Historically, screening for celiac disease was performed with a diagnostic test for malabsorption (D-xylose) or serological tests for antigliadin antibodies (AGA) and antiendomysial antibodies (anti-EMA). Antitissue transglutaminase (anti-tTG) tests followed and, more recently, antideaminated gliadin peptide (DGP) antibodies have been introduced (). These serological tests have become invaluable in the identification, diagnosis and management of patients with gluten-sensitive disorders that affect the bowel or the skin. The ability of human leukocyte antigen (HLA) typing for HLA-DQ2 and HLA-DQ8 associations with celiac disease to exclude disease is well described but not often used clinically.
Diagnostic performance of available serological tests for classical celiac disease
Diagnostic test accuracy has shown that the AGA tests are less sensitive and less specific than EMA and tTG (11
) to diagnose celiac disease. Sensitivity for the AGA-IgA test is in the range of 46% to 87%, with a specificity of 70% to 98%. The AGA-IgG test has sensitivity from 42% to 93%, and specificity in the region of 84% to 97%. However, a systematic review (12
) revealed many issues related to heterogeneity in this test group.
Early in the decade, tests for DGP were described. These tests are derived from the understanding that the deamidation of gluten by tTG makes gliadin peptides more potent activators of T-cells in celiac disease patients (13
). DGPs are formed by the action of tTG on gliadin. Both IgG and IgA tests are available. A number of commercial ELISA tests have been made available for research studies and are beginning to appear in the test menu of some clinical laboratories. Diagnostic test accuracy for DGP-IgA is not better than tTG-IgA, with pooled sensitivity of 87.8% (range 85.6% to 89.9%) and specificity of 94.1% (range 92.5% to 95.6%) (14
). However, there is some preliminary data indicating that the DGP-IgG test may become a useful test in IgA deficiency (15
). Sensitivity for DGP-IgG is reasonable, with results between 69% and 81%, while specificity is excellent, with results between 97.8% and 99.6%. The diagnostic accuracy of DGP-IgA does not outperform tTG-IgA (17
). Until now, there are no recommendations that tTG tests be replaced by DGP-AGA, but these tests may aid in the diagnosis of patients who are symptomatic and seronegative for tTG, and in children younger than two years of age. The use of the IgG DGP-AGA seems to be gaining support in the literature in both adults and pediatric populations (18
). The diagnostic utility of DGPs will require further investigation.
The pathophysiology of gluten-related disorders and serological testing
The various serological tests for celiac disease relate to different aspects of the pathophysiological process, whereby exposure of the gastrointestinal tract to dietary gluten leads to the development of antibodies (IgA and IgG) against gliadin. In addition, however, there is the development of antibodies to host tTG (tTG-IgA and tTG-IgG). Host tTG is the antigen also recognized by EMA, which subsequently forms the basis for another serological test. More recent work has led to the development of serological tests that can identify antibodies to deamidated gliadin – the product of tTG action on dietary gliadin. Additional blood tests will help clarify the results of serological tests, either by determining whether a test may be falsely negative because the patient is IgA deficient, or whether the patient has a high pretest probability of gluten-related disease based on the presence of HLA-DQ2 or HLA-DQ8 genes.
Although histology is considered to be the ‘gold standard’ for the diagnosis of gluten-related disease, the degree of gluten-related injury ranges from Marsh I to Marsh IV. However, the severity of gluten-related injury may vary from one biopsy to another, and only Marsh stages III and IV have generally been considered to be diagnostic of celiac disease. Thus, patients may experience less severe histological changes that are, nonetheless, gluten related. Under these circumstances, they will not be considered to have a ‘gluten-related disorder’, and a positive serological test will be considered to have been ‘falsely positive’. It is important to note that younger patients with positive serology and no histological changes, as well as those who have only Marsh I and Marsh II changes, have all been shown to have increased morbidity and mortality rates compared with healthy controls (2
). Although increased mortality was not confirmed in a study of elderly individuals with undiagnosed celiac disease (22
), a limited increase in morbidity related to reduced bone density was observed.
Serological tests: Variability in celiac disease
All of the tests mentioned above are used to diagnose celiac disease. The specificities and sensitivities reported are usually derived from studies using duodenal histology as the diagnostic gold standard. All of the serological tests have some degree of limitation from a laboratory perspective. First, there are no universal standards for the target antibodies. Second, each type of test (ELISA, radioimmuno assay and immunofluorescent assay) has high variability within lots, among lots and among methods. This variability makes the clinical interpretation more challenging when sequential results are compared. This was highlighted in a recent report from an international proficiency survey (23
Use of celiac serology tests in other conditions
In addition to celiac disease, there are several well-described conditions that have been reported in association with positive AGA-IgA and AGA-IgG tests, although it remains to be proven whether any or all of these conditions are caused by gluten exposure.
Routine serological screening is now recommended for celiac disease in patients with IBS symptoms, particularly those with diarrhea-predominant and mixed phenotypes of IBS (24
). A positive IgA-tTG test will identify celiac disease as a potential cause of symptoms in a patient presenting with IBS symptoms. In the event of a positive biopsy, the patient will be diagnosed with celiac disease, and likely no longer considered to have IBS. However, a different clinical problem presents to the physician when a patient describes IBS-like symptoms that are triggered by gluten ingestion and improved after dietary exclusion, but who has a negative tTG and a normal or borderline biopsy (6
). Serological testing using anti-AGA in these patients may help identify gluten sensitivity in patients who do not fulfill the diagnostic criteria for ‘classic’ celiac disease but who may, nonetheless, benefit from a gluten-exclusion diet (6
No significant association has been reported between dyspepsia and positive serological tests for antibodies to gliadin, tTG or endomysium. Although positive serology was found in 7.9% of patients with dyspepsia in one study (26
), there was a 3.9% positivity rate in the controls. Thus, the current data do not support routine serology screening for gluten-related disorders in patients with dyspepsia.
The prevalence of celiac disease ranges from 1.5% to 9.0% in patients with elevated transaminase levels of unknown cause, from 2.9% to 6.4% in patients with autoimmune hepatitis, and up to 6.0% in those with primary biliary cirrhosis (27
). Thus, screening for celiac disease in patients with liver disease should be considered.
Inflammatory bowel disease (IBD):
The prevalence of IBD in celiac disease, as diagnosed by AGA, EMA and tTG antibodies, and total IgA level, is increased 10-fold compared with that in healthy individuals whose serology was negative, while the prevalence of celiac disease in IBD patients was comparable with that in controls (29
Other conditions reported to be associated with positive AGA serology:
A higher prevalence of positive AGA serology has been reported in patients with RhA (30
), Sjogren’s syndrome (31
), systemic lupus erythematosus, sarcoidosis and type 1 diabetes (32
). Dermatitis herpetiformis is a well-recognized, gluten-sensitive skin disorder associated with a positive tTG test. There have also been reports of an association between celiac markers and pemphigoid (35
) and atopic eczema (36
). IgA nephropathy has also been associated with positive AGA serology (37