Celiac Disease: Which Blood Tests Confirm or Rule It Out?

Celiac disease is an autoimmune disorder triggered by gluten — the protein found in wheat, barley, and rye. When someone with celiac eats gluten, their immune system attacks the small intestine's lining, damaging the villi (the small finger-like projections that absorb nutrients). Over time, this damage causes nutrient malabsorption and a wide range of symptoms that can affect virtually any body system.

The complication: celiac symptoms are wildly variable. The "classic" presentation (chronic diarrhea, weight loss, malabsorption) is actually the minority of cases. Most adults with celiac present with:

• Atypical or "silent" symptoms — fatigue, brain fog, mood changes, joint pain, headaches, infertility, anemia (iron-deficiency that won't respond to iron supplementation), osteoporosis, dental enamel defects, or skin issues (dermatitis herpetiformis is celiac-specific)
• Subtle digestive symptoms — intermittent bloating, occasional diarrhea or constipation, mild abdominal discomfort that doesn't seem alarming enough to investigate
• Asymptomatic with nutritional deficiencies — found incidentally through routine bloodwork showing low iron, low B12, low vitamin D, or osteoporosis

Prevalence: about 1% of the US population has celiac disease. An estimated 60-80% are undiagnosed. The average time from symptom onset to diagnosis is 6-10 years. Most people who have it haven't been tested because the symptoms didn't seem dramatic enough — but the long-term consequences of untreated celiac (nutritional deficiencies, osteoporosis, increased lymphoma risk, infertility, neurological issues) are significant. Testing is high-yield, especially for people with any of the risk factors below.

Who should get tested:

• Persistent digestive symptoms (bloating, intermittent diarrhea, abdominal discomfort)
• Iron-deficiency anemia that doesn't respond to iron supplementation
• Unexplained chronic fatigue or brain fog
• Osteoporosis under age 60
• First-degree relative with celiac disease (10% chance you have it too)
• Type 1 diabetes (5-10% prevalence of celiac)
• Hashimoto's thyroiditis or other autoimmune conditions (celiac frequently coexists)
• Unexplained infertility or recurrent pregnancy loss
• Dermatitis herpetiformis (an itchy, blistering skin rash that's celiac-specific)
• Children with delayed growth, dental enamel defects, or chronic GI symptoms

This is the single most important thing to know about celiac testing: you must be eating gluten regularly when you do the blood test. The antibodies that the test measures (tTG, DGP, EMA) only get produced when your immune system is actively reacting to gluten. If you've been gluten-free for weeks or months before the test, the antibody levels drop and the test will be falsely negative — even if you have celiac disease.

The specific requirement: eat at least 1-2 servings of gluten daily (a slice of bread, a serving of pasta, a few crackers) for at least 6-8 weeks before testing. Some experts recommend longer for already-on-a-gluten-free-diet patients (up to 12 weeks of "gluten challenge").

If you're already gluten-free and don't want to do a gluten challenge: you have two options. (1) Genetic testing for HLA-DQ2/HLA-DQ8 — these genes are present in essentially everyone with celiac (and ~30-40% of the general population). A negative result rules out celiac with high confidence; a positive result means celiac is possible but not diagnostic. (2) Stay gluten-free and proceed based on symptom response — practical but doesn't confirm diagnosis. The downside of going gluten-free without a formal diagnosis is that it's harder to motivate strict adherence (which celiac requires) without the diagnosis to anchor it.

Why this matters: we frequently see patients order celiac blood tests after already trying a gluten-free diet for a few months. They get a negative result and assume they don't have celiac. But the negative result is meaningless because they weren't eating gluten. The right approach is either: (a) eat gluten regularly before testing, or (b) accept that a definitive celiac answer will require either genetic testing or a formal gluten challenge with retesting.

Modern celiac serology uses multiple antibody markers for sensitivity and specificity. The recommended panel:

1. tTG-IgA (Tissue Transglutaminase IgA antibody). The first-line screening test — highly sensitive (95%) and specific (95-97%) for celiac. Reflects active immune response to gluten exposure. The single best test if only one can be ordered.

