What is the most definitive test to diagnose celiac disease in a patient presenting with gastrointestinal symptoms and a rash?

The most definitive test to diagnose celiac disease is a duodenal biopsy. This procedure allows for direct visualization of the small intestine and enables the assessment of the intestinal mucosa for characteristic changes associated with celiac disease, such as villous atrophy, crypt hyperplasia, and increased intraepithelial lymphocytes. These findings are crucial, as they confirm the diagnosis by showing the damage caused by gluten in susceptible individuals.

Although serological tests such as tissue transglutaminase IgA antibody can indicate celiac disease by detecting the presence of specific antibodies, they are not definitive on their own. A positive result should always be supported by histological evidence from a biopsy to confirm the diagnosis.

HLA-DQ genotyping can be used as a supporting test, particularly in cases where the diagnosis is unclear, but it does not confirm the disease directly. A significant percentage of the general population carries the necessary HLA-DQ2 or HLA-DQ8 haplotypes without having celiac disease, meaning this test is not definitive.

Measuring 25-hydroxyvitamin D is related to assessing vitamin D status, which can be relevant in patients with malabsorption but is not specific for diagnosing celiac disease. In summary,