There are several tests that can be used to assist in diagnosis. The level of symptoms may determine the order of the tests, but all tests lose their usefulness if the patient is already taking a gluten-free diet. Intestinal damage begins to heal within weeks of gluten being removed from the diet, and antibody levels decline over months. For those who have already started on a gluten-free diet, it may be necessary to perform a re-challenge with 10 g of gluten (four slices of bread) per day over 2–6 weeks before repeating the investigations. Those who experience severe symptoms (e.g. diarrhoea) earlier can be regarded as sufficiently challenged and can be tested earlier.
Combining findings into a prediction rule to guide use of endoscopy reported a sensitivity of 100% (it would identify all the cases) and specificity of 61% (it would be incorrectly positive in 39%). The prediction rule recommends that patients with high-risk symptoms or positive serology should undergo endoscopy. The study defined high-risk symptoms as weight loss, anaemia (haemoglobin less than 120 g/l in females or less than 130 g/l in males), or diarrhoea (more than three loose stools per day).
Serological blood tests are the first-line investigation required to make a diagnosis of coeliac disease. Serology for anti-tTG antibodies has superseded older serological tests and has a high sensitivity (99%) and specificity (>90%) for identifying coeliac disease. Modern anti-tTG assays rely on a human recombinant protein as an antigen.
Because of the major implications of a diagnosis of coeliac disease, professional guidelines recommend that a positive blood test is still followed by an endoscopy/gastroscopy and biopsy. A negative serology test may still be followed by a recommendation for endoscopy and duodenal biopsy if clinical suspicion remains high due to the 1 in 100 “false-negative” result. As such, tissue biopsy is still considered the gold standard in the diagnosis of coeliac disease.
Historically three other antibodies were measured: anti-reticulin (ARA), anti-gliadin (AGA) and anti-endomysium (EMA) antibodies. Serology may be unreliable in young children, with anti-gliadin performing somewhat better than other tests in children under five. Serology tests are based on indirect immunofluorescence (reticulin, gliadin and endomysium) or ELISA (gliadin or tissue transglutaminase, tTG).
Guidelines recommend that a total serum IgA level is checked in parallel, as coeliac patients with IgA deficiency may be unable to produce the antibodies on which these tests depend (“false negative”). In those patients, IgG antibodies against transglutaminase (IgG-tTG) may be diagnostic.
HLA genetic typing
Antibody testing and HLA testing have similar accuracies.
An upper endoscopy with biopsy of the duodenum (beyond the duodenal bulb) or jejunum is performed. It is important for the physician to obtain multiple samples (four to eight) from the duodenum. Not all areas may be equally affected; if biopsies are taken from healthy bowel tissue, the result would be a false negative.
Most patients with coeliac disease have a small bowel that appears normal on endoscopy; however, five concurrent endoscopic findings have been associated with a high specificity for coeliac disease: scalloping of the small bowel folds (pictured), paucity in the folds, a mosaic pattern to the mucosa (described as a “cracked-mud” appearance), prominence of the submucosa blood vessels, and a nodular pattern to the mucosa.
Until the 1970s, biopsies were obtained using metal capsules attached to a suction device. The capsule was swallowed and allowed to pass into the small intestine. After x-ray verification of its position, suction was applied to collect part of the intestinal wall inside the capsule. One often-utilised capsule system is the Watson capsule. This method has now been largely replaced by fibre-optic endoscopy, which carries a higher sensitivity and a lower frequency of errors.
The classic pathology changes of coeliac disease in the small bowel are categorised by the “Marsh classification”:
- Marsh stage 0: normal mucosa
- Marsh stage 1: increased number of intra-epithelial lymphocytes, usually exceeding 20 per 100 enterocytes
- Marsh stage 2: proliferation of the crypts of Lieberkuhn
- Marsh stage 3: partial or complete villous atrophy
- Marsh stage 4: hypoplasia of the small bowel architecture
The changes classically improve or reverse after gluten is removed from the diet, so many official guidelines recommend a repeat biopsy several (4–6) months after commencement of gluten exclusion.
In some cases, a deliberate gluten challenge, followed by biopsy, may be conducted to confirm or refute the diagnosis. A normal biopsy and normal serology after challenge indicates the diagnosis may have been incorrect. Patients are warned that one does not “outgrow” coeliac disease in the same way as childhood food intolerances.
Other diagnostic tests
Other tests that may assist in the diagnosis are blood tests for a full blood count, electrolytes, calcium, renal function, liver enzymes, vitamin B12 and folic acid levels. Coagulation testing (prothrombin time and partial thromboplastin time) may be useful to identify deficiency of vitamin K, which predisposes patients to hemorrhage. These tests should be repeated on follow-up, as well as anti-tTG titres
Some professional guidelines recommend screening of all patients for osteoporosis by DXA/DEXA scanning.
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