Laboratory Workup for Dark Green Stool
Dark green stool is typically benign and caused by dietary factors (iron supplements, green vegetables, food dyes) or rapid intestinal transit, but initial laboratory testing should focus on excluding inflammatory bowel disease, infection, and occult gastrointestinal pathology when clinically indicated.
Initial Risk Stratification
Before ordering extensive laboratory testing, assess for alarm features that would warrant comprehensive evaluation versus minimal testing:
- Alarm features requiring full workup: fever, significant weight loss, rectal bleeding, anemia, abnormal physical findings, nocturnal symptoms, acute onset in previously well patients, age >50 years at symptom onset, family history of IBD or colorectal cancer 1
- Absence of alarm features: Consider limited screening approach focused on excluding common pathologies
Recommended Laboratory Panel
Core Screening Tests (All Patients)
- Complete blood count (CBC) to screen for anemia and exclude inflammatory processes 1, 2
- C-reactive protein (CRP) or erythrocyte sedimentation rate (ESR) to assess for occult inflammation 1, 2
- Fecal calprotectin (if patient <45 years with diarrhea) to exclude inflammatory bowel disease 1
- Stool occult blood testing for screening purposes 1
Important caveat: Fecal occult blood tests can be falsely positive due to food additives and dietary factors 3, which is particularly relevant in patients with green stool who may be consuming foods with artificial coloring or high iron content.
Infection Exclusion
- Stool culture for bacterial pathogens (Salmonella, Shigella, Campylobacter) if diarrhea present, especially with fever or bloody stools 4
- Clostridium difficile testing (glutamate dehydrogenase antigen and toxin A/B enzyme immunoassays, or nucleic acid amplification) - particularly important in patients with recent antibiotic use, hospitalization, or colonic symptoms 4, 5
- Stool examination for ova, cysts, and parasites based on travel history or endemic area exposure 4, 1
Additional Testing Based on Clinical Context
- Fecal lactoferrin or fecal leukocytes if inflammatory diarrhea suspected - these neutrophil-derived markers indicate intestinal inflammation 4, 6, 7
- Basic metabolic panel to assess electrolyte disturbances if significant diarrhea present 2
- Celiac serology (anti-tissue transglutaminase IgA with total IgA) if chronic symptoms present 1, 2
- Serum albumin if malabsorption or chronic disease suspected 4
Interpretation Considerations
Fecal inflammatory markers (leukocytes, lactoferrin, occult blood) share certain features with bacterial infections like shigellosis, salmonellosis, and campylobacteriosis - including fever, abdominal pain, and bloody stools 4. In developed countries, fecal leukocytes have positive and negative likelihood ratios of 4.56 and 0.32 respectively when >5 cells/high-power field are present 7.
CRP limitations: Approximately 20% of patients with active Crohn's disease may have normal CRP levels 4, so normal inflammatory markers do not completely exclude IBD.
Stool testing approach: If the differential diagnosis is broad, stool studies should categorize diarrhea as watery, fatty, or inflammatory to guide further evaluation 2.
When Minimal Testing is Appropriate
If the patient has isolated green stool without alarm features, recent dietary history of iron supplementation or green vegetables, no diarrhea, and no systemic symptoms, extensive laboratory testing may not be necessary. However, at minimum, CBC and CRP should be obtained to exclude occult inflammatory processes 1.