Watery diarrhea + nausea/vomiting + sick contacts + cruise/daycare/nursing home exposure + low-grade fever

→ Viral gastroenteritis (Norovirus, Rotavirus)

Confirmatory test:

  • Usually clinical diagnosis

  • GI pathogen PCR if hospitalized/severe/immunocompromised

Watery diarrhea + recent antibiotics (<8 weeks) + hospitalization/SNF/PPI use + fever + abdominal cramping

→ C. difficile colitis

Confirmatory test:

  • Stool C. difficile toxin/PCR (preferred)

  • Consider CT A/P if fulminant disease suspected

Bloody diarrhea + fever + abdominal pain + recent travel/undercooked food/reptile exposure

→ Invasive bacterial gastroenteritis
(Shigella, Salmonella, Campylobacter, Yersinia)

Confirmatory test:

  • GI pathogen PCR panel (preferred)

  • Stool culture

Bloody diarrhea + severe abdominal pain (often out of proportion to exam) + elevated lactate + AFib/PVD/elderly patient

→ Mesenteric ischemia / Ischemic colitis

Confirmatory test:

  • CTA Abdomen/Pelvis (gold standard imaging)

  • Colonoscopy may confirm ischemic colitis if stable

Bloody diarrhea + chronic symptoms (>4 weeks) + weight loss + anemia + young patient

→ IBD (UC/Crohn Disease)

Confirmatory test:

  • Colonoscopy with biopsy (gold standard)

  • Fecal calprotectin supports diagnosis

Chronic watery diarrhea + normal CBC/CMP/CRP + symptoms worsen with stress + improves after BM + no nocturnal symptoms

→ IBS-D

Confirmatory test:

  • No definitive test

  • Diagnosis of exclusion after normal:

    • CBC

    • CRP

    • Fecal calprotectin

    • Colonoscopy (if indicated)

Chronic watery diarrhea + elderly female + PPI/NSAID/SSRI use + normal colonoscopy

→ Microscopic colitis

Confirmatory test:

  • Colon biopsy (required)

  • Colonoscopy often appears normal

Greasy, floating, foul-smelling stools + weight loss + chronic pancreatitis history

→ Pancreatic exocrine insufficiency

Confirmatory test:

  • Fecal elastase <200 mcg/g stool

  • CT pancreas supports etiology

Chronic diarrhea + iron deficiency anemia + weight loss + bloating + dermatitis herpetiformis

→ Celiac disease

Confirmatory test:

  • tTG-IgA + total IgA

  • Duodenal biopsy (gold standard)

Diarrhea + tachycardia + weight loss + tremor + heat intolerance + anxiety

→ Hyperthyroidism

Confirmatory test:

  • TSH ↓

  • Free T4 ↑

Diarrhea begins after starting medication

→ Medication-induced diarrhea

Confirmatory test:

  • Medication withdrawal/improvement after discontinuation

  • No specific diagnostic test

Chronic diarrhea + iron deficiency anemia + weight loss + age >50 + occult blood

→ Colon cancer

Confirmatory test:

  • Colonoscopy with biopsy (gold standard)

Bloody diarrhea + minimal fever + severe abdominal cramps + undercooked beef exposure

→ EHEC (Shiga toxin E. coli)

Confirmatory test:

  • Stool Shiga toxin assay

  • GI pathogen PCR

Diarrhea + flushing + wheezing + episodic symptoms

→ Carcinoid syndrome

Confirmatory test:

  • 24-hour urine 5-HIAA

  • Chromogranin A

Diarrhea + HIV/immunosuppression/transplant

→ Opportunistic infection
(CMV, Cryptosporidium, Microsporidia)

Confirmatory test:

  • Stool O&P

  • Stool Cryptosporidium antigen

  • Colonoscopy with biopsy (CMV)