Acute diarrhea ± blood/fever
First: Rehydrate (oral/IV)
Bloody / severe / febrile → stool culture + WBC + O&P
Bacterial (Campylobacter most common) → Azithromycin 500 mg OD ×3 d preferred; ciprofloxacin only if susceptible (rising FQ resistance — IDSA 2017)
C. difficile → fidaxomicin 200 mg PO BID ×10 d preferred; vancomycin 125 mg PO QID ×10 d alternative (IDSA/SHEA 2021); metronidazole only if neither available and non-severe
Order set
- Rehydrate
- Stool culture/PCR + C. diff if indicated
- U&E
- Avoid empirical antibiotics in most
Criteria
AdmitSevere dehydration, sepsis, or HUS features
ICUSeptic shock / HUS with complications
DialysisHUS-AKI with AEIOU
DischargeRehydrated, tolerating oral, cause addressed
Never
- Give antibiotics for EHEC O157:H7 (↑ HUS risk)
Clinical detail — differentials, red flags, pitfalls, disposition
Red flags
- Bloody diarrhoea + AKI (HUS)
- Sepsis
- Severe dehydration
Differentials
- Viral gastroenteritis
- C. diff
- IBD flare
- Ischaemic colitis
Common mistakes
- Antibiotics in EHEC
- Antimotility agents in invasive/toxic colitis
Disposition & follow-up
Mostly supportive; targeted antibiotics selectively.
Discharge package
MedicationsRehydration; targeted antibiotics only if indicated
Follow-upPublic health if notifiable
LifestyleHand hygiene; food-handler/exclusion advice
Warning symptomsBloody stools, reduced urine, drowsiness, high fever
💊 Treatment detail — doses & preparation
Oral rehydration solutionrehydration
DoseFrequent small volumes; 200–250 mL after each loose stool
PreparationWHO sachet in 1 L clean water (Na⁺ 75, glucose 75 mmol/L)
MonitorUrine output, dizziness; IV LR if >10% dehydrated/shock
Loperamideantimotility
Dose4 mg then 2 mg after each loose stool (max 16 mg/day) — NON-bloody diarrhoea only
Preparation2 mg capsules
MonitorAVOID in dysentery/C. diff (toxic megacolon)
Azithromycinmacrolide
DoseCAP: 500 mg OD ×3–5 d; chlamydia 1 g once; dysentery 500 mg OD ×3 d
PreparationPO tablets/suspension; IV 500 mg in 250 mL over 1 h if NBM
MonitorQT prolongation; resistance rising in gonorrhoea (not monotherapy)
Vancomycin POC. difficile therapy
Dose125 mg PO QID ×10 d (alternative — fidaxomicin preferred, IDSA/SHEA 2021)
PreparationCapsules or IV solution reconstituted for oral use — NOT absorbed systemically
MonitorStool frequency; fidaxomicin 200 mg BD ×10 d PREFERRED first-line and for recurrence (IDSA/SHEA 2021)
Metronidazolenitroimidazole
Dose400 mg PO TDS ×10 d (mild C. diff alternative); 500 mg IV q8h (intra-abdominal)
PreparationPO tablets; IV ready bag 500 mg/100 mL
MonitorMetallic taste; avoid alcohol (disulfiram), neuropathy if prolonged
📖 IDSA Infectious DiarrhoeaReviewed July 2026