The 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).

Inclusion criteria:

• Patients, on gluten-containing diet, with TGt-IgA antibodies > Upper Limit of Normal (ULN) and intestinal biopsy confirming celiac disease (CD).
• Patients with TCRγδ+ >10% combined with decrease NK-like intraepithelial lymphocytes in the intestinal epithelium.
• Paediatric patients who meet ESPGHAN criteria (TGt-IgA antibodies >10xULN) without duodenal biopsy performed.

Exclusion criteria:

• Patients already diagnosed with CD.
• Patients who refused duodenal biopsy.
• Patients with primary or secondary immunodeficiencies.
• Patients with malignancy.
• Recruited patients whose remaining serum cannot be sent to the reference centre, whose biopsies are of poor quality or who nod not sign the informed consent from.

Patients will be consecutively recruited from the gastroenterology departments (either adult or pediatric) of the participating hospitals.