Diagnosis and Tests for Diarrhea

Most diarrhea needs no tests at all - the diagnosis is the story and the hydration check. Tests enter when specific red flags appear.

What the doctor assesses without tests

• The story: onset, stool frequency and character (watery, bloody, fatty), travel, food history, antibiotics recently, others sick at home
• The dehydration exam: eyes, tongue moisture, skin pinch, pulse, blood pressure, urine output - this, not a lab, decides the urgency
• In children: weight, activity, tears, fontanelle - the pediatric dehydration grade

When tests are actually needed

• Blood or mucus in stool, or high fever: stool examination and stool culture - looking for Shigella, Salmonella, amoeba and friends
• Diarrhea over 2 weeks: stool tests for parasites (Giardia), then blood tests - celiac serology, thyroid, inflammation markers
• Severe dehydration or hospital care: electrolytes and kidney function - to guide IV fluids
• Recent antibiotics with severe diarrhea: testing for C. difficile
• Suspected outbreaks (several people, one food source): public-health testing matters beyond the individual

Tests NOT routinely needed

• Most acute watery diarrhea: no stool test, no scan, no scope - it resolves before results return
• Colonoscopy is for chronic or alarming patterns (blood, weight loss, age, family history), not for a two-day stomach bug

What you can track at home

• Stool count and character, urine frequency and colour - the two most useful numbers for the doctor
• Fluid intake versus vomiting - can the patient keep ORS down?

One-line summary

Diagnosing diarrhea is mostly the story plus the dehydration exam; stool tests and blood work are for blood in stool, high fever, the 2-week mark, severe dehydration or recent antibiotics - and most acute cases need no tests, just fluids and watching.