Fellow's quick read · Infectious Diseases
Clostridioides difficile infection
IDSA/SHEA 2017 + 2021 focused update · with ACG 2021, AGA 2024 (FMT), Michigan & our institution CPGs 2025 · reviewed 2026-06-17
Personal study digest for a new ID fellow. Recommendations are drawn from the cited guidelines. The "What's changed since 2021" section is reviewer synthesis of newer evidence, each claim cited. Not a substitute for the full guideline.
In one line
CDI = ≥3 unformed stools/24 h + a positive toxigenic C. difficile stool test in a patient not on laxatives — never test the asymptomatic or the formed stool. The 2021 IDSA update made FIDAXOMICIN the preferred agent for both the initial episode and recurrence; metronidazole is now a fallback only.◆When to suspect / diagnose
- Test only symptomatic patients: ≥3 unformed stools in 24 h, with no laxatives in the prior 48 h. ~20% of hospitalized adults are asymptomatically COLONIZED — testing them generates false "infections."
- Diagnostic method: NAAT alone, OR a multistep algorithm (GDH + toxin EIA, arbitrated by NAAT). A positive toxin EIA supports active infection; a NAAT-positive / toxin-negative result may be colonization.
- No test of cure, and no repeat testing within 7 days of a negative (or 14 days of a positive) result.
- Risk factors: recent antibiotics (fluoroquinolones, clindamycin, broad β-lactams), PPI/H2RA, age ≥65, immunocompromise, IBD, prior CDI.
◆Severity — the definitions that pick the drug
- Nonsevere: WBC ≤15,000 cells/µL and creatinine <1.5 mg/dL.
- Severe: WBC ≥15,000 cells/µL OR creatinine ≥1.5 mg/dL (our institution also counts >50% rise from baseline).
- Fulminant: hypotension/shock, ileus, or toxic megacolon — this is the category that changes the whole regimen.
◆Therapy (IDSA/SHEA 2021)
| Scenario | Drug / dose / route / duration | Notes & strength |
|---|---|---|
| Initial episode (non-severe OR severe) | Fidaxomicin 200 mg PO BID × 10 d (preferred) Alt: vancomycin 125 mg PO QID × 10 d | Fidax conditional/moderate; lower recurrence, sustained-cure RR 1.16. Vanc remains acceptable. |
| Initial, if fidax/vanc unavailable (non-severe only) | Metronidazole 500 mg PO TID × 10–14 d | Demoted in 2021 — fallback only, not for severe disease. |
| First recurrence | Fidaxomicin 200 mg PO BID × 10 d, OR extended-pulsed (200 mg BID × 5 d, then once every other day × 20 d) Alt: vancomycin tapered/pulsed | ± bezlotoxumab if recurrence within 6 mo (see flag). Use vanc taper if metronidazole treated episode 1. |
| ≥2 recurrences (3+ episodes) | Fidax / vanc taper / vanc→rifaximin / fecal microbiota–based therapy | FMT/microbiome product after appropriate antibiotics for ≥2 recurrences fail. |
| Fulminant | Vancomycin 500 mg PO/NG QID + metronidazole 500 mg IV q8h. Add rectal vancomycin (500 mg in 100 mL NS as retention enema) if ileus. | No role for fidaxomicin. Surgery + ID consults; consider colectomy. |
◆Key decisions a fellow owns
- Stop the inciting antibiotic whenever possible — the single most important non-drug move.
- Diagnostic stewardship: refuse testing on patients with formed stool, recent laxatives, or no symptoms. Don't treat colonization.
- Risk-stratify for recurrence (age ≥65, prior severe CDI, recurrence within 6 mo, immunocompromise, ribotypes 027/078/244) — this drives fidaxomicin-first and adjunct decisions.
- Escalate early in fulminant disease — ID + general surgery, and GI for microbiome therapy in multiply-recurrent disease. At our institution, fidaxomicin and microbiome products require ID/GI approval.
◆Special populations
- IBD: high recurrence risk; microbiome-based therapy is effective (PUNCH CD3-OLS, 8-wk success 78.9%). AGA suggests against conventional FMT in the severely immunocompromised.
- Don't stop a needed PPI reflexively (ACG: continue if appropriately indicated).
- Probiotics: ACG recommends against for primary or secondary prevention.
What's changed since 2021
Reviewer synthesis of newer evidence — not the guideline. Each claim cited; flags where IDSA 2021 lags current practice.
- Two FDA-approved live biotherapeutic products (LBPs) now exist for PREVENTION of recurrence — they postdate the 2021 IDSA update. Rebyota (fecal microbiota, live-jslm; formerly RBX2660), rectal single dose, first LBP, approved 30 Nov 2022; and Vowst (fecal microbiota spores, live-brpk; formerly SER-109), oral capsules, first ORAL microbiome therapeutic, approved 26 Apr 2023. ECOSPOR III: SER-109 8-wk recurrence 12% vs 40% placebo, RR 0.32 — Feuerstadt, N Engl J Med 2022;386:220-229, doi:10.1056/NEJMoa2106516. Rebyota/PUNCH CD3 — doi:10.1097/MCG.0000000000001947 (per PubMed)
- AGA 2024 FMT guideline endorses fecal microbiota–based therapies (conventional FMT + the two LBPs) after standard-of-care antibiotics for recurrent CDI in immunocompetent adults; suggests against in the severely immunocompromised and against FMT for IBD/IBS. AGA Clinical Practice Guideline, Gastroenterology 2024
- Bezlotoxumab was DISCONTINUED by Merck, effective 31 Jan 2025 — no longer marketed in the US, despite still appearing in IDSA 2021 and our CPG. Recency-sensitive — confirm before relying on it. FDA/ASHP shortage tracker; Infectious Disease Advisor, 2025
- Metronidazole demotion holds — the 2017→2021 shift (fidaxomicin preferred, metronidazole fallback only for nonsevere when fidax/vanc unavailable) remains current; real-world fidaxomicin uptake has lagged the guideline. per IDSA/SHEA 2021; ContagionLive uptake data
- Still no newer IDSA society version — the 2021 focused update remains the operative IDSA document as of June 2026.
◆Anki cards minted this run
- FDA-approved live biotherapeutics for recurrence prevention (Rebyota live-jslm; Vowst spores live-brpk).
- Fulminant CDI regimen (oral vanc 500 QID + IV metronidazole; no fidaxomicin).
- Severity definitions (severe vs fulminant).
- Diagnostic discriminator: toxin EIA positive vs NAAT-only colonization.
- Fidaxomicin-preferred, metronidazole-demoted, and bezlotoxumab cards — already in deck, skipped as duplicates.
Sources: IDSA/SHEA 2017 (Clin Infect Dis 2018;66:e1, doi:10.1093/cid/cix1085); IDSA/SHEA 2021 Focused Update (Clin Infect Dis 2021;73:e1029, doi:10.1093/cid/ciab549); ACG 2021 (Am J Gastroenterol 2021;116:1124); AGA FMT 2024 (Gastroenterology 2024); ECOSPOR III/SER-109 (N Engl J Med 2022;386:220-229, doi:10.1056/NEJMoa2106516); Rebyota/PUNCH CD3 (doi:10.1097/MCG.0000000000001947); bezlotoxumab discontinuation (ASHP/FDA shortage tracker, 2025); Michigan & our institution CDI CPGs 2025. Source attribution for trial data: PubMed.