Serologic and Ancillary Testing Guidelines for Celiac Disease

Primary Screening

IgA tissue transglutaminase antibody (tTG‑IgA) is the preferred first‑line test. In adults, a 15 U/mL cutoff yields ≈ 90 % sensitivity and 87 % specificity; in children, a 20 U/mL cutoff yields ≈ 98 % sensitivity and 70 % specificity. 1
Total IgA measurement must be performed concurrently with tTG‑IgA to detect IgA deficiency, which occurs in about 1–3 % of individuals with celiac disease and can cause false‑negative IgA‑based results. 1

IgA endomysial antibody (EMA‑IgA) is used as a second‑line confirmatory assay, offering very high specificity (≈ 99.6 % in adults and ≈ 93.8 % in children). 1
When tTG‑IgA exceeds 10 × the upper limit of normal and EMA‑IgA is positive on a repeat sample, the positive predictive value approaches 100 %. 1

IgG deamidated gliadin peptide (DGP‑IgG) is the preferred IgG‑based assay, showing ≈ 94 % sensitivity and ≈ 99 % specificity in adult cohorts. 1
IgG tissue transglutaminase (tTG‑IgG) has lower performance (sensitivity ≈ 41–85 %; specificity ≈ 78–89 %) and is less reliable than DGP‑IgG. 1
IgG EMA demonstrates near‑universal positivity in IgA‑deficient celiac patients in research studies. 1
IgG‑based testing should not be used for primary screening when total IgA is normal because of reduced specificity and a high rate of false‑positive results. [1][2]

HLA‑DQ2/DQ8 typing provides > 99 % negative predictive value; absence of both alleles essentially excludes celiac disease. 1
HLA testing is reserved for special scenarios such as equivocal intestinal histology, discordant serology, patients already on a gluten‑free diet without prior testing, or strong clinical suspicion despite negative serology. [1][2]

Complete blood count (CBC) to screen for anemia and assess overall nutritional status. 3
Iron studies (serum iron, ferritin, transferrin saturation) to evaluate iron deficiency. 3
Folate and vitamin B12 measurements to detect malabsorption of these nutrients. 3
Broad micronutrient panel (calcium, vitamin D, zinc, fat‑soluble vitamins) to monitor nutritional repletion. 3
Liver function tests to identify hepatic involvement that may accompany celiac disease. 3

All serologic assays must be performed while the patient consumes ≥ 10 g of gluten daily for 6–8 weeks to avoid false‑negative results. [1][4]2
If the patient has already started a gluten‑free diet, they should resume a normal gluten intake (e.g., three slices of wheat bread per day) for 1–3 months before repeat testing. [1][4]

Do not initiate a gluten‑free diet before completing diagnostic testing, as this invalidates both serologic and histologic assessments. [1][4]
Do not rely on serology alone to diagnose celiac disease in adults; intestinal biopsy confirmation remains required. 1
Use the same tTG‑IgA assay for serial monitoring, because different assay platforms vary in sensitivity for detecting suboptimal treatment response. 3

The American Academy of Family Physicians recommends IgA tissue transglutaminase antibody (tTG-IgA) as the preferred first-line screening test with high sensitivity (90-96%) and specificity (>95%) 5, 6
Total IgA levels should be measured simultaneously to rule out IgA deficiency, which could cause false-negative results 7, 5
Testing must be performed while the patient is on a gluten-containing diet (at least 10g daily for 6-8 weeks) to avoid false-negative results 5, 6
In patients with tTG-IgA levels >10x upper limit of normal, IgA endomysial antibody (EMA-IgA) should be performed as a confirmatory test with excellent specificity (99.6%) 5, 8

In IgA-deficient patients, IgG-based tests should be used instead, such as IgG deamidated gliadin peptide (DGP-IgG) and IgG tissue transglutaminase (tTG-IgG) 5, 8
In children under 2 years of age, tTG-IgA testing should be combined with IgG and IgA deamidated gliadin peptides for improved sensitivity 7, 6

Upper endoscopy with small bowel biopsy remains the gold standard for diagnosis in adults 5, 6
Multiple duodenal biopsies are required, including 1-2 samples from the duodenal bulb and at least 4 samples from the distal duodenum 7, 5
Characteristic histologic findings include villous atrophy, crypt lengthening, and increased intraepithelial lymphocytes 5

HLA-DQ2 and HLA-DQ8 testing has high negative predictive value (>99%) and is useful in specific scenarios, such as when celiac disease is strongly suspected despite negative serology 5, 6
HLA-DQ2 and HLA-DQ8 testing is also useful in patients with equivocal small-bowel histologic findings, patients already on a gluten-free diet without prior testing, and patients with discrepant celiac-specific serology and histology 7, 5

In children with tTG-IgA ≥10× upper limit of normal, positive EMA-IgA, and positive HLA-DQ2/DQ8, biopsy may be avoided according to pediatric guidelines 5, 6
During the COVID-19 pandemic, similar non-biopsy protocols were temporarily adopted for adults with high tTG-IgA levels 9

Initiating a gluten-free diet before completing diagnostic testing can lead to false-negative results 5, 8
Relying solely on serology without biopsy confirmation can lead to misdiagnosis in adults 5, 8
Lymphocytic infiltration of the intestinal epithelium without villous atrophy is not specific for celiac disease and requires consideration of other causes 7
Improvement of symptoms after starting a gluten-free diet has a low positive predictive value for celiac disease and should not be used for diagnosis without supportive evidence 7

Follow-up serology should be performed at 6 and 12 months after diagnosis, and yearly thereafter 5
Negative serology after treatment does not guarantee intestinal mucosal healing 5
Persistently elevated antibodies usually indicate ongoing gluten exposure 5, 8