2. Total IgA. Critical accompaniment to tTG-IgA. About 2-3% of celiac patients have selective IgA deficiency (a separate immune condition), which means their tTG-IgA will be falsely negative because they can't produce IgA antibodies at all. Total IgA confirms that you CAN produce IgA, validating that the negative tTG-IgA is a true negative. If you're IgA-deficient, additional testing (DGP-IgG, tTG-IgG) is needed.

3. DGP-IgA + DGP-IgG (Deamidated Gliadin Peptide antibodies). Newer markers with high sensitivity. Particularly useful in children under 2 (where tTG-IgA has lower sensitivity) and in IgA-deficient patients. The DGP-IgG specifically catches IgA-deficient cases.

4. EMA (Endomysial Antibodies). Very specific (close to 100%) but more expensive and operator-dependent than tTG-IgA. Sometimes used as confirmatory testing when tTG-IgA is positive but biopsy isn't planned, or when results are equivocal.

The Celiac Disease Comprehensive Panel ($76.99) at TestWell includes tTG-IgA, DGP-IgA, DGP-IgG, and total IgA — the most useful combination for screening and avoiding the IgA-deficiency false negative.

Interpreting results:

• tTG-IgA positive (and total IgA normal): Likely celiac. Standard next step is upper endoscopy with small intestine biopsy to confirm and assess damage severity. Strongly positive tTG-IgA (10x upper limit of normal) in symptomatic patients is sometimes sufficient for diagnosis without biopsy, per recent European guidelines.
• tTG-IgA negative + total IgA normal: Celiac unlikely (high negative predictive value). If symptoms persist and suspicion remains high, consider genetic testing (HLA-DQ2/DQ8) to definitively rule out, or repeat testing in 6-12 months.
• tTG-IgA negative + total IgA low (IgA deficient): The tTG-IgA result is unreliable. Check DGP-IgG and tTG-IgG instead. If positive, proceed to biopsy.
• Equivocal or weakly positive: Repeat testing in 3-6 months on a gluten-containing diet, or proceed to endoscopy depending on symptoms.

Important note on confirmation: The gold standard for celiac diagnosis is small intestinal biopsy via upper endoscopy. Positive blood tests warrant referral to a gastroenterologist for biopsy in most adult cases. This is true even if symptoms are mild — biopsy assesses how much damage is present and provides baseline data for monitoring response to a gluten-free diet.

If celiac is confirmed: Treatment is a strict, lifelong gluten-free diet. This isn't gluten-light or "mostly gluten-free" — celiac requires complete elimination because even tiny amounts of gluten (50 mg/day, or 1/100th of a slice of bread) can cause intestinal damage. A registered dietitian with celiac expertise is invaluable for navigating gluten-free living, hidden gluten sources, and ensuring nutritional adequacy.

Monitoring on a gluten-free diet:

• tTG-IgA every 6-12 months for the first 2 years, then annually. Should fall and normalize over 12-24 months on a strict gluten-free diet. Persistently elevated tTG-IgA despite gluten-free diet usually indicates gluten contamination (intentional or accidental).
• CBC + iron studies (Iron Panel) annually. Iron-deficiency anemia is the most common nutritional complication of celiac and often persists for 12-24 months after starting GFD as the intestine heals.
• Vitamin D, B12, folate annually. All commonly deficient in newly diagnosed celiac and require active supplementation during healing.
• TSH + Free T4 annually. Hashimoto's thyroiditis frequently coexists with celiac (autoimmune cluster).
• Bone density (DEXA scan) at diagnosis and follow-up — celiac patients have increased osteoporosis risk due to long-term nutrient malabsorption.
• Lipid panel — interestingly, untreated celiac is associated with low cholesterol; cholesterol levels often rise after gluten-free diet starts. Monitor for new dyslipidemia.

Other workups to consider given autoimmune clustering: Celiac frequently coexists with other autoimmune conditions. Worth screening if symptoms suggest: TPO antibodies (Hashimoto's thyroiditis), ANA (lupus and related), GAD antibodies (type 1 diabetes risk), if you have new or worsening symptoms beyond celiac itself.

Family screening: First-degree relatives (parents, siblings, children) have ~10% risk of celiac vs 1% general population. Screening with tTG-IgA + total IgA is recommended at age 3+ for at-risk family members, repeated every 2-3 years through adulthood since celiac can develop at any age.