What serum and fecal studies should be ordered to evaluate a 14-year-old with rapid weight gain and chronic abdominal and bowel symptoms?

Diagnostic Workup for 14-Year-Old with Rapid Weight Gain and Chronic Abdominal/Bowel Issues

Order celiac serology (tissue transglutaminase IgA with total IgA level), complete blood count, erythrocyte sedimentation rate, comprehensive metabolic panel, thyroid function tests, iron studies, and fecal calprotectin immediately—this combination identifies the most common serious pathology in adolescents with these symptoms while avoiding unnecessary invasive procedures.

Essential Serum Studies

First-Tier Blood Tests (Order All)

• Celiac disease screening with tissue transglutaminase IgA antibodies plus total IgA level is mandatory because celiac disease affects 0.5-1% of the population and presents with chronic abdominal symptoms in 43-85% of cases 1
• Measure total IgA simultaneously because 2.6% of celiac patients have selective IgA deficiency, which causes false-negative IgA-based tests 1
• Complete blood count with differential to detect anemia (common in IBD and celiac disease) and evaluate for infection or malignancy 1
• Erythrocyte sedimentation rate (ESR) is specifically recommended in younger patients to screen for inflammatory bowel disease 1
• Comprehensive metabolic panel including albumin to assess nutritional status, electrolyte abnormalities, and liver function 1
• Thyroid-stimulating hormone (TSH) and free T4 because hyperthyroidism causes weight changes and altered bowel habits, while hypothyroidism occurs in 17-30% of type 1 diabetics and can present with bowel symptoms 1
• Iron studies (serum ferritin, transferrin saturation, total iron-binding capacity) because ferritin <30 µg/L indicates iron deficiency, and chronic GI blood loss may be occult 2, 3

Important Caveats for Celiac Testing

• If IgA deficiency is present (total IgA low), order IgG tissue transglutaminase or IgG deamidated gliadin peptide antibodies instead 1
• Do not start a gluten-free diet before testing—this causes false-negative results 3
• Negative celiac serology does not exclude the disease—6.4-7% of celiac cases are antibody-negative, so maintain clinical suspicion 1

Essential Fecal Studies

First-Tier Stool Tests

• Fecal calprotectin is the single most important stool test to exclude inflammatory bowel disease in adolescents with chronic abdominal complaints 3, 4
• Stool for ova and parasites (three separate samples) has 60-90% sensitivity for detecting giardiasis and other parasitic infections that cause chronic symptoms 1
• Fecal occult blood test or fecal immunochemical test to detect occult GI bleeding 1, 2
• Clostridioides difficile toxin if there is any history of recent antibiotic use or hospitalization 1, 2

When to Add Stool Culture

• Send bacterial stool culture (Salmonella, Shigella, Campylobacter) only if diarrhea is a prominent feature 2

Critical Decision Points Based on Initial Results

If Celiac Serology is Positive

• Proceed directly to upper endoscopy with small bowel biopsies for definitive diagnosis before dietary changes 1, 3
• Do not rely on serology alone—biopsy confirmation is mandatory, especially in asymptomatic or mildly symptomatic patients 1

If Fecal Calprotectin is Elevated (≥250 µg/g)

• Elevated calprotectin indicates mucosal inflammation and necessitates colonoscopy with biopsies to diagnose or exclude IBD 3, 4
• Note that combining symptoms with blood markers (ESR, hemoglobin) and calprotectin achieves 99.7% accuracy for predicting IBD in pediatric patients 4

If Weight Loss or Anemia is Present

• These are alarm features requiring colonoscopy regardless of age or other test results to exclude colorectal pathology including malignancy 2, 3
• Do not attribute microcytic anemia to dietary deficiency alone—GI pathology must be excluded first 3

Common Pitfalls to Avoid

• Do not skip ESR in adolescents—it is specifically recommended for younger patients and helps detect IBD, though 20% of active Crohn's disease cases have normal inflammatory markers 1, 2
• Do not delay endoscopy if alarm features exist (weight loss, anemia, blood in stool, family history of IBD or cancer)—these override normal laboratory results 1, 3
• Do not order hormone panels (VIP, gastrin, glucagon) in this age group—these tumors are extraordinarily rare (1 per 10 million population) and should only be considered with high-volume watery diarrhea after excluding all other causes 1
• Do not perform thyroid function tests at the time of acute metabolic derangement—wait until metabolic stability is achieved if initial presentation includes severe symptoms 1

Symptom-Specific Additional Testing

If Constipation Predominates

• Consider therapeutic trial of fiber before extensive testing 1
• If symptoms persist, colonic transit studies or anorectal manometry may be indicated 3

If Diarrhea Predominates

• Add lactose hydrogen breath test if lactose intolerance is suspected based on symptom pattern 1
• Consider glucose hydrogen breath test for small bowel bacterial overgrowth if malabsorption is suspected 1

Regarding Rapid Weight Gain

• This atypical feature (weight gain rather than loss) suggests the primary issue may be endocrine or metabolic rather than malabsorptive—thyroid dysfunction and insulin resistance should be prioritized 1