Tissue Transglutaminase IgA Testing is the Most Appropriate Next Step
The most appropriate next diagnostic step is to order tissue transglutaminase IgA (tTG-IgA) with total IgA level (Option C). This child presents with classic features of celiac disease: chronic diarrhea (>4 months), poor weight gain with significant percentile drop (36th to 3rd percentile), intermittent abdominal pain, and abdominal distention—all hallmark manifestations of malabsorption 1.
Why Celiac Disease Screening is Priority
Celiac disease is the most common small bowel enteropathy in Western populations, affecting 1:200 to 1:559 individuals, with many presenting with diarrhea, poor weight gain, growth failure, and abdominal pain 1.
Guidelines explicitly recommend routine serological testing for celiac disease in all patients presenting with chronic diarrhea as part of the initial primary care assessment 1.
The British Society of Gastroenterology states there is "a strong case for routine serological testing for coeliac disease for all patients presenting with 'diarrhoea'" 1.
This child's clinical presentation—chronic diarrhea, poor weight gain, abdominal distention, and declining growth percentiles—matches the classic symptom profile described in guidelines: "diarrhea, weight loss or poor weight gain, growth failure, abdominal pain, and malnutrition due to malabsorption" 1.
Recommended Testing Strategy
Order IgA tissue transglutaminase antibodies (tTG-IgA) with total IgA level measurement 1.
The total IgA level is essential because selective IgA deficiency occurs in 1:500 in the general population but in 2.6% of celiac disease patients, causing false-negative IgA-based tests 1.
If IgA deficiency is detected, IgG-based tests (IgG tTG or IgG deamidated gliadin peptide antibodies) should be used instead 1.
The sensitivity of tTG-IgA is 90.9% and specificity is 90.9%, with a negative predictive value of 99.6% 2.
Why Other Options Are Inappropriate
Blood gas (Option A):
- Blood gas analysis has no role in evaluating chronic diarrhea with poor weight gain in a hemodynamically stable child 1.
- This would only be indicated if there were signs of severe dehydration or metabolic derangement, which are not present 1.
CBC (Option B):
- While CBC can detect anemia suggesting organic disease, it is non-specific and does not establish a diagnosis 1.
- Guidelines recommend CBC as part of initial screening alongside celiac serology, not instead of it 1.
- A normal CBC would not exclude celiac disease, making it insufficient as the sole next step 1.
Genetic test (Option D):
- Genetic testing (HLA-DQ2/DQ8) is not a first-line diagnostic test for celiac disease 1.
- Genetic testing is used for risk stratification or in ambiguous cases, not for initial diagnosis 1.
- Approximately 30-40% of the general population carries celiac disease-associated genes, making genetic testing alone inadequate for diagnosis 1.
Critical Pre-Testing Requirement
- The child must remain on a gluten-containing diet until testing is complete to avoid false-negative results 1, 3.
- Never start a gluten-free diet before completing the diagnostic workup, as this leads to false-negative serology and inconclusive biopsies 3.
Follow-Up Based on Results
If tTG-IgA is positive: Refer to pediatric gastroenterology for upper endoscopy with duodenal biopsy (at least 6 specimens from the second part of the duodenum or beyond) for histological confirmation 1, 3.
If tTG-IgA is very elevated (≥10 times the upper limit of normal): The positive predictive value is 91.4%, and the child should be referred for biopsy confirmation, especially given the symptomatic presentation 4, 5.
If tTG-IgA is negative: Consider other causes of chronic diarrhea including thyroid dysfunction, inflammatory bowel disease, or functional disorders, but celiac disease is effectively excluded with a negative predictive value of 99.6% 2.
Additional Considerations
While thyroid function testing (TSH) is also recommended in the initial workup of chronic diarrhea, celiac disease is more likely given this specific constellation of symptoms (malabsorption pattern with growth failure) 1.
The significant weight percentile drop from 36th to 3rd over one year strongly suggests an organic cause requiring specific diagnosis rather than empiric treatment 1.
Early diagnosis allows intervention before complications develop, including severe malnutrition, vitamin deficiencies, and associated autoimmune conditions 1, 3.