Celiac Disease and Non-Celiac Gluten Sensitivity Screening in the Sports Nutrition Intake: The tTG-IgA Plus Total-IgA Workup, the Elimination-and-Rechallenge Protocol, and Why the Athlete Presenting With GI Distress, Iron-Refractory Ferritin, and Chronic Fatigue Deserves the Screen Before the Diet

A 26-year-old collegiate 10,000-meter runner presents at intake with what her coach describes as "the same problem, year three." Her ferritin, drawn six weeks ago, is 18 ng/mL — the third value under 25 across the last 14 months, all three drawn after an oral iron trial her sports-medicine physician wrote for the previous low reading. She has been on ferrous sulfate 325 mg once daily for the last 90 days and her recheck moved the ferritin from 14 to 18. Her hemoglobin is 12.4 g/dL with an MCV of 79 fL. She reports bloating she describes as "only bad on long-run days" and loose stools two to four times a week that she has attributed to the coach's mid-camp pasta feeds. Her fatigue is her third-year complaint; her coach has begun using the words "overtraining syndrome" in her weekly check-in. She tried a gluten-free eight-week experiment last May on the recommendation of a teammate, felt "noticeably better," ate a plate of pasta at family Thanksgiving in November and reported "the worst bloating of the year," and has been eating a normal gluten-containing diet through the winter track season. She is sitting in your intake asking whether she should "just cut gluten again."

The answer, if you give it as a diet recommendation, is the single most common preventable misstep the sports RD makes in this presentation. Recommending a gluten-free trial before serologic celiac screening in a gluten-consuming patient collapses your ability to test her at all. The celiac serology depends on the immune response to gluten exposure; a patient who removes gluten for six to eight weeks before serologic draw will produce a false-negative tTG-IgA in a substantial fraction of true celiac cases, and a diagnostic-quality small-bowel biopsy after gluten withdrawal is not reliable either. Once the athlete has been off gluten for weeks and "feels better," the diagnostic window has closed without a confirmed diagnosis, and the athlete is now on an indefinite restrictive diet that may or may not be medically necessary and that carries downstream nutritional consequences the RD is now responsible for managing without a diagnosis to justify them.

The correct workflow is the inverse. When the intake surfaces any of the three high-yield presentations — GI distress, iron-refractory ferritin, or unexplained fatigue that the standard sports-medicine workup has not explained — the sports RD's job is to (a) hold the diet-change recommendation, (b) run or order the celiac serologic screen against a gluten-containing diet, (c) refer to gastroenterology for biopsy confirmation when the serology is positive, and (d) after celiac is ruled out, run a structured gluten elimination-and-rechallenge protocol under the RD's supervision to test for non-celiac gluten sensitivity. Below is the structured workup the sports RD should embed in the intake when any of the three trigger presentations shows up: the epidemiology and pre-test probability calculation, the tTG-IgA plus total-IgA primary screen, the reflex panel when the primary screen is equivocal, the gastroenterology referral pathway, the elimination-and-rechallenge protocol for NCGS, the nutritional consequences to manage after a positive celiac diagnosis, and the SOAP pattern that documents the workup defensibly.

Why the Sports RD Is the Right Screener

Celiac disease affects roughly 1% of the U.S. general population, with substantially higher prevalence in the specific subgroups the sports-nutrition intake sees weekly: patients with iron-deficiency anemia refractory to oral iron have celiac prevalence around 3 to 6%, patients with chronic diarrhea or unexplained GI complaints run 3 to 15% prevalence depending on the study population, and first-degree relatives of celiac patients run 5 to 10%. The distance-runner population in particular has an elevated pre-test probability for celiac because iron loss through foot-strike hemolysis, GI microbleeding, and sweat losses make the athlete more sensitive to any concurrent absorptive defect than a matched non-athlete.

