Diagnostic tests

Blood tests

Blood tests or coeliac serology are an important part of the diagnostic process. These tests look for the presence of certain antibodies that circulate in the blood when a person with coeliac disease ingests gluten.

Tests used to screen for coeliac disease include-

• Tissue transglutaminase IgA antibodies (tTG-IgA) with total serum IgA
• Deamidated gliadin peptide (DGP-IgG)
• Endomysial antibody (EMA-IgA) – less commonly performed now

In adults, positive coeliac antibody results are not enough to confirm a diagnosis of coeliac disease on their own. Sometimes, a positive antibody result (e.g. tTG-IgA) can be due to other autoimmune issues (e.g. autoimmune liver disease), causing a false positive. Similarly, false negatives (i.e. when a test erroneously suggests that someone with coeliac does not have the condition) are also possible. A positive coeliac antibody test usually means your doctor will organise a gastroscopy and small bowel bioipsies to definitively confirm coeliac disease.

Diagnostic recommendations for coeliac disease in children have evolved in recent years. Guidelines published by the European Society for Paediatric Gastroenterology, Hepatology and Nutrition (ESPGHAN) in 2020 indicate that when children meet specific criteria a specialist (paediatric gastroenterologist) can diagnose coeliac disease without the need for small intestinal biopsies. Currently, this criteria requires a positive transglutaminase-IgA level greater than 10-times the upper limit of normal, and on a second blood test, a positive EMA result. If the transglutaminase-IgA is elevated but these specific criteria are not met, a gastroscopy and small intestinal biopsies are still needed to establish the diagnosis just as they are currently recommended for adults.

Gastroscopy and small bowel biopsy

A gastroscopy is a procedure that involves inserting an endoscope (i.e. a long, flexible tube with a camera on the end) through the mouth, down the oesophagus, into the stomach, and into the first section of the small bowel (i.e. the duodenum). Gastroscopy allows for your doctor to examine the signs of illness. Coeliac-related damage is usually only microscopic, thus a tissue sample is taken from the small bowel and examined under a microscope for signs of villous atrophy (Examining tissue samples under a microscope for signs of disease is called histological examination).

A gastroscopy requires you to fast prior. Before the procedure begins, a sedating drug is administered. You will have limited or no memory of the procedure. Residual sedative effects can take some time to disappear, so you cannot drive yourself home afterwards, operate heavy machinery, consume alcohol, or make legal decisions.

Genetic testing

Under some circumstances, your doctor may order human leukocyte antigen (HLA) testing. Virtually all people with coeliac disease have either the HLA-DQ2 and/or HLADQ-8 genes. Due to the rarity of someone with coeliac disease not having either of these genes, a negative HLA test effectively rules out the possibility of coeliac disease. Approximately a half of the general population have either of these genes, but the vast majority will not go on to develop coeliac disease.

HLA testing may also be informative in situations where coeliac serology and small bowel histology return conflicting results or in people who are not improving on a gluten-free diet, raising questions about their diagnosis. HLA typing may also be used in cases where the person being screened has already begun a gluten-free diet. In this case, it allows for an opportunity to potentially rule out coeliac disease prior to the gluten challenge. A negative HLA test may indicate another condition, such as non-coeliac gluten sensitivity or wheat allergy. A positive test, on the other hand, provides further evidence that a gluten challenge followed by blood testing is necessary.

Post-diagnosis

After diagnosis, there are several additional tests that may be performed to investigate for conditions often associated with coeliac disease. These can include-

• Blood tests to examine for nutritional deficiencies (e.g. iron deficiency anaemia) or other autoimmune diseases e.g. autoimmune thyroid disease, diabetes, liver disease
• Bone density scan (i.e. Dual energy X-ray absorptiometry [DEXA]). Coeliac disease is associated with osteopenia (i.e. low bone density) and osteoporosis (i.e. a more severe form of low bone mineral density).
• If you have a red itchy bumpy rash on your skin, a tissue biopsy may be taken to see if you have the condition dermatitis herpetiformis.

Seeing a dietitian with expertise in teaching a gluten-free diet is very important. This will provide information on the safe and unsafe foods, how to read and interpret food labels, how to safely eat out and avoid being gluteneded.

Bone density scanning (DEXA)

Dual energy x-ray absorptiometry (DEXA) scans are a way for your doctor to examine the density of your bones. Coeliac disease is often associated with thinning of the bones (i.e. osteopenia, or the more severe form called osteoporosis). This thinning weakens the bones and makes them more likely to break (fracture). Reduced bone density in coeliac disease occurs due to the chronic inflammation that causes bone breakdown and this can be further compounded by nutritional deficiencies (e.g. vitamin D, calcium) that result from intestinal malabsorption.

The scan is quick (i.e. around 10–30 minutes), painless, and involves a dose of X-ray radiation that is significantly less than a conventional X-ray. You will be asked to lie flat on the X-ray table and to hold still while the scan is conducted.