General MedicineTier 1Approach to a presentation

Undifferentiated presentations - diarrhoea

Red flags

Bloody diarrhoea + fever + abdominal pain - invasive bacterial colitis or IBD flare
Profound dehydration/haemodynamic compromise
Diarrhoea + recent antibiotic use - C. difficile
Diarrhoea + severe abdominal pain out of proportion - ischaemic colitis, toxic megacolon
Chronic diarrhoea + weight loss + nocturnal symptoms - organic cause (IBD, malignancy), not IBS

Acute (<2 weeks)

Infective - viral (norovirus, rotavirus) commonest overall; bacterial (Campylobacter, Salmonella, Shigella, E. coli including STEC); parasitic (Giardia - foreign travel/prolonged course)
C. difficile - antibiotic exposure in preceding weeks
Drug-induced (see pharm-tox ADR-diarrhoea note)
Food poisoning (toxin-mediated, rapid onset, short course)

Chronic (>4 weeks)

Functional: IBS-D (diagnosis of exclusion, no alarm features)
Inflammatory: Crohn's disease, ulcerative colitis, microscopic colitis
Malabsorptive: coeliac disease, pancreatic insufficiency, bile acid malabsorption, bacterial overgrowth
Endocrine: hyperthyroidism, diabetic autonomic neuropathy
Neoplastic: colorectal cancer, neuroendocrine tumour (carcinoid - flushing, wheeze)

Duration, blood/mucus, nocturnal symptoms, weight loss, travel, sick contacts, recent antibiotics
Food history, unpasteurised dairy/undercooked meat/seafood
Extraintestinal symptoms - joint pain, rash, eye symptoms (IBD)
Family history of IBD/coeliac/colorectal cancer

Hydration status, abdominal tenderness/distension/peritonism
Perianal exam (fistula/skin tags - Crohn's), extraintestinal signs (erythema nodosum, uveitis, arthritis)

Stool MCS + C. difficile toxin/PCR if antibiotic exposure; faecal calprotectin to distinguish IBD from IBS
FBE, UEC, CRP/ESR, TFT, coeliac serology
Colonoscopy with biopsy if chronic/alarm features/suspected IBD

Rehydration is the priority in acute diarrhoea - oral rehydration solution, IV fluids if severe
Most acute infective diarrhoea is self-limiting - antibiotics not routinely indicated, reserved for specific pathogens/severe or invasive disease/immunosuppression
Avoid anti-motility agents (loperamide) in bloody diarrhoea/suspected C. difficile/STEC - risk of toxic megacolon and, with STEC, HUS
C. difficile - oral vancomycin/fidaxomicin (see pharm-tox note)
IBD - induction/maintenance therapy per disease activity (gastroenterology-led)
IBS-D - dietary modification (low FODMAP), antispasmodics, loperamide (appropriate here, once organic disease excluded)

Loperamide is appropriate in IBS-D but dangerous in invasive/bloody bacterial diarrhoea - the same drug, opposite advice depending on cause
STEC (E. coli O157) with antibiotic treatment can precipitate haemolytic uraemic syndrome - avoid empirical antibiotics for suspected STEC
Chronic diarrhoea attributed to "IBS" without alarm-feature screening risks missing IBD or coeliac disease
C. difficile risk persists for weeks after antibiotic exposure - always ask beyond "current medications"

"Bloody diarrhoea changes my whole approach - no loperamide, and antibiotics only if clearly indicated by the pathogen."
"Chronic diarrhoea gets a calprotectin and coeliac screen before I call it IBS."

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