Celiac diagnosis is chronically delayed in the general adult population — mean time from symptom onset to diagnosis in U.S. adults is 6 to 10 years — because the presenting symptom cluster (fatigue, GI variability, iron issues) is common across many conditions and the specific test to order is not on the primary-care physician's standard workup for a young athlete complaining of training-related fatigue. The sports RD sees the exact patient population most likely to be missed by the standard primary-care pathway: a young, symptomatic athlete whose complaints get attributed to "training stress" or "low iron" without the further-down-the-differential celiac screen. The RD's advantage over the primary-care physician is threefold. First, the intake is where the diet-symptom correlation is captured in the level of detail that surfaces the celiac pattern — the "pasta-day bloating" and "gluten-free trial felt better" data points are the kind of history-of-present-illness detail a 15-minute primary-care visit does not have time to elicit. Second, the RD is the practitioner the athlete asks about diet changes, which makes the RD the last-and-only checkpoint before the athlete self-initiates the gluten-free trial that collapses the diagnostic window. Third, the RD can order or recommend the correct panel — tTG-IgA plus total IgA — with the correct pre-analytic condition (gluten-containing diet for at least six weeks before the draw) and can hold the diet recommendation until the workup is complete, which is exactly the discipline the athlete's self-directed experiment did not provide.

The Three Trigger Presentations That Should Fire the Screen

Not every athlete presenting to a sports-nutrition intake needs a celiac serologic screen. The screen belongs in the workup when any of the following three presentations is on the table.

Trigger one: iron-refractory anemia or ferritin. An athlete whose ferritin has failed to climb above 30 ng/mL after 90 days of adherent oral iron at a therapeutic dose (typically ferrous sulfate 325 mg one to three times daily depending on tolerance), or whose hemoglobin has failed to normalize after the same trial, has an absorptive problem that oral iron alone will not fix and warrants the celiac screen before the next iron trial. Foot-strike hemolysis, GI microbleeding from NSAID use, and sweat losses can all contribute to iron loss in the athlete population, but if the iron intake and dose are documented adequate and the ferritin response is inadequate, the differential must include a proximal small-bowel malabsorptive process. Celiac damages the duodenal villi that are the primary site of iron absorption, and iron-refractory anemia is the second-most-common presenting picture for adult celiac after the classic GI-symptom triad.

Trigger two: unexplained GI symptoms that correlate with gluten-containing meals. The athlete who reports bloating, loose stools, abdominal cramping, or a subjective "heavy" post-meal feeling that clusters around bread-, pasta-, or beer-heavy meals warrants the screen even if the symptom pattern is intermittent. The classic celiac presentation of chronic steatorrhea and weight loss is uncommon in modern diagnosed cases; the more typical presentation is milder GI variability, and the athlete population in particular can misattribute the symptoms to training-load-related GI distress or to a specific race-week fueling protocol. The correlation with gluten-containing meals is a soft signal but not a strong one for celiac specifically — many other food intolerances (FODMAP-sensitive IBS, fructan-specific sensitivity, gluten-independent wheat sensitivities) produce the same pattern — and the serologic screen is the tool that separates the celiac cases from the rest.

Trigger three: chronic fatigue that has resisted the standard sports-medicine workup. An athlete whose fatigue complaint has been evaluated for RED-S, overtraining, thyroid dysfunction, iron deficiency, and vitamin D insufficiency without a clear answer, and whose training and fueling logs read as adequate, warrants a celiac screen. The mechanism connecting celiac to fatigue is multifactorial — malabsorption of iron and B12, folate depletion from villous atrophy, chronic low-grade systemic inflammation, and the metabolic cost of the immune response — but the empirical observation is that adult celiac patients frequently present with fatigue as the dominant complaint, and the fatigue resolves within three to six months of gluten withdrawal in the biopsy-confirmed cases.

An athlete with any one of these three triggers has a pre-test probability high enough to justify the serologic screen. An athlete with two or three of the three has a pre-test probability that makes the screen a defensible standard-of-care recommendation.

