Method	Potential abnormalities
Laboratory	Specific disease indicators _ Detection of IgA antibodies to transglutaminase > endomysium > gliadin _ HLA-subtype classification positive for DQ2/DQ8 Nonspecific signs of malassimilation _ Protein, nutrient and vitamin deficiency _ Positive D-xylose test _ Positive hydrogen breath tests (lactose, fructose etc.)
Ultrasound	Nonspecific findings _ Fluid-filled intestinal loops and wall thickening _ To and fro peristalsis (“washing machine phenomenon”)
Endoscopy	Nonspecific findings _ Loss of Kerckring’s folds, increased or absent mucosal vascular markings, mosaic structure, fissure-like lesions, fine nodular mucosa, partial or total villous atrophy
Histology	Diagnostic criteria according to the Marsh classification _ Increased intraepithelial lymphocytes (> 40/100 epithelial cells), crypt hyperplasia and villous atrophy (partial, subtotal, complete)

The D-xylose test (25 g D-xylose administered orally with fluids), which is no longer used routinely in primary diagnosis, generally only helps identify malabsorption in the jejunum if less than 4 g of the xylose administered is measured in urine after a 5-h test period, assuming kidney function is normal. This test has low specificity and is not suited to establishing a celiac disease-specific diagnosis. However, it can be used as a quantitative measure of the jejunum’s absorptive capacity during disease follow-up in individual patients. The 13C sorbitol breath test, which has shown superior sensitivity compared to the H ₂-sorbitol breath test during disease follow-up, can also be used to this end in future [33]. H₂, hydrogen; HLA, human leukocyte antigen