What initial laboratory and imaging studies, and what additional tests later if needed, should I order for a vegetarian patient with chronic, food‑triggered diarrhea (e.g., chocolate intolerance) who is not taking any medications?

Initial Laboratory Testing for Chronic Food-Triggered Diarrhea

Order a focused panel of blood tests and stool studies now, reserving endoscopy and advanced testing for cases with alarm features or abnormal initial results.

Immediate Testing to Order Now

Blood Tests (First-Line Panel)

The British Society of Gastroenterology strongly recommends a comprehensive initial blood workup for all patients with chronic diarrhea 1:

• Complete blood count (CBC) – screens for anemia suggesting celiac disease, inflammatory bowel disease, or malignancy 1, 2
• C-reactive protein (CRP) – elevated levels indicate inflammatory processes 1, 2
• Comprehensive metabolic panel – assesses electrolytes, kidney function, and albumin (low albumin has high specificity for organic disease) 1
• Liver function tests – screens for hepatobiliary causes 1
• Iron studies, vitamin B12, and folate – identifies malabsorption and nutritional deficiencies common in celiac disease and inflammatory bowel disease 1, 2
• Thyroid function tests (TSH) – hyperthyroidism is a common endocrine cause of chronic diarrhea 1
• Anti-tissue transglutaminase IgA (tTG-IgA) with total IgA level – mandatory screening for celiac disease, which has prevalence of 1:200-1:559 in Western populations and frequently presents with diarrhea 1, 2

Critical caveat: Always order total IgA alongside tTG-IgA, as selective IgA deficiency occurs in 2.6% of celiac patients (versus 0.2% general population) and causes false-negative IgA antibody tests 1.

Stool Studies (First-Line)

The British Society of Gastroenterology recommends targeted stool testing based on clinical suspicion 1:

• Fecal calprotectin – strongly recommended to exclude colonic inflammation in patients under age 40; levels >50 mg/g indicate inflammatory bowel disease with >90% sensitivity 1, 2
• Stool culture and ova/parasites – only if infectious etiology suspected (recent travel, antibiotic use, or immunocompromise) 1, 2
• Giardia antigen – high diagnostic accuracy for this common chronic diarrhea cause 3
• Fecal immunochemical test (FIT) – screens for occult blood loss to guide urgency of colonoscopy 1

Testing to Reserve for Later (If Initial Workup Abnormal or Symptoms Persist)

Endoscopic Evaluation

Age-stratified approach is critical 1:

• Patients ≥45 years: Full colonoscopy with biopsies is mandatory due to colorectal cancer risk, regardless of initial test results 1, 4
• Patients <40 years without alarm features and normal fecal calprotectin: Avoid immediate colonoscopy and consider positive IBS diagnosis after basic screening 1, 4
• Any age with alarm features: Urgent colonoscopy within 2-4 weeks 4, 3

Alarm features requiring urgent endoscopy 1, 4:

• Nocturnal diarrhea (strongly suggests organic disease)
• Unintentional weight loss
• Blood in stool or iron deficiency anemia
• Fever
• Recent onset (<3 months duration)
• Family history of colorectal cancer or inflammatory bowel disease

Biopsy protocol: If colonoscopy is performed, obtain biopsies from right and left colon (not rectum) even if mucosa appears normal, as microscopic colitis has entirely normal endoscopic appearance but characteristic histologic changes 1, 4.

Advanced Testing (Third-Line, If Colonoscopy Negative)

• Bile acid diarrhea testing: SeHCAT scan or serum 7α-hydroxy-4-cholesten-3-one (not empiric trial) – common treatable cause missed by Rome criteria 1, 4
• Hydrogen breath testing for lactose maldigestion – or empiric lactose withdrawal trial 1
• Fecal elastase – if fat malabsorption/pancreatic insufficiency suspected (bulky, pale, malodorous stools) 1
• Upper endoscopy with duodenal biopsies – only if celiac serology positive or small bowel enteropathy suspected 1
• MR enterography or video capsule endoscopy – for small bowel abnormalities if other testing unrevealing 1

Clinical Context for This Patient

This vegetarian patient with food-triggered diarrhea (chocolate intolerance, tolerates fruit and cheese) has no documented alarm features based on the note. The systematic food diary approach is appropriate, but laboratory screening is essential before assuming functional disease 1.

Key considerations:

• Vegetarian diet increases risk of B12 deficiency, making nutritional screening particularly important 1
• Food triggers suggest possible carbohydrate malabsorption (lactose, fructose) or IBS, but organic disease must be excluded first 1, 2
• Rome criteria alone have only 52-74% specificity and miss 26-48% of organic diseases including microscopic colitis, inflammatory bowel disease, and bile acid diarrhea – all treatable conditions 1, 4

Do not diagnose IBS until basic blood and stool screening is complete 1, 4. The British Society of Gastroenterology emphasizes that functional diagnosis requires negative workup, not just symptom pattern 1.