The tTG-IgA Plus Total-IgA Primary Screen

The primary serologic screen for celiac disease in the adult population is IgA-class tissue transglutaminase antibody (tTG-IgA), ordered alongside a total serum IgA level. Both matter. The tTG-IgA is the sensitivity-optimized, specificity-optimized single test with sensitivity around 93% and specificity around 97% in adult populations with untreated celiac who have been on a gluten-containing diet for at least six weeks before the draw. The total IgA is not itself a celiac test — it is the check for selective IgA deficiency, a condition that affects roughly 1 in 500 in the general population but is 10 to 15 times more common in celiac patients (roughly 2 to 3% of adult celiacs have selective IgA deficiency). A patient with IgA deficiency will produce a false-negative tTG-IgA regardless of celiac status because they cannot mount the IgA-class antibody response the test is measuring. Ordering the tTG-IgA without a total IgA is a common pre-analytic error that produces a false-negative rate high enough to matter.

Two pre-analytic conditions determine whether the tTG-IgA result is interpretable. First, the patient must be on a gluten-containing diet for at least six weeks before the draw; the IgA-class response requires ongoing antigenic exposure, and a patient who has removed gluten will produce a falsely low or negative tTG-IgA within four to eight weeks of withdrawal. Second, the lab reference range and cutoff should be from the specific commercial assay the lab is running; commercial tTG-IgA assays vary in their cutoff units (some report in U/mL, others in a normalized "index" with a cutoff at 1.0), and the interpretation must reference the specific assay's cutoff. A tTG-IgA that is more than 10x the assay's upper limit of normal has a specificity for celiac disease so high in the adult population that some pediatric guidelines have moved to a biopsy-free diagnostic pathway on that value alone; in adults, biopsy confirmation is still the standard.

Result interpretation runs on a simple decision tree. If the total IgA is normal and the tTG-IgA is above the assay's cutoff, refer to gastroenterology for endoscopic small-bowel biopsy to confirm the diagnosis. If the total IgA is normal and the tTG-IgA is below the cutoff, celiac is unlikely and the workup pivots to the NCGS elimination-and-rechallenge or to other differentials depending on the presentation. If the total IgA is low (selective IgA deficiency), the primary screen is uninterpretable and the reflex panel — IgG-class deamidated gliadin peptide (DGP-IgG) and tTG-IgG — is ordered instead. If the tTG-IgA is equivocal (borderline positive) and the total IgA is normal, the reflex panel is endomysial antibody IgA (EMA-IgA) plus DGP-IgG; the EMA-IgA has a specificity around 99% and is the test that resolves an equivocal primary screen.

The one test to avoid ordering as a primary screen is a stand-alone anti-gliadin antibody (AGA) IgA or IgG. The older AGA tests were superseded by the tTG and EMA panels a decade ago because AGA has both a substantially lower specificity and a substantially lower sensitivity than the tTG in adult celiac. If the athlete arrives with an outside AGA result showing a positive value and nothing else, the AGA does not establish celiac and does not rule it out; the workup restarts with a tTG-IgA plus total IgA on a documented gluten-containing diet.

The Gastroenterology Referral and Biopsy Confirmation

A positive tTG-IgA in an adult with a compatible presentation is the trigger for gastroenterology referral for endoscopic small-bowel biopsy. The biopsy is the confirmatory diagnostic in adult celiac; the serologic screen does not diagnose the disease on its own in adults regardless of how high the tTG-IgA runs. The biopsy quantifies the degree of villous atrophy on the Marsh classification (Marsh 0 through 3c), documents intraepithelial lymphocyte infiltration, and provides the histologic baseline against which post-gluten-free-diet response will be measured on the follow-up biopsy at one year.

The gastroenterology referral requires two pieces of information from the RD: the tTG-IgA result with the specific assay's cutoff and reference range, and confirmation that the patient has been on a gluten-containing diet for at least six weeks before the serologic draw and remains on gluten pending the biopsy. If the patient has removed gluten at any point between the serologic screen and the biopsy, the biopsy sensitivity drops meaningfully — the villous atrophy begins to heal within weeks of gluten withdrawal — and the gastroenterologist may require a gluten-challenge protocol (typically 3 g of gluten per day for six weeks) before rebiopsy. The RD's job through this window is to hold the gluten-free diet recommendation until the biopsy is done. This is the moment the athlete is most likely to jump to the diet change on their own; the RD's counseling script is: "The serologic screen came back positive. To get to a confirmed diagnosis you need the biopsy, and the biopsy needs you to keep eating gluten until it happens. If you go gluten-free now, you will feel better in two to four weeks, but you may lose the ability to get a clean diagnosis and you may end up on a lifelong restrictive diet without medical confirmation that it is necessary. Give the biopsy 30 to 60 days. Then we start the gluten-free diet with a confirmed diagnosis behind it."

