RecruitingNo-biopsy Approach in Celiac Disease: Cut-off Points for IgA Anti-tissue Transglutaminase AssaysThe main objective of this multicenter and observational study is to define the optimal threshold of different commercially available IgA anti-transglutaminase (tTG-IgA) antibody assays for celiac disease diagnosis (CD) avoiding the need for an intestinal biopsy. The main questions to be answered are: * Is the anti-tTG-IgA titer cut-off above 10 times the upper limit of normal (ULN) useful in all anti-tTG IgA assays? * Is the diagnostic performance of the newly defined cut-offs of anti-tTG-IgA the same in all the evaluated assays? * Is the dynamic of the anti-tTG-IgA levels after the introduction of the gluten-free diet (GFD) similar across the different assays included in the study? This is a prospective multicenter study that will enroll pediatric and adult patients with new-onset CD during the years 2023 and 2024. Serum from these patients will be collected for the determination of anti-tTG-IgA according to the local methodology (participating hospital) and by the anti-tTG IgA assays most commonly used in our country, which will be centralized in the same reference center (Hospital Universitario La Paz).