Celiac disease (CeD) is a chronic multisystem immune-mediated disorder that affects the small intestine of genetically predisposed individuals (). It is triggered by gluten ingestion, a protein found in wheat, barley, and rye, and managed with strict gluten elimination. The gluten-free diet (GFD) has exponentially grown in popularity -- partially from increased recognition of CeD, but also because of perceived health benefits (Figure 1). Between 2009 and 2014, the number of individuals in the United States following a GFD without a diagnosis of CeD more than tripled to more than 6 million (1.7%) people. This number has continued to rise in both the United States and globally. Further highlighting this interest is the astronomical rise of gluten-free products -- a market estimated to be >$7 billion in 2024 with projected growth to $27 billion by 2032 (). Thus, assessing patients following a GFD before a CeD diagnosis is a common clinical scenario. This article aims to provide gastroenterologists with a comprehensive approach tailored to evaluating for CeD in patients already adhering to a GFD.
CASE STUDY
John is a 28-year-old man who has been struggling with chronic gastrointestinal (GI) symptoms such as bloating, abdominal pain, and intermittent diarrhea for several years without a definitive diagnosis. After hearing about the potential benefits of a GFD for similar issues, he decided to try it himself. Within a few weeks of eliminating gluten, he experienced significant relief, with reduced pain and fewer episodes of diarrhea. Motivated by these improvements, John adhered strictly to the diet despite the challenges of navigating social situations and increased food costs. Certain that gluten had been causing his symptoms, he continues the GFD without having consulted a specialist or any medical provider for a formal diagnosis. This case illustrates the common scenario of self-prescribed dietary changes in response to uninvestigated GI symptoms.
INITIAL ASSESSMENT
History should be focused on symptoms, current diet, and directed questioning about potential risk factors for CeD. Ask about onset, frequency, duration, and severity of symptoms including how symptoms are modified by the GFD. In addition, special care should be made to investigate for intestinal and extraintestinal nonclassic manifestations of CeD including dermatitis herpetiformis (cluster of pruritic papules on an erythematous base), dental enamel changes, anemia, unexplained infertility, ataxia, neuropathy, and osteoporosis (). Given the necessary genetic susceptibility and known elevated risk in family members, always ask about first-degree relatives with definitive CeD ().
Regarding the GFD, inquire about motivation, duration, response, and strictness. Ascertain why the diet was initiated; often motives are related to ameliorating GI symptoms, but also can be rooted in perceived health benefits, weight loss strategies, curiosity, advice from friends, or for management of other known, potentially overlapping conditions such as irritable bowel syndrome (IBS). Ask them to gauge symptom improvement on the GFD as either a percent change or on the Likert scale. Dietary evaluation, a week-long journal or formal dietitian review, is crucial to understand what is meant by a "GFD" on a case-by-case basis. With strict gluten avoidance, there is well documented symptomatic improvement in CeD; observational and controlled trials show significant improvement within 8 weeks in up to 80% of patients with CeD (). However, symptom improvement with gluten avoidance does not have a strong positive predictive value (just 36%) for CeD because other conditions (IBS, nonceliac gluten sensitivity, wheat allergy, and food intolerance) benefit from gluten avoidance ().
LABORATORY TESTING
Although the first-line test for diagnosis of CeD is the measurement of the tissue transglutaminase immunoglobulin A antibodies (TTG-IgA) (highly sensitive and specific), patients following a GFD need to be evaluated differently from those consuming gluten (Figure 2). Adherence to a GFD can result in a decrease or complete normalization of TTG-IgA levels because the immune response that triggers the production of these antibodies is halted. It is for this reason that we suggest primary care providers recommend against self-initiated dietary changes before formal serologic CeD evaluation because this complicates a correct gastroenterological evaluation. The GFD also reduces inflammation in the small intestine, which complicates the biopsy-based diagnosis in individuals who have already eliminated gluten before testing (). TTG-IgA antibodies can be checked as the first step because they are still helpful if positive but anticipate these will likely be negative if the patient is strict in the GFD (sensitivity 16%). Given the limited precision of serology when following a GFD, the next step in the diagnostic process is genetic testing for HLA-DQ2 and HLA-DQ8 alleles, which are necessary for CeD development (Table 1). Over 99% of individuals with CeD possess one of these genetic markers, but they are also found in up to 40% of the general population (). With permissible genetics, the next step in the adept clinician's handbook is the gluten challenge.
THE GLUTEN CHALLENGE
Without an effective re-exposure to gluten, individuals may remain undiagnosed or misdiagnosed, particularly if they have nonceliac gluten sensitivity or other conditions (e.g., IBS) that may symptomatically respond to a GFD. A gluten challenge typically involves reintroducing at least 3 g of gluten per day, although some studies suggest that up to 10 g may be required for there to be significant biochemical marker changes (Table 2) (). Ideally, patients should consume >3 g/d for 8 weeks, if tolerated, although the exact dose and duration are up to clinician discretion and patient tolerance (). It is the authors' practice to recommend consumption of 6-10 g/d. The amount of gluten required to provoke a detectable immune response can vary between individuals; antibody titers can increase as early as 2 weeks, but markedly rise by 4 weeks ().
