Coeliac Serology

Coeliac disease is a chronic immune mediated enteropathy triggered by dietary gluten in genetically susceptible people (HLA-DQ2 or HLA-DQ8). Serology is the first line test and has high accuracy when the patient is eating gluten.

The main assays are IgA anti-tissue transglutaminase (tTG-IgA), IgA anti-endomysial antibody (endomysial IgA, by immunofluorescence on primate oesophagus or umbilical cord), IgG and IgA anti-deamidated gliadin peptide (DGP), and total IgA to detect IgA deficiency. Anti-gliadin antibodies (native) are obsolete because of poor specificity.

Coeliac disease affects about 1% of the population worldwide, with higher rates in first degree relatives (5 to 15%), type 1 diabetes (3 to 10%), autoimmune thyroid disease, Down syndrome, Turner syndrome and selective IgA deficiency. Most adults are diagnosed after years of non-specific symptoms, and 75% remain undiagnosed. Untreated disease causes anaemia, osteoporosis, infertility, neurological disease and, rarely, enteropathy associated T-cell lymphoma.

Indications. Chronic diarrhoea, steatorrhoea, weight loss, bloating, unexplained iron, folate or B12 deficiency, osteoporosis, persistent transaminase rise, dermatitis herpetiformis, recurrent aphthous ulcers, unexplained infertility, neuropathy or ataxia, faltering growth and delayed puberty in children.

Screening of first degree relatives and of high risk groups (type 1 diabetes, autoimmune thyroid disease, Down syndrome, Turner syndrome, Williams syndrome). Monitoring adherence to a gluten free diet.

Specimen. Serum 1 mL, no fasting. Stable for days at 2 to 8 °C. The patient must have been eating gluten daily (at least one meal containing gluten each day) for 6 weeks before the test.

Reference values (assay dependent).

• tTG-IgA: negative under 4 U/mL (some assays under 7 or under 20 U/mL), weak positive 4 to 10 times the upper limit, strong positive over 10 times the upper limit.
• Endomysial IgA: negative (screening dilution 1 in 5).
• DGP-IgG: negative under 20 U/mL.
• Total IgA: adult 0.7 to 4.0 g/L (age dependent in children).

Interpretation.

• tTG-IgA positive with normal total IgA: coeliac disease is likely. Sensitivity is 90 to 98% and specificity 95 to 99%. Duodenal biopsy (at least four from the second part and two from the bulb) confirms villous atrophy and intraepithelial lymphocytosis (Marsh grading).
• tTG-IgA over 10 times the upper limit with positive endomysial IgA on a second blood sample and HLA-DQ2 or DQ8 in a symptomatic child: diagnosis without biopsy is accepted in paediatric practice. In adults biopsy is generally still obtained.
• tTG-IgA negative with low total IgA (under 0.2 g/L): IgA deficiency, so IgG-based tests (tTG-IgG, DGP-IgG) are used and biopsy considered.
• tTG-IgA negative and endomysial IgA negative with typical symptoms: coeliac disease unlikely, but biopsy may be required if suspicion is high (seronegative disease is 2 to 5%).
• DGP-IgG is useful in children under 2 years (tTG-IgA is less sensitive) and in IgA deficiency.
• HLA-DQ2/DQ8 negative: coeliac disease virtually excluded (negative predictive value over 99%). HLA positivity is present in 30 to 40% of the general population and is not diagnostic.
• Falling tTG titres on a gluten free diet: dietary adherence, with normalisation in 6 to 24 months. Persistently raised titres indicate continued gluten exposure or refractory disease.

Critical values and findings needing immediate action.

• Positive serology with severe malabsorption, hypoalbuminaemia, hypocalcaemia or anaemia: urgent dietitian referral and nutritional replacement.
• Persistent symptoms despite a strict gluten free diet and negative serology: refractory coeliac disease and enteropathy associated lymphoma must be excluded (capsule endoscopy, imaging, clonality studies).
• Coeliac crisis (profuse diarrhoea, dehydration, electrolyte disturbance, hypoproteinaemia, metabolic acidosis): admit, with intravenous fluids and replacement.
• New abdominal pain, fever, weight loss and bowel obstruction in a known patient: complications (ulcerative jejunoileitis, lymphoma).

Interferences and pitfalls.

• Testing after starting a gluten free diet gives false negatives. Patients on a diet before testing are asked to resume gluten (the equivalent of 3 to 6 g gluten, about two slices of bread, daily for 6 to 8 weeks, or 2 weeks in some practices) before serology and biopsy.
• IgA deficiency (2% of coeliac patients, 10 to 15 times more frequent than in the general population) gives false negative IgA tests, so total IgA is always measured.
• Immunosuppressive drugs (steroids, azathioprine) and young age (under 2 years) lower sensitivity.
• Low level positive tTG-IgA (under 3 times the upper limit) has lower predictive value and is seen in other conditions (liver disease, type 1 diabetes, heart failure, infections, inflammatory bowel disease), so biopsy is needed.
• Assays from different manufacturers give different units and thresholds.
• Point-of-care tests are less accurate and are not used to exclude disease.
• Genetic testing cannot diagnose, but is useful to exclude disease in patients who have already started a gluten free diet.
• Other causes of villous atrophy are excluded: giardiasis, tropical sprue, small intestinal bacterial overgrowth, Crohn disease, common variable immunodeficiency, autoimmune enteropathy, olmesartan and other drugs, HIV enteropathy.

Special groups.

• Children: tTG-IgA is the first test, with total IgA. Symptoms may be gastrointestinal or extra-intestinal (short stature, anaemia, irritability).
• Pregnancy: untreated disease raises risks of miscarriage, low birth weight and anaemia. Serology can be requested.
• Older adults: atypical or silent disease is common, with anaemia or osteoporosis only.
• Type 1 diabetes: screen at diagnosis and every 1 to 3 years, as disease is often silent.
• Regions: coeliac disease is common in North Africa, the Middle East and South Asia, and is under recognised where wheat is a staple.

Follow up tests. Total IgA, duodenal biopsies, HLA-DQ2/DQ8 typing, full blood count, ferritin, folate and vitamin B12, calcium, vitamin D, bone density scan, liver function, thyroid function, HbA1c or glucose, faecal calprotectin, stool microscopy for Giardia, and dietitian assessment. Repeat serology 6 to 12 months after starting diet, then yearly.