Is stool pH and reducing substances a reliable low-cost test for screening malabsorption?

Stool pH and Reducing Substances as Screening Tests for Malabsorption

Stool pH and reducing substances are poor screening tests for malabsorption with low sensitivity (9-28%) and should not be used as primary diagnostic tools. 1

Limitations of Stool pH and Reducing Substances Testing

• Stool pH and reducing substances tests have very low sensitivity (9-28%) and only moderate specificity (74-81%) for detecting intestinal disaccharidase deficiencies, which are common causes of carbohydrate malabsorption 1
• These tests perform poorly even in infants, where the highest percentage of abnormal results occur across all malabsorption tests 1
• The positive predictive values for stool reducing sugars and stool pH are only 54% and 50% respectively, making them unreliable for clinical decision-making 1

Preferred Diagnostic Approaches for Malabsorption

Initial Assessment

• Confirmation of true diarrhoea through stool inspection and measurement of stool weight is recommended before pursuing extensive malabsorption workup 2
• Initial laboratory investigations should include full blood count, erythrocyte sedimentation rate, electrolytes, liver function tests, iron studies, vitamin B12, folate, and thyroid function 2

Recommended Testing for Malabsorption

• Antiendomysium antibody testing is the preferred first-line test for coeliac disease, the most common small bowel enteropathy in European populations 2
• If small bowel malabsorption is suspected and serological tests are negative, upper gastrointestinal endoscopy with distal duodenal biopsies should be performed 2
• For suspected pancreatic insufficiency, newer specific tests such as stool elastase are preferred over older methods 2

Fat Malabsorption Assessment

• Quantification of three-day faecal fat, while historically used as a standard test, is poorly reproducible, unpleasant, and non-diagnostic; its use is discouraged 2
• Breath tests for fat malabsorption (using 14C-triolein or 13C-labeled mixed triglyceride) have low sensitivity for mild/moderate fat malabsorption but may serve as alternatives to faecal fat collection where available 2

Carbohydrate Malabsorption Assessment

• Carbohydrate malabsorption is predominantly associated with mucosal disease or dysfunction 2
• For suspected carbohydrate malabsorption, direct measurement of intestinal disaccharidase activity through tissue biopsy provides more reliable results than stool tests 1
• Hydrogen breath tests, while not perfect, offer better diagnostic value for specific carbohydrate intolerances than stool pH or reducing substances 2

Pitfalls and Caveats

• Many clinicians still rely on stool pH and reducing substances due to their simplicity and low cost, despite their poor diagnostic performance 1
• The intestine has a large reserve capacity for digestion and absorption of nutrients, which means mild malabsorption may not be detected by simple screening tests 3
• No single test can diagnose all forms of malabsorption; the diagnostic approach should be guided by clinical suspicion of specific deficiencies 4
• Early recognition of malabsorption is key for proper diagnostic workup, but relying on poor screening tests may delay diagnosis 4

Alternative Low-Cost Approaches

• For suspected malabsorption, a therapeutic trial of specific interventions (e.g., gluten-free diet for suspected celiac disease) may be more informative than non-specific stool tests 5
• When resources are limited, focusing on specific symptoms and targeted testing based on the most likely diagnosis is more efficient than using poor screening tests 4