If the biopsy confirms Marsh 2 or higher villous atrophy or intraepithelial lymphocytosis with a positive serology, the diagnosis is confirmed and the treatment is a lifelong strict gluten-free diet — the only currently approved treatment for celiac disease. The follow-up serologic tTG-IgA at 6 and 12 months tracks the immune response to the diet (the number should trend down toward the cutoff within 6 to 12 months of adherent gluten avoidance), and the follow-up biopsy at 12 to 24 months documents mucosal healing. The RD's role transitions from screening to long-term co-management: gluten-free diet education, nutritional-adequacy monitoring in the context of the restriction, cross-contamination coaching, and the specific micronutrient panel to run at 6-month intervals post-diagnosis.

Non-Celiac Gluten Sensitivity: The Diagnosis of Exclusion, After the Celiac Screen Is Negative

Non-celiac gluten sensitivity (NCGS) is a distinct clinical entity from celiac disease. It has no validated serologic marker, no histologic finding on biopsy, and no known autoimmune mechanism. It is diagnosed clinically by symptomatic improvement on a structured gluten elimination and symptomatic recurrence on a blinded or open rechallenge, after celiac disease and wheat allergy have been ruled out on a gluten-containing diet. The prevalence in the general population is uncertain but likely in the 0.5 to 6% range depending on the study definition; the self-reported prevalence of "feeling better without gluten" is substantially higher, and one of the RD's jobs is to separate the small subset of true NCGS from the much larger group whose subjective improvement on gluten-free eating traces to FODMAP reduction, fructan avoidance, generalized dietary quality improvement, or placebo effect.

The elimination-and-rechallenge protocol runs in three phases. Phase one (baseline symptom tracking, 2 weeks): the athlete tracks GI symptoms, energy, and any other complaint targets on a standardized daily symptom scale (a validated instrument such as the Gastrointestinal Symptom Rating Scale [GSRS] or a simpler numeric rating scale for the athlete's top three symptoms works) while remaining on a normal gluten-containing diet. The baseline establishes what the symptom pattern looks like when gluten is present. Phase two (elimination, 4-6 weeks): the athlete removes all gluten-containing foods (wheat, barley, rye, and their derivatives) and continues the daily symptom tracking. Six weeks is the standard elimination window in the NCGS literature — long enough to catch delayed responders, short enough to avoid the athlete anchoring on the restriction. Phase three (rechallenge, 1-2 weeks): the athlete reintroduces gluten in a controlled way — typically 10 to 20 g of gluten per day, roughly the equivalent of 2 slices of bread or a serving of pasta — and continues the symptom tracking. A meaningful symptom recurrence during rechallenge that did not appear during elimination is the clinical signal for NCGS.

The blinded-rechallenge version of the protocol is the research-grade standard but rarely practical in the sports-RD clinic setting; the open rechallenge, with the RD's explicit acknowledgment to the athlete that the placebo response is well-documented and that a modest symptom recurrence on rechallenge should not be over-interpreted, is what most clinical practice runs. The two most common failures of the open-rechallenge protocol are (a) the athlete's expectation of symptoms during rechallenge inflating the reported symptom score and (b) the athlete not completing the full 6-week elimination and interpreting a partial response as diagnostic. The RD's role is to hold the protocol to spec: no early rechallenge, no partial elimination, no diagnostic conclusion drawn from a 2-week trial.

