What is the recommended diagnostic approach for celiac disease, including serologic testing, duodenal biopsy, and HLA‑DQ2/DQ8 typing?

Diagnosis of Celiac Disease

Initial Serologic Testing

Begin with IgA tissue transglutaminase antibody (tTG-IgA) combined with total serum IgA measurement as the first-line diagnostic test, performed while the patient consumes at least 10g of gluten daily for 6–8 weeks. 1, 2

• tTG-IgA demonstrates sensitivity of 90.7% and specificity of 87.4% in adults at the 15 U/mL threshold, making it the single most efficient serologic screening test 1, 3
• Measuring total IgA simultaneously is mandatory because selective IgA deficiency occurs in 1–3% of celiac patients and causes false-negative IgA-based results 1, 2
• In children younger than 2 years, combine tTG-IgA with deamidated gliadin peptide (DGP) IgG/IgA testing to improve sensitivity 1
• Inadequate gluten intake before testing is the leading cause of false-negative results; patients must not reduce gluten consumption before completing the diagnostic workup 1, 2

Confirmatory Serologic Testing

When tTG-IgA is positive, perform endomysial antibody (EMA) testing on a second sample to confirm the diagnosis before proceeding to biopsy. 1, 4

• EMA has specificity of approximately 99.6% in adults and 93.8% in children, providing superior confirmation compared to tTG-IgA alone 1, 4
• If tTG-IgA exceeds 10 times the upper limit of normal and EMA is positive on a second sample, the positive predictive value approaches 100% 1
• The two-step approach (tTG-IgA followed by EMA) yields sensitivity of 85.7%, specificity of 98.6%, and negative predictive value of 99.7% 3

Testing in IgA-Deficient Patients

If total IgA is low or absent, switch to IgG-based testing using IgG deamidated gliadin peptide (DGP-IgG) as the preferred test. 1, 4

• IgG DGP demonstrates superior diagnostic accuracy with sensitivity of 93.6% and specificity of 99.4% in adults 1, 4
• IgG tTG should not be used as a primary screen when total IgA is normal because it lacks specificity and generates frequent false-positive results 1, 5
• Isolated positive tTG IgG with negative tTG IgA has only 3% utility in diagnosing celiac disease and should not prompt biopsy in low-risk patients 5

Mandatory Duodenal Biopsy

Upper endoscopy with duodenal biopsy is mandatory for adult diagnosis and cannot be replaced by serology alone, except in patients with coagulation disorders or pregnancy. 1, 2, 6

• Obtain at least six biopsy specimens: 1–2 from the duodenal bulb and ≥4 from the second portion of the duodenum or more distal segments 1, 2
• Multiple biopsies are essential because mucosal lesions can be patchy; reliance on fewer specimens risks missing the diagnosis 1, 2
• Definitive diagnosis requires villous atrophy (Marsh type 3) with crypt hyperplasia and ≥25 intraepithelial lymphocytes per 100 enterocytes 1, 2
• Probable celiac disease may be diagnosed when biopsies show ≥25 intraepithelial lymphocytes without villous atrophy, combined with positive serology 1
• Specimens must be properly oriented and interpreted by a pathologist with gastroenterology expertise to avoid misinterpretation 2

HLA-DQ2/DQ8 Genetic Testing

HLA-DQ2/DQ8 typing is not a routine screening tool but should be reserved for specific scenarios: equivocal histology, patients already on a gluten-free diet, discrepant serology/histology, or when the original diagnosis is in doubt. 1, 2, 7

• Absence of both HLA-DQ2 and HLA-DQ8 alleles provides >99% negative predictive value and essentially excludes celiac disease 1, 2, 7
• Approximately 95% of celiac patients have HLA-DQ2 and 5% have HLA-DQ8, but HLA-DQ2 is present in 25–30% of the White population, making a positive result insufficient to confirm disease 1, 2
• HLA typing is particularly useful for excluding celiac disease lifelong in individuals with equivocal small bowel histological findings 7

Differential Diagnosis of Villous Atrophy

When villous atrophy is present with negative celiac serology, systematically exclude alternative causes before diagnosing seronegative celiac disease. 1

• Medication-induced enteropathy: olmesartan, NSAIDs, mycophenolate mofetil, chemotherapy agents 1
• Infectious causes: Giardiasis (stool antigen/PCR), small-bowel bacterial overgrowth 1
• Immune-mediated: autoimmune enteropathy 1
• Inflammatory bowel disease: Crohn's disease (look for granulomas, perform colonoscopy with ileoscopy) 1

Distinguishing Celiac Disease from Non-Celiac Gluten Sensitivity

Symptom profile or response to a gluten-free diet alone cannot reliably differentiate celiac disease from non-celiac gluten sensitivity; definitive exclusion requires appropriate serology, duodenal histology, and when indicated, HLA typing. 1, 8

• Accurate differentiation is critical to identify patients at risk for nutritional deficiencies, assess familial risk, and determine the necessary duration and strictness of dietary adherence 1
• Non-celiac gluten sensitivity should only be considered after celiac disease has been ruled out with serology and intestinal biopsies 1, 8

Critical Diagnostic Pitfalls to Avoid

• Never initiate a gluten-free diet before completing serologic and histologic evaluation, as this invalidates both testing modalities 1, 2, 6
• Never rely on serology alone for adult diagnosis; a confirmatory duodenal biopsy is required 1, 2
• Always measure total IgA when using IgA-based serologic tests to detect possible IgA deficiency 1, 2
• Never base clinical decisions on subjective improvement after gluten avoidance alone, as this has very low positive predictive value 1
• Never use non-deamidated IgA/IgG gliadin antibody assays, as they provide no additional diagnostic benefit in adults 1, 3

Post-Diagnosis Management

Immediately after biopsy confirmation, initiate a strict lifelong gluten-free diet (daily gluten intake <10 mg) and refer to a registered dietitian experienced in celiac disease management. 1, 2

• Repeat tTG-IgA at 6 months after starting gluten-free diet, at 12 months, and annually thereafter to monitor adherence 1, 2
• Screen for micronutrient deficiencies including iron, folic acid, vitamin D, and vitamin B12 2
• A normal tTG level does not predict recovery of villous atrophy; 44% of patients with normal tTG on a gluten-free diet may still have persisting villous atrophy 3