Several common household food items may be considered to reach optimal daily gluten dosing (Figure 3); however, crackers or slices of bread are practical, available, and often recommended. For patients reporting to be sensitive to high-FODMAP fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAP) foods, lower-FODMAP alternatives such as vital wheat gluten or gluten powder can be used as a more controlled form of gluten exposure -- it can be blended into lactose-free milk or juice, per preference. This can be achieved by having patients consume an escalating amount of powder (from half teaspoon up to 4 teaspoons per day) for a designated interval before re-checking blood tests.
In addition, careful guidance and close monitoring during these trials are essential to ensuring their effectiveness and minimizing patient discomfort. Medications for symptoms such as simethicone, loperamide, and ondansetron can be used for patients experiencing bothersome bloating, diarrhea, or nausea during the challenge, respectively. Reassurance that any return of symptoms will be temporary (along with consequential damage to the villi in CeD) and that medications are available for symptom relief is typically sufficient to prevent symptomatic patients from terminating the challenge early. A proactive conversation about gluten challenge expectations will also help identify the subset of patients who may refuse to undergo a gluten challenge due to fear of symptom recurrence and perceived benefits of a long-term GFD. If refusal is firm, a risk-benefit discussion should take place about dangers of not confirming a CeD diagnosis and downstream consequences without strict GFD adherence weighed against symptom benefits of continued gluten avoidance.
Dangers of not confirming a diagnosis include lack of monitoring for key nutritional deficiencies, suboptimal bone health, increased risk of certain malignancies, and slightly increased mortality risk (). In addition, following a GFD with or without CeD is expensive. On average, gluten-free foods are up to 183% more expensive than their gluten-containing counterparts. In some countries, stipends or prescriptions are offered for those with a medical diagnosis of CeD to offset these high costs ().
CASE RESOLUTION
Following his initial relief on a GFD but without a formal diagnosis, John's primary care physician referred him to a gastroenterologist. TTG-IgA was negative indicating strict GFD adherence. Genetic profiling revealed HLA-DQ2.5 (alleles DQA1*5:01 and DQB1*2:01). He underwent a medically supervised gluten challenge to evaluate for CeD where he reintroduced gluten for 8 weeks, consuming ∼6 g/d, while under clinical observation. During this period, he experienced a recurrence of his GI symptoms, and subsequent blood tests showed elevated TTG-IgA levels, confirming an immune response to gluten. Duodenal biopsy showed villous atrophy.
John's now definitive CeD diagnosis gave him a clear medical reason to continue the GFD. This formal diagnosis will allow him to access dietitian support, monitor for potential nutrient deficiencies, and have guidance for managing his condition safely.
Had his evaluation revealed a slightly positive serology or increased intraepithelial lymphocytes only, this would not have been diagnostic of CeD. In that instance, considering medications such as nonsteroidal anti-inflammatory drugs, inflammatory bowel disease, and a false positive laboratory result would be the next steps along with the differential diagnosis of food-related symptoms (irritable bowel syndrome, fructan intolerance, and nonceliac gluten sensitivity). Clinical monitoring, especially if the patient has genetic permissibility, with a follow-up visit in 6-12 months would have been reasonable.
NOVEL TECHNIQUES
Recent advances have explored novel techniques which may aid in the diagnosis of CeD regardless of diet, with a particular focus on immune-based assays. One such innovation involves the use of tetramer-based assays, which enable the direct detection of gluten-specific T cells in the blood. These tetramers, designed to bind to HLA-DQ2 molecules, can help identify T cells that have been activated by gluten exposure. This presents a highly specific immune marker for CeD. Even for those on a GFD, sensitivity (97%) and specificity (95%) have been impressive (). Another promising diagnostic technique is the measurement of interleukin-2 (IL-2) expression. Upon gluten exposure, IL-2 is released and detecting this cytokine in patient blood samples offers a real-time marker of immune response to gluten. Studies have shown that IL-2 expression can rise rapidly after gluten exposure, even in GFD patients, making it a valuable tool for diagnosis (). These techniques represent a shift toward more direct and dynamic assessments of the immune response in CeD, potentially offering more accurate diagnostic options in the context of GFD patients. Although currently only used in research settings, we hope that further validation and expanded access will allow use in clinical practice imminently.
CONFLICTS OF INTEREST
Guarantor of the article: Alberto Rubio-Tapia, MD.
Specific author contributions: R.R.S., C.L.J.-K., D.K., and A.R.-T.: concept. R.R.S.: drafting of manuscript. R.R.S., C.J.K., D.K., and A.R.-T.: critical editing of manuscript. A.R.-T.: supervision of review.
Financial support: C.L.J.-K. was supported by AGA Research Foundation's AGA-Takeda Pharmaceuticals Research Scholar Award in Celiac Disease (AGA2022-13-07).
Potential competing interests: None to report.