If the elimination-and-rechallenge is positive — clear symptom reduction on elimination, clear symptom recurrence on rechallenge — the working diagnosis is NCGS and the treatment is a gluten-free or gluten-reduced diet titrated to symptom control. Unlike celiac, NCGS does not require strict avoidance for autoimmune-remission reasons; the diet is titrated to the individual's tolerance threshold, which varies. If the elimination phase produces no symptom improvement or the rechallenge produces no symptom recurrence, the working diagnosis is not NCGS, and the workup pivots to the FODMAP differential (low-FODMAP trial as the next elimination protocol), the SIBO workup (breath testing), or the primary-care GI workup for other differentials.

The Nutritional Consequences the RD Owns After a Confirmed Celiac Diagnosis

A confirmed celiac diagnosis flips the RD's role from screening to long-term co-management. The gluten-free diet is medically necessary and non-negotiable, but a gluten-free diet done poorly carries meaningful nutritional consequences that the RD is the practitioner best positioned to prevent and monitor. The high-yield issues in the celiac athlete population are fiber deficiency (gluten-containing whole grains are a major fiber source in most Western diets), B-vitamin gaps (thiamin, riboflavin, folate, and niacin are added by law to fortified wheat products; most gluten-free flours are not fortified), iron deficiency compounding the pre-diagnosis absorptive issue if the gluten-free-diet iron intake is inadequate, and inadequate carbohydrate intake in the endurance athlete who has not learned to work rice, quinoa, potato, and gluten-free oats into the training diet in the volumes the training load requires.

The 6-month post-diagnosis micronutrient panel to consider ordering (in coordination with the athlete's physician) covers ferritin, vitamin B12, folate (RBC folate is more specific than serum folate), 25-OH vitamin D, and — in the pediatric or high-training-load athlete — the zinc and copper status. The RD's dietary counseling should specifically address the fiber-adequacy question (target 25 g/day minimum from gluten-free whole grains, legumes, fruits, and vegetables), the B-vitamin adequacy question (a gluten-free-diet-specific multivitamin is defensible for the first 6 to 12 months post-diagnosis until dietary adequacy is verified), the training-carbohydrate adequacy question for the endurance athlete (the pre-training and long-session fueling protocols need to be re-engineered around gluten-free carbohydrate sources), and the cross-contamination coaching for the athlete who eats at team training tables, restaurants, and travel meals.

The follow-up cadence for the confirmed celiac athlete is a nutrition visit at 3 months to check adherence and initial adequacy, at 6 months to review the follow-up serology and micronutrient panel, and at 12 months to align with the follow-up biopsy and re-assess. Beyond 12 months, an annual nutrition follow-up with the specific labs above is the standard for the celiac population and belongs on the Calsanova recurring-follow-up template.

The SOAP Pattern That Documents the Workup Defensibly

Under Subjective, document the presenting complaint(s) that triggered the screen (which of the three high-yield presentations — iron-refractory ferritin, GI symptoms with gluten correlation, unexplained fatigue), the athlete's prior gluten-free-diet history in detail (any self-directed trial, its duration, the athlete's report of symptom change), and the current dietary status (gluten-containing versus gluten-free at the time of intake). The prior-trial and current-diet documentation is critical because it determines whether a serologic screen can be ordered now (gluten-containing) or requires a gluten-challenge protocol first (currently gluten-free).

Under Objective, document the trigger labs (ferritin trend across recent draws, hemoglobin, MCV, any prior serologic celiac testing with specific assay and cutoff), the intake symptom-tracking baseline if collected, and the results of any current serologic screen ordered — tTG-IgA value with the assay's cutoff, total IgA value with the reference range, and the reflex-panel results (DGP-IgG, EMA-IgA) if the primary screen was equivocal or the total IgA was low.

Under Assessment, name the working diagnosis based on the specific data — for example, "tTG-IgA 87 U/mL (assay cutoff 20 U/mL, more than 4x cutoff) with normal total IgA (238 mg/dL), on documented gluten-containing diet, in the setting of iron-refractory ferritin (18 ng/mL after 90 days of ferrous sulfate 325 mg daily) and chronic fatigue — high pre-test probability, strong serologic support for celiac disease, biopsy confirmation pending gastroenterology consult." If the workup is at the pre-serology stage, the assessment names the pre-test probability and the plan to screen. If the celiac serology is negative and the workup is pivoting to NCGS elimination-and-rechallenge, the assessment names the negative celiac result and the elimination-protocol plan.

