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Approach to acute diarrhea

The same eight steps, every presentation: threats first, then the history and exam that discriminate, one honest sentence, a mechanism-grouped differential, and the tests read in order.

  1. Immediate threats

    • Shock and profound dehydration, most dangerous at the extremes of age
    • Bloody diarrhea with fever: invasive colitis; with abdominal distension, toxic megacolon
    • Recent antibiotics or hospitalization: C. difficile until tested, isolate first
    • Hemolytic uremic syndrome behind bloody diarrhea in the young: check a film, platelets, and creatinine before writing 'gastroenteritis'

  2. History that discriminates

    • Duration (under two weeks is acute), frequency, and what is actually in the pan: water, blood, fat
    • Food, travel, contacts with the same illness, antibiotics within three months, immunosuppression
    • Medications with diarrhea in their pocket: metformin, PPIs, laxatives quietly continued, chemotherapy
    • Nocturnal diarrhea and weight loss push away from functional and toward organic

  3. Examination

    • Volume status honestly: postural pressures, mucous membranes, urine output
    • Abdomen for distension, tenderness, and the silent tympany of megacolon
    • Temperature; skin turgor at the extremes of age

  4. Problem representation

    Profuse watery diarrhea and vomiting twelve hours after a buffet, afebrile with a soft abdomen (points at toxin-mediated food poisoning) versus a week of bloody stools, fever, and left-sided pain in a patient finishing clindamycin (points at C. difficile or invasive colitis).

  5. Differential, by mechanism

    Infective

    • Viral gastroenteritis
    • Toxin-mediated food poisoning
    • Invasive bacteria (Campylobacter, Salmonella, Shigella, E. coli)
    • C. difficile
    • Parasites after travel

    Inflammatory

    • First presentation of IBD
    • Ischemic colitis in the vasculopath

    Medication / osmotic

    • Antibiotic-associated
    • Laxatives and magnesium
    • Metformin, PPIs, colchicine

    Endocrine / other

    • Thyrotoxicosis
    • Overflow around impaction in the elderly

  6. Investigations

    • Most acute watery diarrhea needs no tests; flags (blood, fever, sepsis, recent antibiotics, immunosuppression, over a week) change that
    • Stool culture and C. difficile toxin where flagged; electrolytes, creatinine, CBC
    • The normal-anion-gap metabolic acidosis of heavy diarrhea is a teaching point the gas will show
    • Imaging only for the sick abdomen: distension, peritonism, megacolon weight

  7. Interpretation

    • Dehydration killed diarrheal patients long before any pathogen was named: assessment of volume IS the assessment
    • Bloody plus febrile changes the class of illness; antimotility agents are unwise there
    • Diarrhea in an elderly patient on opioids may be overflow: examine, do not just prescribe loperamide

  8. Next steps

    • Oral rehydration where tolerated, IV where not; potassium replaced with the losses
    • Antibiotics are the exception, not the rule: reserved for severe invasive disease, sepsis, or C. difficile (which gets oral vancomycin and STOPPING the culprit)
    • Isolate suspected infective diarrhea; public health notification where the jurisdiction requires it

Connected

AlgorithmThe acidosis diarrhea writesApproachWhen fever leadsTopicWhere the potassium went

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