Treatment of Initial C. difficile Infection
For this patient with an initial episode of CDI, oral fidaxomicin 200 mg twice daily for 10 days (Option B) is the preferred treatment, though oral vancomycin 125 mg four times daily for 10 days (Option A) remains an acceptable alternative. 1
Rationale for First-Line Therapy
The 2021 IDSA/SHEA focused update guidelines explicitly prioritize fidaxomicin as the preferred agent for initial CDI episodes, with vancomycin listed as an acceptable alternative 1. This recommendation is echoed by the 2021 ESCMID guidelines 2 and the 2026 AGA Clinical Practice Update 3, which favor fidaxomicin due to its:
- Narrower antimicrobial spectrum - better preservation of gut microbiota
- Lower recurrence rates compared to vancomycin
- Superior outcomes in preventing the cycle of recurrent CDI
The standard fidaxomicin regimen is 200 mg orally twice daily for 10 days 1, 3.
Why Vancomycin Remains Acceptable
Vancomycin 125 mg four times daily for 10 days is still considered appropriate first-line therapy 1. The choice between fidaxomicin and vancomycin often depends on:
- Cost and availability - fidaxomicin is significantly more expensive
- Institutional formulary restrictions
- Insurance coverage limitations
Both agents are effective for treating the acute episode; the primary advantage of fidaxomicin is reduced recurrence risk 3, 2.
Why Other Options Are Inappropriate
Bezlotoxumab (Option C) is an adjunctive monoclonal antibody therapy, not a standalone treatment. It is given intravenously during administration of standard antibiotic therapy (vancomycin or fidaxomicin) and is reserved for patients at high risk of recurrence or those experiencing their first recurrence 1. It should not be used as monotherapy for initial CDI.
Fecal transplantation (Option D) is reserved for patients with multiple recurrences who have failed appropriate antibiotic treatments - specifically after at least 2 recurrences (meaning 3 total CDI episodes) 1. The FDA has issued multiple safety alerts regarding FMT, including transmission of pathogenic organisms and potential SARS-CoV-2 transmission 1. It is not indicated for initial episodes.
Critical Severity Assessment
While not specified in this case, clinicians should classify CDI severity at diagnosis 3:
- Severe CDI indicators: WBC >15,000 cells/μL or serum creatinine >1.5 mg/dL (or >1.5× baseline)
- Fulminant CDI: shock, ileus, or megacolon
For this patient with fever and diarrhea but no mention of these severe features, standard initial therapy is appropriate. Even in severe (non-fulminant) cases, fidaxomicin or vancomycin remain first-line 1, 3.
Common Pitfalls to Avoid
- Do not use metronidazole unless both fidaxomicin and vancomycin are unavailable AND the infection is nonsevere (WBC ≤15,000 cells/μL and creatinine <1.5 mg/dL) 1, 3
- Do not perform test of cure after treatment completion; only retest if symptoms persist or worsen 3
- Do not use cholestyramine concurrently with vancomycin or fidaxomicin as it binds these medications 3
The answer is B (fidaxomicin) as preferred, or A (vancomycin) as an acceptable alternative, depending on institutional resources and availability.