What laboratory tests are recommended for a 3-year-old child with chronic constipation and possible poor growth who is being considered for celiac disease screening?

Laboratory Testing for Celiac Disease Screening in a 3-Year-Old

The recommended initial laboratory test is IgA tissue transglutaminase (tTG) antibodies with concurrent measurement of total serum IgA level. 1, 2

Primary Screening Test

• Measure IgA tissue transglutaminase (tTG) antibodies as the first-line screening test, which has sensitivity of 90-96% and specificity greater than 95% in pediatric populations 2, 3
• Simultaneously measure total serum IgA level to rule out selective IgA deficiency, which occurs in 1-3% of celiac disease patients and would cause falsely negative IgA-based test results 1, 2

Alternative Testing if IgA Deficiency Detected

• If total IgA is low or absent, switch to IgG-based testing: either IgG tissue transglutaminase antibodies OR IgG deamidated gliadin peptide (DGP) antibodies 1, 3
• IgG-DGP has demonstrated 95% sensitivity in pediatric celiac disease when IgA testing cannot be used 4

Critical Pre-Testing Requirement

• The child must remain on a gluten-containing diet (at least 10g daily, equivalent to approximately 3 slices of wheat bread) until all testing is complete to avoid false-negative results 2, 3
• Never initiate a gluten-free diet before completing the diagnostic workup, as this makes serologic testing unreliable and may require prolonged gluten challenge (1-3 months) for accurate future testing 2, 3

Tests NOT Recommended for Initial Screening

• Antigliadin antibodies (AGA) are less specific than tTG or endomysial antibodies and are not recommended for screening 1
• Endomysial antibodies (EMA) have excellent specificity (99.6%) but are more expensive, technically demanding, and operator-dependent compared to tTG, making them better suited as confirmatory rather than initial screening tests 1, 2

Interpretation of Results

• If tTG-IgA is positive: Proceed to upper endoscopy with duodenal biopsy (minimum 6 specimens from second part of duodenum or beyond) for histological confirmation using Marsh classification 2
• Strongly positive titers (tTG >10 times upper limit of normal) have positive predictive value approaching 100% and are highly specific (>98%) for Marsh 3a or greater villous atrophy 2, 5
• The correlation between tTG antibody levels and degree of villous atrophy is strong (r = 0.704), with higher antibody levels predicting more severe intestinal damage 5

Special Considerations for This Clinical Context

Given the presentation of chronic constipation and possible poor growth:

• Monitor growth parameters closely (height and weight), as poor growth or growth failure are key symptoms warranting celiac screening in children 1, 6
• While chronic constipation is less typical than diarrhea for celiac disease, the combination with poor growth increases clinical suspicion and justifies screening 1
• Abdominal distention and diarrhea are more strongly associated with abnormal histology, but symptoms alone are not reliable predictors of celiac disease 5

Common Pitfalls to Avoid

• Never reassure parents without testing when there is clinical suspicion (poor growth) or high-risk factors, as early detection prevents complications including malabsorption, nutritional deficiencies, and associated autoimmune conditions 3
• Do not start empiric gluten-free diet based on symptoms alone without diagnostic confirmation, as this represents a significant lifelong dietary burden and makes future testing unreliable 2, 3
• Do not rely on symptoms alone to rule out celiac disease, as height and weight are not reliable predictors, and many children with positive antibodies are asymptomatic or have subtle symptoms 1, 5