Navigating Celiac Disease Testing

In 2023, the American College of Gastroenterology (ACG) released updates to their guideline on celiac disease. While much remains the same from the 2013 edition, some changes were adopted to more closely reflect EPSGHAN guidelines. For the general population, avoiding gluten has virtually no proven health benefits, the single exception being people with laboratory confirmed celiac disease. It is therefore prudent to understand when a gluten free diet is necessary by assessing prevalence, clinical symptoms, pathogenesis, and diagnostic testing. 

The prevalence of Celiac disease

Celiac disease can occur at any age and is more common among women. Celiac disease occurs at a prevalence of approximately 1.5 percent in developed countries, however, the incidence may be increasing. In patients presenting with symptoms or other familiar risk factors, prevalence can be as high as 4.5 percent.

Celiac disease can also be asymptomatic in some individuals, with one study showing a prevalence of 0.75 percent among blood donors, school children, and patients seen during routine wellness exams. Though asymptomatic, these patients still have serological and histological abnormalities. 

In recent years, testing for celiac disease has improved, providing one plausible explanation for the growing prevalence in developed countries. Additionally, today’s patients and healthcare consumers are now more aware and interested in following up on symptoms than past generations. Whether the increase in prevalence is because of a true increase in disease or simply an increase in awareness and screening is not yet known. 

Among persons who do have celiac disease, a genetic predisposition is well documented. Nearly all patients have alleles encoding for the HLA-DQ2 or HLA-DQ8 genes. With a genetic basis, the prevalence increases to 10 percent among first-degree relatives. 

Pathogenesis

The pathogenesis of celiac disease stems from exposure to prolamines, a storage protein containing high amounts of proline amino acids. Prolamines are found within wheat (gliadin), rye (secalines), and barley (hordeins). When gluten interacts with intestinal enterocytes, gut permeability increases allowing gluten to pass through. Here, the gliadin component of gluten is deaminated by tissue transglutaminase. Antigen presenting cells then present the modified peptide to T-lymphocytes, inducing release of pro-inflammatory cytokines and recruitment of B-lymphocytes. Ultimately, the B-lymphocytes produce anti-gliadin and anti-tissue transglutaminase antibodies. 

The compounded effects of this pro-inflammatory response and autoimmune activation are degradation of the intestinal enterocytes or villous atrophy. Villous atrophy impairs intestinal absorption with potential to cause systemic nutritional deficiency.

Symptoms and associations

Celiac disease contains a milieu of symptomatic presentations resulting from autoimmune response and digestive system impairment. Classical symptoms associated with intestinal manifestation include diarrhea, weight loss, and bloating. Extraintestinal signs and symptoms include edema, muscle wasting, and iron deficiency anemia. Other nonclassical signs include short stature, chronic fatigue, and osteoporosis, and elevated liver enzymes.

Numerous lab tests also appear abnormal in untreated celiac disease. Patients may exhibit anemia, vitamin D deficiency, elevated liver enzymes, hypoproteinemia, or hypocalcemia. 

Importantly, there are several other diseases commonly encountered in celiac disease. These comorbidities include type 1 diabetes, autoimmune thyroid disease, and autoimmune liver disease. The associated diseases are similarly representative of an ill-adapted autoimmune response. 

Laboratory testing

The broad array of symptoms of celiac disease is not indicative of any single etiology, requiring differentiation by laboratory methods. The gold standard diagnostic for celiac disease is histologic evaluation with observance of villous atrophy and crypt hyperplasia. Because tissue specimens are obtained by invasive endoscopic biopsy, a blood-based alternative is preferred prior to the invasive procedure. In some instances, serum testing alone may be suitable for diagnosis.

The principally recommended protocol starts with a serum test for total IgA and subsequent detection of tissue transglutaminase IgA (TTG IgA). As many as 3 percent of celiac disease patients are IgA deficient, which can result in a falsely negative TTG IgA result. In the case of normal total IgA and an elevated TTG IgA, no further laboratory testing is needed and biopsy for histologic confirmation may proceed. However, newer guidance from EPSGHAN and ACG have indicated that, in select circumstances, biopsy is not necessary when TTG IgA results exceed 10,000 IU. When TTG IgA is negative in conjunction with a low pre-test probability based on symptoms and familial history, celiac disease can be ruled out. 

When TTG IgA results are not conclusive or total IgA is deficient, additional tests are recommended. TTG IgG is a second-tier test that can prove useful in these situations. Deamidated gliadin peptide (DGP) IgA and/or IgG add additional value to increase test sensitivity and confirm findings. In pediatric patients, DGP has been shown to perform better than TTG with improved sensitivity. Therefore, consideration for DGP may be warranted as part of the initial battery of tests in combination in pediatric patients to confirm negative TTG IgA results. In both TTG and DGP, it is notable that IgA is more sensitive and specific than the IgG isotype. 

Endometrial antibodies (EMA) also test for celiac disease, though it has been surpassed by the TTG and DGP, which have comparable (or favorable) performance characteristics and are easier for laboratories to access. EMA is still recommended as a confirmatory alternative to endoscopic biopsy for pediatric patients whose TTG IgA value is >10x the upper limit of normal. 

Importantly, any serology-based test must occur while the patient is exposed to gluten. Removal of gluten from the diet in patients who have celiac disease causes a downregulation in the autoimmune response and antibody titers will decline, appearing normal when tested.

Genetic testing for HLA-DQ2 and HLA-DQ8 can add further value as a rule out test. These specific HLA alleles are present in >95 percent of patients with celiac disease, therefore, patients who test negative can be satisfactorily ruled out. However, positive findings are not helpful as 30 percent to 40 percent of the US population has HLA-DQ2 or HLA-DQ8, which may unnecessarily increase healthcare costs, result in referral, and require further serologic testing.

Understanding the options

While guidelines may recommend a preferred model, it is important for patients, providers, and laboratory personnel to understand a one-size fits all algorithm is not always feasible. Knowledge of assay and instrumentation limitations and benefits can improve the diagnostic acumen when navigating the celiac disease testing pathways. While total IgA is a prerequisite in many models, an alternative option exists in the combination of TTG IgA with DGP IgA/IgG to better encompass pediatric and IgA deficient patients. A third option is to start with HLA-DQ2 and HLA-DQ8 to rule out negative patients but is limited by high population prevalence.