A 22-year-old woman with recurrent fractures and chronic diarrhea is evaluated for coeliac disease. Her serum total IgA level is 25 mg/dL (low), and tissue transglutaminase IgA antibody is negative. Duodenal biopsy shows villous atrophy. Which serological test should have been used for diagnosis?

• A Repeat tissue transglutaminase IgA after gluten challenge
• B Tissue transglutaminase IgG antibody ✓
• C Total serum complement C3 level
• D Anti-Saccharomyces cerevisiae antibody

Explanation

Coeliac disease is around ten times more prevalent in selective IgA deficiency, and IgA-based assays give falsely negative results in these patients. When total IgA is deficient, diagnosis relies on IgG-class antibodies, either tTG-IgG or deamidated gliadin peptide IgG, paired with compatible biopsy findings. Repeating tTG-IgA will remain negative because the substrate immunoglobulin class is absent. ASCA is associated with Crohn's disease, and complement levels play no role in coeliac serology.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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