What stool tests are recommended for diagnosing chronic diarrhea?

Stool Tests for Chronic Diarrhea

For chronic diarrhea evaluation, start with faecal calprotectin to exclude inflammatory bowel disease, stool culture/microscopy for infections (especially C. difficile in appropriate settings), and consider laxative screening if other tests are negative. 1

Initial Stool Testing in Primary Care

The British Society of Gastroenterology recommends specific stool tests as part of first-line evaluation:

• Faecal calprotectin is the single most important initial stool test, with levels <50 μg/g faeces making inflammatory bowel disease unlikely, while levels >250 μg/g faeces correlate with active endoscopic inflammation 1
• Stool culture and microscopy should be performed even though infectious causes are uncommon in immunocompetent patients with chronic symptoms in developed countries 1
• For C. difficile, use two-stage testing: first confirm organism presence with glutamate dehydrogenase enzyme immunoassay or PCR, then demonstrate toxin production with toxin EIA 1

Categorizing Diarrhea Type Through Stool Analysis

Once basic screening is complete, additional stool tests help categorize the mechanism:

For Inflammatory Diarrhea

• Faecal immunochemical testing (FIT) can guide investigation priority in patients without rectal bleeding 1
• Elevated calprotectin warrants colonoscopy with biopsies from right and left colon (not rectum) to exclude microscopic colitis 1

For Malabsorptive/Fatty Diarrhea

• Faecal elastase is recommended when fat malabsorption or pancreatic insufficiency is suspected 1
• Stool inspection may reveal bulky, malodorous, pale stools suggesting steatorrhea, though milder malabsorption may not show obvious abnormalities 1

For Factitious Diarrhea

This becomes increasingly common in specialist practice (20% at tertiary centers vs 4% in district clinics) 1:

• Faecal osmolality <290 mosmol/kg indicates dilutional diarrhea from added water or hypotonic solution 1
• Faecal magnesium >45 mmol/l strongly suggests magnesium-induced diarrhea 1
• Laxative screening should include spectrophotometric or chromatographic detection of anthraquinones, bisacodyl, and phenolphthalein in urine, plus magnesium and phosphate in stool, performed by specialist laboratories 1, 2
• Repeated testing is essential as patients may ingest laxatives intermittently 1

Common Pitfalls to Avoid

Do not rely on alkalinization assays (where phenolphthalein turns stool red, bisacodyl purple-blue) as they lack sufficient sensitivity and should be abandoned 1

Do not measure stool weights as they are no longer recommended due to limited clinical value 1, 3

Use the Bristol Stool Chart (Types 5-7 define diarrhea) to standardize stool consistency assessment rather than subjective descriptions 1, 3

When Infection Testing is Critical

• Immunocompromised or elderly patients require testing for common infections including Giardia, which can now be diagnosed with stool antigen testing (92% sensitivity, 98% specificity) rather than wet preparations 1
• HIV testing should be performed in immunocompromised patients presenting with chronic diarrhea 1

Testing Sequence Based on Clinical Suspicion

The strength of evidence supports this algorithmic approach:

1. All patients: Faecal calprotectin + stool culture/microscopy 1
2. If calprotectin elevated: Proceed to colonoscopy with biopsies 1
3. If malabsorption suspected: Faecal elastase 1
4. If multiple negative investigations: Screen for laxative abuse 1
5. If functional symptoms predominate: Consider bile acid diarrhea testing (SeHCAT or serum 7α-hydroxy-4-cholesten-3-one) 1

The 2018 British Society of Gastroenterology guidelines represent the most current evidence and supersede the 2003 recommendations, particularly regarding the central role of faecal calprotectin and the two-stage C. difficile testing approach 1.