Under Plan, document the specific next steps — the labs ordered (or recommended to the athlete's physician to order, if the RD's scope of practice or state licensure does not permit direct lab ordering), the gastroenterology referral with the specific referral partner and the target appointment window, the pending-biopsy dietary hold (the athlete stays on gluten until the biopsy), the RD follow-up appointment scheduled for post-biopsy diet initiation, and the specific micronutrient labs to schedule at the 3- and 6-month follow-up windows if the diagnosis is confirmed.

The three things the SOAP note must not do: it must not recommend a gluten-free trial before the celiac serology is completed on a gluten-containing diet (this collapses the diagnostic window and shifts the athlete to an indefinite restrictive diet without confirmation), it must not diagnose celiac disease on serology alone in an adult without biopsy confirmation (the diagnosis is a two-test standard in adult practice), and it must not conflate NCGS with celiac in the counseling record (they are distinct entities with distinct treatment implications — NCGS is symptom-titrated, celiac is strict lifelong avoidance).

The Adjacent Screens Worth Adding to the Intake

Three additional screens fold into the same 90-minute intake and complement the celiac workup by covering the closest differentials.

Wheat allergy versus celiac. Wheat allergy is a distinct IgE-mediated food allergy that presents with acute symptoms (urticaria, angioedema, wheeze, or anaphylaxis within minutes to hours of wheat exposure) and is diagnosed with skin prick testing or wheat-specific IgE serology; it is not celiac disease and is not diagnosed by tTG-IgA. In the athlete population, wheat-dependent exercise-induced anaphylaxis (WDEIA) is a specific presentation — the athlete tolerates wheat at rest but reacts to wheat consumed within 4 hours of vigorous exercise. WDEIA is the differential to keep in mind when the athlete reports an acute reaction pattern tied to training meals rather than a chronic bloating-and-fatigue pattern, and the referral is to allergy rather than gastroenterology.

FODMAP-driven IBS as the alternative diagnosis. The largest group of patients whose symptoms improve on a self-directed gluten-free diet are the FODMAP-sensitive IBS patients whose real trigger is the fructans in wheat, not the gluten. If the celiac serology is negative and the NCGS elimination-and-rechallenge is also negative, the next protocol is a low-FODMAP trial with a structured reintroduction phase, run in coordination with a gastroenterologist. The distinction between NCGS and fructan-sensitive IBS matters for long-term dietary management because the fructan-sensitive patient can tolerate other gluten-containing but low-fructan foods (spelt, sourdough) whereas the NCGS patient may not.

SIBO as the other alternative. Small intestinal bacterial overgrowth (SIBO) can produce a symptom picture — bloating, altered bowel pattern, malabsorption — that overlaps substantially with celiac and NCGS, and the workup is a breath-hydrogen or breath-methane test after a specific pre-test carbohydrate load, ordered by a gastroenterologist. SIBO is a common finding in the endurance athlete population and often coexists with the other diagnoses; a negative celiac screen and a negative NCGS trial in a patient with ongoing symptoms should trigger the SIBO workup rather than an indefinite continuation of restrictive dietary trials.

The 26-year-old runner in the intake vignette does not need a repeat iron trial and does not need an early diet-change recommendation. She needs a tTG-IgA plus total IgA on her current gluten-containing diet, a gastroenterology referral if the serology is positive, and a structured NCGS elimination-and-rechallenge under the RD's supervision if the celiac workup is negative. The screen belongs in every adult sports nutrition intake that surfaces the three trigger presentations. The Calsanova intake template captures the trigger-presentation flags as discrete fields, prompts the tTG-IgA plus total IgA panel automatically when any trigger is checked, and rolls the pre-test-probability calculation into the SOAP note — the RD gets the defensible chart trail and the athlete gets the workup done in the correct order. The single largest preventable error the sports RD makes on this presentation is the pre-serology gluten-free-diet recommendation. Hold the diet. Run the screen. Then decide.