American Association for Clinical Chemistry
Better health through laboratory medicine
NACB - Scientific Shorts
NACB - Scientific Shorts (formerly NACB Blog)
By Julie Rosser, DO, and Ronald Whitley, PhD

Historically, a diagnosis of celiac disease was reserved for children with diarrhea, short stature and failure to thrive who ultimately had a high mortality rate. Prevalence in North America was estimated to be around 1 in 3000-10,000 people. Celiac is now known to be one of the “great mimickers” presenting with vague symptoms such as bloating, diarrhea, skin rash, muscle cramps, weight loss and neuropathy. Recent research unveiling the relationship between serum autoantibodies and specific HLA haplotypes with gluten sensitive individuals is aiding clinicians with this diagnostic conundrum. The advent of serologic testing allowed expansion of the diagnosis to encompass adult patients and patients previously thought to have irritable bowel, Crohn’s, or even parasite infections.  Previously thought of as a rare disease, celiac prevalence is more accurately estimated around 1 in 80-300 with many individuals undiagnosed.
Celiac disease is a malabsorption syndrome caused by antibodies to gluten, found in wheat, barley and rye. Affected individuals mount an inappropriate T-cell response to small intestinal mucosa. Co-morbidities coincide with vitamin deficiency and include growth retardation and behavioral disturbances in children, defects in dental enamel, osteoporosis, infertility and lymphoma. Celiac disease is often associated with dermatitis herpetiformis, autoimmune thyroiditis and type I diabetes mellitus. Histologically, it is defined by villous atrophy of small intestine mucosa and a lymphocytic inflammatory response.
Endoscopy is considered the gold-standard and is done to evaluate the intestinal mucosa. However, not all persons with the disease manifest the characteristic biopsy changes. Laboratory evaluations also play an essential role in diagnosing celiac disease, particularly serologic testing for anti-tissue transglutaminase antibodies (tTG), endomysial antibodies (EMA), and deaminated gliadin peptide antibodies (DGP), with testing available for both IgA and IgG isotypes.  EMA is measured by indirect immunofluorescence and is a subjective, labor-intensive process with high clinical specificity (99-100%) but variable sensitivity (75-98%). Tissue transglutaminase (tTG), on the other hand, is measured by an enzyme-linked immunosorbent assay (ELISA), which is more sensitive (96-98%), cheaper and easier to perform but less specific (88-100%) than an EMA. Both of these assays may produce false negative results, particularly in IgA-deficient individuals in whom IgG testing should then be performed. There are also assays for IgA and IgG antibodies against gliadin, an antigen related to dietary gluten. Antibodies to unmodified gliadin proteins have been replaced by a new generation of antibodies against deamidated gliadin.  The latter assay is a bit less sensitive than tTG with comparable specificity. The combination of both tests adds to the sensitivity without substantially reducing the specificity. Recently, multiplex assays have been developed that offer the advantage of testing for multiple antigen specificities in a single reaction (for example, detection of IgA and IgG isotypes of both tTG and DGP in a single assay). 
Guidelines from the North American Society for Pediatric Gastroenterology, Hepatology and Nutrition and recommendations of the American Gastroenterological Association both advocate starting with the tTG assay.  They reason that this is a more sensitive test than EMA, yielding a lower false positive rate which is more appropriate for patient screening. Both organizations also highlight that tTG testing is accomplished via plate-based ELISA, a quicker and less expensive process than the immunofluorescence assay used for EMA. Confirmation with EMA is advised when there are indeterminate or weak positive tTG results. In the context of IgA deficiency, testing with tTG-IgG and/or EMA-IgG is recommended. The routine use of anti-gliadin antibody testing is ill-advised due to comparatively poor sensitivity and specificity of the assay. Intestinal biopsy should be performed following a positive serology, or if suspicion is high and serology testing was negative. However, biopsy changes consistent with celiac disease but with negative serologies should trigger the search for a different etiology.
There seems to be a general consensus on the screening of high risk individuals: obtain serology and if positive, perform small intestinal biopsy (or if suspicion remains high despite negative serology). Unfortunately, there are many individuals plagued with mild celiac disease who have a “negative” intestinal biopsy. Undiagnosed individuals who demonstrate mild disease continue to suffer and ultimately maintain the chance of acquiring the co-morbidities associated with long-term, untreated celiac disease. Two large questions arise:  1) is there an argument for screening the general population, and 2) should biopsy continue to be the mandatory gold standard in diagnosis?
A plausible response to the first challenge may be a lower threshold for serologic testing of patients in the primary care setting presenting with any of the known major or minor symptoms (abdominal pain, diarrhea, anemia, etc) despite minimal suspicion of actually having the disease. Since the serologic markers carry such high sensitivity and specificity, it is reasonable to conclude negative serology equates to a different etiology.  As recommendations currently state, an intestinal biopsy should follow any positive serology.
The second question is a much more difficult undertaking. It would require reversal of years of thought and clinical implementation. However, the arguments against biopsy are mounting; the patchiness of mucosal manifestations produce many “normal” biopsies in known celiac patients, tangential placement of the specimen can alter pathologists’ interpretation, high costs of the procedure and most importantly the procedure is invasive. On the other hand, biopsy may identify the few serologic false negative individuals. But is this enough to keep biopsy part of the gold-standard to diagnosis? Can it be deemed merely an adjunct in difficult cases, similar to HLA detection? Perhaps it is time to reorganize our thoughts and step into a new era of celiac diagnosis. Serologic testing is precise and accurate and with clinical symptom analysis should provide the information required to produce an educated diagnosis.




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