What further tests are indicated for a 25-year-old patient with chronic abdominal complaints, constipation, microcytic anemia, and weight loss, with normal liver function tests (LFT), urea, and electrolytes, and a normal abdominal X-ray?

Further Diagnostic Testing for a 25-Year-Old with Chronic Abdominal Complaints, Constipation, Microcytic Anemia, and Weight Loss

This patient requires urgent investigation for celiac disease and gastrointestinal blood loss, with colonoscopy being essential to exclude colorectal malignancy despite the young age, given the combination of alarm features (weight loss and microcytic anemia). 1, 2

Immediate Priority Testing

Celiac Disease Screening

• Tissue transglutaminase IgA antibodies with total IgA level must be checked immediately, as celiac disease commonly presents with microcytic anemia, weight loss, and chronic abdominal symptoms in young adults 1, 2, 3
• IgA deficiency testing is essential because it can cause false-negative celiac serology 1
• If serology is positive, small bowel biopsy via upper endoscopy is required for definitive diagnosis before starting treatment 1

Iron Studies and Anemia Workup

• Serum ferritin is the most powerful test for iron deficiency and should be measured first 1, 4
  • Ferritin <12 μg/dL is diagnostic of iron deficiency 1
  • Ferritin may be falsely elevated (>12-15 μg/dL) with concurrent inflammation or malignancy, though if >100 μg/dL, iron deficiency is unlikely 1
• If ferritin is equivocal, obtain transferrin saturation, serum iron, and total iron-binding capacity 1, 4
• In a 25-year-old with microcytic anemia and weight loss, gastrointestinal blood loss from malignancy must be excluded 1

Inflammatory Markers

• Fecal calprotectin should be tested to exclude inflammatory bowel disease, particularly given the chronic abdominal complaints and constipation 1, 2, 3
• C-reactive protein (CRP) or erythrocyte sedimentation rate (ESR) to assess for occult inflammation 2, 3

Essential Structural Evaluation

Colonoscopy with Biopsies

• Colonoscopy is mandatory despite the patient's young age because the combination of weight loss and microcytic anemia constitutes alarm features requiring exclusion of colorectal cancer 1
• Right and left colon biopsies (not rectal) should be obtained to exclude microscopic colitis, which can present with chronic abdominal complaints and altered bowel habits 1
• Blood loss from the gastrointestinal tract is the most common cause of iron deficiency anemia in adults, and asymptomatic colonic and gastric carcinoma may present with microcytic anemia 1

Upper Endoscopy Considerations

• Upper endoscopy should be performed if celiac serology is positive or if there are upper gastrointestinal symptoms 1
• Consider upper endoscopy regardless to evaluate for gastric cancer, angiodysplasia, or other sources of occult bleeding given the microcytic anemia 1

Additional Testing Based on Initial Results

If Initial Tests Are Normal

• Stool occult blood testing (fecal immunochemical test) if not already performed 1, 2, 3
• Complete blood count review for red cell distribution width (RDW), which may be elevated in combined deficiencies 1
• Thyroid function tests (TSH) to exclude hyperthyroidism as a cause of weight loss and altered bowel habits 1

If Constipation Persists Despite Normal Structural Evaluation

• Consider colonic transit studies if symptoms persist after treating any identified causes 1
• Anorectal manometry may be considered if defecatory disorder is suspected, though this is typically reserved for refractory cases 1

Critical Pitfalls to Avoid

• Do not attribute microcytic anemia to dietary deficiency alone without excluding gastrointestinal pathology, as borderline deficient diets are common but should not be presumed as the sole cause 1
• Do not delay colonoscopy based on age alone when alarm features (weight loss, anemia) are present 1
• Do not perform celiac testing after starting a gluten-free diet, as this will cause false-negative results 1
• Do not assume normal X-ray excludes significant pathology; plain radiographs have limited sensitivity for many gastrointestinal conditions 1

Algorithmic Approach Summary

1. First-line blood tests: Celiac serology (tTG-IgA + total IgA), serum ferritin, CBC with indices, CRP/ESR 1, 2, 3
2. First-line stool test: Fecal calprotectin 1, 2, 3
3. Structural evaluation: Colonoscopy with biopsies (mandatory given alarm features) 1
4. Upper endoscopy: If celiac serology positive or to evaluate upper GI bleeding source 1
5. Thyroid function: TSH to exclude hyperthyroidism 1
6. Further testing: Based on results of above, consider colonic transit studies or anorectal manometry if constipation persists 1

The combination of weight loss and microcytic anemia in any adult mandates aggressive investigation for gastrointestinal malignancy and celiac disease, regardless of age. 1