoning
Key Diagnostic Features Supporting C. difficile Infection
Recent antibiotic exposure is the single most important risk factor. The patient's recent UTI treatment disrupted her normal colonic microbiota, allowing C. difficile to proliferate and produce toxins. 1 Antibiotics increase CDI risk up to sixfold during therapy and in the subsequent month. 1
The clinical presentation is characteristic:
- Watery, non-bloody diarrhea (≥3 unformed stools per 24 hours meets diagnostic criteria) 1
- Lower abdominal cramping (consistent with colonic inflammation) 1
- Systemic inflammatory response with leukocytosis (13,000) and elevated CRP 1
- Tachycardia (pulse 100/min, suggesting volume depletion or systemic inflammation) 1
Additional risk factors present:
- Age 60 years (older age increases CDI risk) 1
- Methotrexate use (immunosuppression increases CDI risk) 1
- Elevated creatinine 1.5 (chronic kidney disease is an associated risk factor) 1
Why Not the Other Options?
Microscopic colitis (option C) is less likely despite some overlapping features. While microscopic colitis does present with chronic watery non-bloody diarrhea and can occur in this age group, several key differences argue against it: 2, 3, 4
- Microscopic colitis typically has a more insidious onset over weeks to months, not the acute one-week presentation described here 3, 4
- The marked systemic inflammatory response (leukocytosis 13,000, elevated CRP) is not characteristic of microscopic colitis, which shows normal or near-normal inflammatory markers 2, 3
- Recent antibiotic exposure strongly points toward C. difficile rather than microscopic colitis 1
- The acute renal impairment (creatinine 1.5) suggests volume depletion from severe diarrhea, more consistent with infectious colitis 1
"Antibiotic-associated (chemical) colitis" (option A) is not a distinct diagnostic entity. This term historically referred to benign antibiotic-associated diarrhea, but when symptoms are this severe with systemic inflammation, C. difficile must be ruled out first. 5, 6, 7
"Inflammatory colitis" (option B) is too vague and non-specific. While C. difficile does cause inflammatory colitis, this term alone doesn't represent a specific diagnosis. If this refers to inflammatory bowel disease (IBD), the acute presentation following antibiotic use makes CDI far more likely than new-onset IBD. 3
Diagnostic Approach
Immediate stool testing is mandatory:
- Stool test for C. difficile toxin or toxigenic C. difficile using either multistep algorithms with PCR or single-step PCR (sensitivity 0.68-1.00, specificity 0.92-1.00) 1
- Only test patients with diarrhea (≥3 unformed stools in 24 hours) - testing asymptomatic patients leads to false positives from colonization 1
Do not wait for test results to initiate empirical therapy in this case. Given the strong clinical suspicion for severe CDI (leukocytosis, elevated creatinine suggesting volume depletion, systemic symptoms), empirical treatment should be started while awaiting results. 1
Critical Pitfalls to Avoid
Do not order a "test of cure" after treatment. Up to 56% of successfully treated patients continue shedding C. difficile spores for up to six weeks asymptomatically. 1
Do not repeat testing if the first test is positive and symptoms persist during appropriate treatment - this does not change management. 1
Discontinue the offending antibiotic immediately if clinically feasible, as continued antibiotic use significantly increases recurrence risk. 1
Consider CT imaging if the patient deteriorates or shows signs of severe/complicated disease (though not immediately necessary in this stable patient). CT findings would include colonic wall thickening >4mm, accordion sign, or peri-colonic stranding. 1