The goal of this observational study is to learn about an adult's chance of having celiac disease based on blood testing and symptoms. The main question it aims to answer is:

Can a blood test and symptom information separate patients into 3 groups of low, intermediate, and high risk for celiac disease?

Participants already being evaluated for celiac disease as part of their regular medical care will answer online survey questions about their symptoms and have laboratory data collected from their charts.

We hypothesize that a clinical prediction model integrating clinical data with TTG-IgA antibody levels can accurately identify patients with celiac disease offering a personalized approach. We anticipate this prediction model would classify patients into 3 risk groups for celiac disease: 1) Low likelihood (no further testing required), 2) Intermediate likelihood (biopsy required for confirmation), and 3) High likelihood (biopsy can be avoided based on the model's accuracy) thereby reserving endoscopy and biopsies for cases of intermediate probability to improve diagnosis, reduce invasive testing, increase patient focus, and decrease costs.

The specific aims of this study are to: 1) develop and validate a clinical prediction model for celiac disease probability (external validation will be performed by site and time), 2) evaluate the implementation potential of the model, and 3) pilot the model and determine its impact on patient experience.

For Aim 1:

Inclusion Criteria:

• Patients ≥18 who underwent duodenal biopsy during upper endoscopy and had a TTG-IgA antibody test 3 months before or 1 month after biopsy

Exclusion Criteria:

• Patients with a prior diagnosis of celiac disease undergoing biopsy and TTG-IgA antibody testing for follow-up care
• Children and vulnerable populations (e.g. pregnant women or prisoners)
• Patients with IgA deficiency
• Patients already following a gluten-free diet

For Aim 2:

Inclusion Criteria:

• Physicians (primary care or subspeciality) who test or evaluate patients for celiac disease
• Patients already diagnosed with celiac disease or undergoing evaluation for celiac disease

Exclusion Criteria:

- Children and vulnerable populations (e.g. pregnant women or prisoners)

For Aim 3:

Inclusion Criteria:

• Patients ≥18 with a standard-of-care celiac disease evaluation (both TTG-IgA antibody and upper endoscopy with duodenal biopsy)
• Willing to participate and able to provide informed consent

Exclusion Criteria:

- Children and vulnerable populations (e.g. pregnant women or prisoners)

The study population for Aim 1 is all adult patients ≥18 who underwent duodenal biopsy during upper endoscopy and had a TTG-IgA antibody test 3 months before or 1 month after biopsy. First pathology data will be reviewed to identify duodenal biopsies conducted during the study period then TTG-IgA antibody availability will be confirmed. We aim to study incident cases. The study population is similar for Aim 3 - patients being evaluated for celiac disease. The study population for Aim 2 includes primary care, gastroenterology, and subspeciality physicians diagnosing celiac disease and patients with or being evaluated for celiac disease.