Testing diarrhoea safely and effectively
What to test, when to test, and how to avoid harm
Why testing strategy matters
Most diarrhoea does not require extensive testing. Poorly targeted investigations increase anxiety, delay appropriate care, and expose patients to avoidable risk.
Stool testing
Multiplex stool PCR (preferred where available)
Multiplex PCR detects multiple pathogens rapidly from a single sample (bacteria, viruses, parasites). Advantages: rapid and sensitive; limitations: detects DNA and may reflect colonisation.
When stool testing is appropriate
- Severe illness
- Fever or blood
- Persistent diarrhoea (>7 days)
- High-risk patients
- Recent travel
- Suspected C. diff
C. difficile testing
- Test only if diarrhoea is present and recent antibiotics/healthcare exposure
- Do not test formed stool
Blood tests
- Full blood count, CRP, electrolytes/renal function, LFTs
Pancreatic enzymes
- Amylase and lipase are not routine; useful if pancreatitis suspected
Coeliac disease testing
- tTG antibodies; test before gluten restriction
Pancreatic elastase (faecal elastase)
Faecal elastase assesses exocrine pancreatic function.
- >200 µg/g normal
- 100–200 µg/g borderline
- <100 µg/g pancreatic exocrine insufficiency
Watery stool can dilute elastase and give falsely low results; repeat/clinical correlation may be required.
Hormonal and rare causes (selected cases only)
Not routine. May include gastrin, VIP, thyroid tests, and medication/hormonal effects (e.g. GLP‑1 receptor agonists).
Imaging
- Ultrasound: biliary disease
- CT abdomen: severe pain, complications, colitis, malignancy
- MRCP: biliary/pancreatic duct detail
Colonoscopy
When indicated
- Chronic diarrhoea, suspected IBD, weight loss/anaemia, persistent symptoms
Safety note: Colonoscopy carries perforation risk in active colitis. Avoid or defer in severe colitis/toxic megacolon; flexible sigmoidoscopy may be safer in selected cases.
Key takeaway: Good testing answers the right question at the right time. More tests do not mean better care.