Coeliac Disease Diagnosis

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.

Blood tests

Antibody testing

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 Serology for anti-tTG antibodies has superseded older serological tests and has a high sensitivity (99%) and specificity (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. 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.

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.

Pathology

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. This gene is part of the MHC class II antigen-presenting receptor (also called the human leukocyte antigen) system and distinguishes cells between self and non-self for the purposes of the immune system. The gene is located on the short arm of the sixth chromosome, and as a result of the linkage, this locus has been labeled CELIAC1.

There are seven HLA DQ variants (DQ2 and DQ4–DQ9). Over 95% of coeliac patients have the isoform of DQ2 or DQ8, which is inherited in families. The reason these genes produce an increase in risk of coeliac disease is that the receptors formed by these genes bind to gliadin peptides more tightly than other forms of the antigen-presenting receptor. Therefore, these forms of the receptor are more likely to activate T lymphocytes and initiate the autoimmune process. Some individuals inherit DQ2.5 from one parent and portions of the haplotype (DQB1*02 or DQA1*05) from the other parent, increasing risk. Less commonly, some individuals inherit the DQA1*05 allele from one parent and the DQB1*02 from the other parent, called a trans-haplotype association, and these individuals are at similar risk for coeliac disease as those with a single DQ2.5-bearing chromosome 6, but in this instance, disease tends not to be familial. Among the 6% of European coeliacs that do not have DQ2.5 (cis or trans) or DQ8 (encoded by the haplotype DQA1*03:DQB1*0302), 4% have the DQ2.2 isoform, and the remaining 2% lack DQ2 or DQ8.

The frequency of these genes varies geographically. DQ2.5 has high frequency in peoples of North and Western Europe (Basque Country and Ireland with highest frequencies) and portions of Africa and is associated with disease in India, but is not found along portions of the West Pacific rim. DQ8, spread more globally than DQ2.5, is more prevalent from South and Central America (up to 90% phenotype frequency).

Prolamins

The majority of the proteins in food responsible for the immune reaction in coeliac disease are the prolamins. These are storage proteins rich in proline (''prol-'') and glutamine (''-amin'') that dissolve in alcohols and are resistant to proteases and peptidases of the gut. Prolamins are found in cereal grains with different grains having different but related prolamins: wheat (gliadin), barley (hordein), rye (secalin), corn (zein) and as a minor protein, avenin in oats. One region of α-gliadin stimulates membrane cells, enterocytes, of the intestine to allow larger molecules around the sealant between cells. Disruption of tight junctions allow peptides larger than three amino acids to enter circulation. Membrane leaking permits peptides of gliadin that stimulate two levels of immune response, the innate response and the adaptive (T-helper cell mediated) response. One protease-resistant peptide from α-gliadin contains a region that stimulates lymphocytes and results in the release of interleukin-15. This innate response to gliadin results in immune-system signalling that attracts inflammatory cells and increases the release of inflammatory chemicals. However, not all prolamins will cause this immune reaction, and there is ongoing controversy on the ability of avenin (the prolamin found in oats) to induce this response in coeliac disease.

Tissue transglutaminase

Anti-transglutaminase antibodies to the enzyme tissue transglutaminase (tTG) are found in an overwhelming majority of cases. Tissue transglutaminase modifies gluten peptides into a form that may stimulate the immune system more effectively. Deamidation is the reaction by which a glutamate residue is formed by cleavage of the epsilon-amino group of a glutamine side chain. Transamidation, which occurs three times more often than deamidation, is the cross-linking of a glutamine residue from the gliadin peptide to a lysine residue of tTg in a reaction which is catalyzed by the transglutaminase. Crosslinking may occur either within or outside the active site of the enzyme. The latter case yields a permanently, covelently linked complex between the gliadin and the tTg. This results in the formation of new epitopes which are believed to trigger the primary immune response by which the autoantibodies against tTg develop

Stored biopsies from suspected coeliac patients have revealed that autoantibody deposits in the subclinical coeliacs are detected prior to clinical disease. These deposits are also found in patients who present with other autoimmune diseases, anaemia or malabsorption phenomena at a much-increased rate over the normal population. Endomysial components of antibodies (EMA) to tTG are believed to be directed toward cell-surface transglutaminase, and these antibodies are still used in confirming a coeliac disease diagnosis. However, a 2006 study showed that EMA-negative coeliac patients tend to be older males with more severe abdominal symptoms and a lower frequency of "atypical" symptoms including autoimmune disease. In this study, the anti-tTG antibody deposits did not correlate with the severity of villous destruction. These findings, coupled with recent work showing that gliadin has an innate response component, suggests that gliadin may be more responsible for the primary manifestations of coeliac disease, whereas tTG is a bigger factor in secondary effects such as allergic responses and secondary autoimmune diseases. In a large percentage of coeliac patients, the anti-tTG antibodies also recognise a rotavirus protein called VP7. These antibodies stimulate monocyte proliferation, and rotavirus infection might explain some early steps in the cascade of immune cell proliferation. Indeed, earlier studies of rotavirus damage in the gut showed this causes a villous atrophy. This suggests that viral proteins may take part in the initial flattening and stimulate self-crossreactive anti-VP7 production. Antibodies to VP7 may also slow healing until the gliadin-mediated tTG presentation provides a second source of crossreactive antibodies.

Villous atrophy and malabsorption

The inflammatory process, mediated by T cells, leads to disruption of the structure and function of the small bowel's mucosal lining and causes malabsorption as it impairs the body's ability to absorb nutrients, minerals and fat-soluble vitamins A, D, E and K from food. Lactose intolerance may be present due to the decreased bowel surface and reduced production of lactase but typically resolves once the condition is treated.

Alternative causes of this tissue damage have been proposed and involve release of interleukin 15 and activation of the innate immune system by a shorter gluten peptide (p31–43/49). This would trigger killing of enterocytes by lymphocytes in the epithelium. or human intestinal adenovirus. Some research has suggested that smoking is protective against adult-onset coeliac disease.

A 2005 prospective and observational study found that timing of the exposure to gluten in childhood was an important risk modifier. People exposed to wheat, barley, or rye before the gut barrier has fully developed (within the first three months after birth) had five times the risk of developing coeliac disease relative to those exposed at four to six months after birth. Those exposed even later than six months after birth were found to have only a slightly increased risk relative to those exposed at four to six months after birth. A study conducted in 2006 showed that early introduction of grains was protective against grain allergies; however, this study explicitly excluded any participants found to have coeliac disease and therefore offers no help in this regard. Breastfeeding may also reduce risk. A meta-analysis indicates that prolonging breastfeeding until the introduction of gluten-containing grains into the diet was associated with a 52% reduced risk of developing coeliac disease in infancy; whether this persists into adulthood is not clear.