What labs should be checked in a patient with chronic diarrhea?

Laboratory Evaluation for Chronic Diarrhea

For patients with chronic diarrhea, comprehensive laboratory testing should include stool studies for Giardia, fecal calprotectin or lactoferrin, and serological testing for celiac disease, along with basic blood work to evaluate for inflammatory, metabolic, and nutritional abnormalities. 1

Initial Laboratory Workup

Blood Tests

• Complete blood count (CBC), C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), basic metabolic panel, liver function tests, calcium, vitamin B12, folate, iron studies, and thyroid function tests 1
• While ESR and CRP have high specificity for organic disease, they have lower sensitivity for inflammatory bowel disease (IBD) compared to fecal markers 1
• Total IgA levels should be checked when testing for celiac disease to identify potential IgA deficiency that could lead to false-negative results 1

Stool Studies

• Fecal calprotectin or fecal lactoferrin to screen for inflammatory bowel disease (IBD) - using a threshold value of 50 mg/g for calprotectin and 4.0-7.25 mg/g for lactoferrin to optimize sensitivity 1
• Giardia testing using antigen detection or polymerase chain reaction (PCR) - strongly recommended due to high prevalence and excellent test performance 1
• Stool culture for bacterial pathogens (Salmonella, Shigella, Campylobacter, Yersinia) 2
• C. difficile toxin testing, especially with recent antibiotic use 2
• Testing for ova and parasites is generally not recommended in patients without travel history to or recent immigration from high-risk areas 1

Celiac Disease Testing

• IgA tissue transglutaminase (tTG) antibody test with a second test to detect celiac disease in the setting of IgA deficiency 1
• For IgA-deficient patients, IgG-tTG or tests for IgG deaminated gliadin peptides should be used 1

Additional Testing Based on Stool Characteristics

For Watery Diarrhea

• Stool electrolytes (Na+, K+) to calculate fecal osmotic gap and differentiate between osmotic and secretory diarrhea 2
• Consider testing for bile acid malabsorption if suspected 2
• Screen for laxative abuse with spectrophotometric or chromatographic analysis of stool for magnesium, phosphate, and urine for laxative compounds 1

For Fatty Diarrhea (Malabsorption)

• Fecal elastase to assess for pancreatic insufficiency 2
• Qualitative fecal fat if malabsorption is suspected 2

For Bloody or Inflammatory Diarrhea

• Stool for occult blood 2
• Testing for Shiga toxin-producing E. coli 2

Special Considerations

• For travelers or those with risk factors, more extensive parasite testing may be warranted 2
• In immunocompromised patients, additional testing for opportunistic pathogens should be considered 2
• Patients over 45 years with chronic diarrhea typically require endoscopic evaluation 2

Common Pitfalls to Avoid

• Relying solely on ESR or CRP to screen for IBD when fecal calprotectin or lactoferrin are more sensitive markers 1
• Failing to check total IgA levels when testing for celiac disease, which could lead to false-negative results in IgA-deficient patients 1
• Overlooking factitious diarrhea, which becomes increasingly common in specialist referral practice 1
• Not considering microscopic colitis in older patients with chronic watery diarrhea 3, 4
• Forgetting to test for C. difficile in patients with recent antibiotic use, even if they haven't been hospitalized 2

By systematically approaching laboratory evaluation based on diarrhea characteristics and clinical presentation, clinicians can efficiently diagnose the underlying cause and initiate appropriate treatment to improve patient outcomes.