What laboratory tests are recommended for diagnosing malabsorption?

Laboratory Tests for Malabsorption

A basic screen for evidence of malabsorption should include full blood count, urea and electrolytes, liver function tests, vitamin B12, folate, calcium, ferritin, erythrocyte sedimentation rate, and C-reactive protein, with mandatory serological testing for coeliac disease in all patients presenting with suspected malabsorption. 1

Initial Blood Tests

The British Society of Gastroenterology provides clear guidance on the essential initial laboratory workup 1:

• Complete blood count (CBC): Identifies anaemia, which has high specificity for organic disease when present alongside other abnormalities 1
• Iron studies (ferritin, serum iron): Iron deficiency is a sensitive indicator of small bowel enteropathy, particularly coeliac disease 1
• Vitamin B12 and folate: Essential markers of malabsorption, particularly in small bowel disease 1
• Calcium: Screens for malabsorption of fat-soluble nutrients 1
• Liver function tests: Detects hepatobiliary involvement and albumin levels 1
• Urea and electrolytes: Assesses for dehydration and electrolyte disturbances 1
• Inflammatory markers (ESR, CRP): Abnormal results combined with anaemia or low albumin have high specificity for organic disease 1
• Thyroid function tests (TSH): Hyperthyroidism can cause diarrhoea and must be excluded 1

Mandatory Coeliac Disease Screening

Serological testing for coeliac disease is mandatory in all patients with suspected malabsorption, given that coeliac disease affects 0.5-1% of the population and is present in 3-10% of patients with chronic diarrhoea referred to secondary care 1, 2:

• Anti-endomysial antibody (EMA): Preferred first-line test 2
• Anti-tissue transglutaminase IgA (anti-TG2 IgA): Alternative first-line option 1
• If serological tests are negative but malabsorption is still suspected, upper gastrointestinal endoscopy with distal duodenal biopsies should be performed 2, 3

Additional Micronutrient Assessment

Beyond the basic screen, additional testing should be considered based on clinical context 1:

• Vitamin D (25-hydroxyvitamin D): Deficiency occurs in 16-95% of patients with malabsorption, particularly in inflammatory bowel disease 1
• Fat-soluble vitamins (A, E, K): Particularly important in patients with fat malabsorption 1
• Magnesium and phosphorus: Essential for metabolic bone disease assessment 1
• Vitamin B6, vitamin C, zinc, selenium: Consider in patients with small bowel disease, previous resection, or those receiving nutritional supplementation 1

Specialized Testing for Specific Scenarios

Pancreatic Insufficiency

• Stool elastase: Preferred over older methods for suspected pancreatic insufficiency 2

Fat Malabsorption

• Avoid quantitative 3-day faecal fat collection: Poor reproducibility, unpleasant, and non-diagnostic 2
• Breath tests using 14C-triolein or 13C-labeled mixed triglyceride have low sensitivity for mild/moderate fat malabsorption but may serve as alternatives where available 2

Carbohydrate Malabsorption

• Hydrogen breath tests: Offer better diagnostic value for specific carbohydrate intolerances than stool pH or reducing substances 2

Monitoring Frequency in Established Malabsorption

For patients with known small bowel disease or previous resection 1:

• Vitamin B12 and folic acid: Every 3-6 months 1
• Anaemia screening (CBC, ferritin, CRP): Every 3 months in symptomatic patients 1
• Bone density assessment: Every 2-3 years in short bowel syndrome patients 1

Critical Interpretation Pitfalls

• Ferritin in active inflammation: Values up to 100 μg/L may still indicate iron deficiency in active disease, especially with transferrin saturation <20% 1
• Albumin is NOT an appropriate test for malabsorption: It is an acute phase protein that does not correlate with nutritional status in otherwise healthy individuals 1
• Coeliac serology has poor sensitivity (11.8-30%) for detecting persistent mucosal damage in patients already on a gluten-free diet 1