Celiac Disease

USMLE Step 1 trap: Selects anti-gliadin antibodies as the preferred serologic test rather than anti-tTG IgA. Anti-tissue transglutaminase IgA (anti-tTG IgA) is the most sensitive and specific initial serologic test for celiac disease; anti-gliadin antibodies are less specific.

Celiac disease is a T-cell mediated autoimmune enteropathy triggered by dietary gluten (specifically its component gliadin) in genetically susceptible individuals carrying HLA-DQ2 or HLA-DQ8. The result is progressive villous atrophy in the proximal small bowel, leading to malabsorption of fat-soluble vitamins, iron, calcium, and folate. On USMLE Step 1, this topic shows up across all three test formats — pure recall on HLA associations and serology, application questions asking you to explain why a patient has anemia or osteoporosis, and vignette interpretation where you have to recognize celiac from indirect clues like dermatitis herpetiformis or unexplained iron-deficiency anemia that doesn't respond to oral iron.

The exam loves to probe the gap between what students think is the gold standard serologic test versus what actually is. Anti-gliadin antibodies are a classic trap — they were used historically but are far less specific than anti-tTG IgA, which is the correct first-line test. A second major trap is the IgA deficiency pitfall: celiac patients have a higher-than-average prevalence of selective IgA deficiency, which will cause a false-negative anti-tTG IgA result if you don't check total serum IgA first. This is a high-yield detail that distinguishes strong test-takers.

The pathologic anatomy also trips people up. Students often guess the ileum as the biopsy site because 'that's where absorption happens,' but the duodenum and proximal jejunum are where gluten exposure is highest and villous atrophy is most pronounced — that's where you biopsy. USMLE Step 1 will also test the malignancy associations: enteropathy-associated T-cell lymphoma (EATL) and small bowel adenocarcinoma are the relevant risks, not colorectal cancer. Tying all of this to dietary management and serology-based compliance monitoring (anti-tTG titers fall with adherence) rounds out the high-yield picture.

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Common misconceptions

Common mistake

Wrong: Anti-gliadin antibodies are the most specific serologic test for celiac disease.

Right: Anti-tissue transglutaminase IgA (anti-tTG IgA) is the most sensitive and specific initial serologic test for celiac disease; anti-gliadin antibodies are less specific.

Anti-gliadin antibodies were part of older celiac panels but have poor specificity — many people without celiac can have them. Anti-tissue transglutaminase IgA (anti-tTG IgA) is the current first-line serologic test because tTG is the autoantigen that deamidated gliadin peptides actually activate, making it both more sensitive and more specific. On Step 1, if a question asks which test to order first or which is most specific, the answer is anti-tTG IgA, not anti-gliadin.

Common mistake

Wrong: Celiac disease biopsy should be taken from the ileum because that is where most absorption occurs.

Right: Celiac disease biopsy is taken from the second portion of the duodenum or proximal jejunum, where villous atrophy and crypt hyperplasia are most pronounced.

Gluten is encountered and digested in the proximal small bowel, so the inflammatory damage — villous atrophy, crypt hyperplasia, intraepithelial lymphocytosis — is most severe in the second portion of the duodenum and proximal jejunum. The ileum is where vitamin B12 and bile acids absorb, but it's largely spared in celiac disease. Biopsy from the wrong site will miss the diagnosis entirely, which is why this anatomic distinction is testable.

Common mistake

Wrong: Celiac disease primarily increases risk of colorectal adenocarcinoma.

Right: Celiac disease most significantly increases risk of enteropathy-associated T-cell lymphoma (EATL) and small bowel adenocarcinoma, not colorectal cancer.

Celiac disease causes chronic immune activation in the gut epithelium, which predisposes specifically to enteropathy-associated T-cell lymphoma (EATL) — a T-cell lymphoma arising from the chronically stimulated intraepithelial lymphocytes — and small bowel adenocarcinoma. Colorectal adenocarcinoma risk is not meaningfully elevated. When a celiac vignette mentions a patient who relapses despite a gluten-free diet or presents with constitutional symptoms, think EATL first.

Common mistake

Gap: Missing that IgA deficiency causes false-negative anti-tTG IgA serology in celiac disease workup

Selective IgA deficiency (more common in celiac patients) causes false-negative anti-tTG IgA results; total serum IgA should be checked, and IgG-based tests used if IgA is deficient.

Selective IgA deficiency occurs at roughly 10–15x the rate in celiac patients compared to the general population. If a patient has IgA deficiency and you run anti-tTG IgA, you'll get a false negative — the test literally can't fire without adequate IgA. The fix is to check total serum IgA alongside the anti-tTG IgA, and if IgA is low, switch to IgG-based testing (anti-tTG IgG or anti-deamidated gliadin peptide IgG). USMLE Step 1 will present this as a patient with classic celiac symptoms and a negative initial serology — your next move is checking total IgA.

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What the exam tests

Explain the immune mechanism underlying villous destruction in celiac disease — specifically the roles of gliadin, tissue transglutaminase, HLA-DQ2/DQ8, and CD4+ T-cell activation leading to crypt hyperplasia and villous atrophy.
Identify the correct serologic workup for celiac disease, including why anti-tTG IgA is the preferred initial test, when to check total serum IgA, and what biopsy site and histologic findings confirm the diagnosis.
Recognize extraintestinal manifestations of celiac disease (dermatitis herpetiformis, iron-deficiency anemia, osteoporosis, neurologic symptoms) and correctly identify the associated malignancy risks — specifically EATL and small bowel adenocarcinoma.
Apply knowledge of dietary therapy (strict gluten-free diet) and understand how anti-tTG IgA titers are used to monitor treatment compliance over time.

Can you avoid these mistakes?

A 28-year-old woman presents with chronic diarrhea, iron-deficiency anemia, and a pruritic blistering rash on her elbows. Anti-tTG IgA comes back negative. What is the most important next step in the serologic workup, and why might the initial test be falsely negative?

On small bowel biopsy, you see flattened villi, elongated crypts, and increased intraepithelial lymphocytes. What site was this biopsy taken from, what diagnosis does this confirm, and what HLA alleles predispose to this condition?

A celiac patient who was initially compliant with a gluten-free diet presents with 15-lb weight loss, night sweats, and abdominal pain despite continued dietary adherence. What complication should be highest on your differential, and why does celiac disease predispose to this?

A 32-year-old woman with suspected celiac disease is in front of you. Your intern suggests ordering anti-gliadin IgA. Which test should you order instead and why? What antigen does the preferred test target, and how does that antigen relate to the pathogenesis of the disease?

Celiac Disease

Common misconceptions

What the exam tests

Can you avoid these mistakes?

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