The primary aim of the study is to detect the presence of gluten immunogenic peptides (GIP) in urine samples from celiac patients on a gluten-free diet clinically responsive and non-responsive to dietary treatment and from suspected celiac patients already on a gluten-free diet.

Once a year, celiac patients perform a clinical (gastroenterological visit) and blood chemistry (blood tests) check. At the Center for the Prevention and Diagnosis of Celiac Disease, about 1400 check-ups of celiac patients are carried out per year. During this annual control visit, consecutive patients afferent to the center and suffering from celiac disease who have been treated for more than 6 months, clinically "responsive" and "non-responsive" to the gluten free diet (GFD), will be invited to participate in the proposed study. Those who participate will be given the symptom questionnaires (VAS), the food questionnaire in order to evaluate the relevance of the symptoms reported and the type of diet followed by the patient and the adherence test to the gluten-free diet (CDAT). Urine samples will be collected to test the presence of GIP (gluten immunogenic peptides) and for patients whose samples test positive, will be asked for a stool sample to detect GIP.

Subjects with suspected celiac disease but already on a gluten-free diet will undergo a challenge, or the reintroduction of gluten in incremental steps of three weeks (routinely performed as per the instructions of the Italian Health Care System).

Inclusion Criteria:

• Age: 18 to 65 years old
• Gender: Males and Females
• Patients: suffering from Celiac Disease and suspected, but already on a gluten-free diet

Exclusion Criteria:

• Celiac patients less than 6 months after diagnosis
• Celiac patients reporting intentional gluten ingestion

The study population will be patients with celiac disease and suspected celiac disease (about 300)