What are the appropriate screening tests for celiac disease?
The tests of choice are antibody measurements in the blood, ideally performed before the patient has removed gluten from the diet. However, patients and physicians must remember that no screening test is perfect, and that the keys to confirming the diagnosis of CELIAC DISEASE remain a small intestinal biopsy combined with the patient’s subsequent clinical response to a gluten-free diet. Thus, a patient (especially a young child) with symptoms of CELIAC DISEASE should have a small bowel biopsy, even if the antibodies are not highly suggestive.
What are the different antibody tests available? Can there be errors in testing?
The blood tests can be divided into 2 different types of antibodies: those which are “anti-gluten”, and those that “anti-self”. The “anti-gluten” antibodies are the anti-gliadin IgG and IgA. Ig stands for “immunoglobulin” or “antibody”. The “anti-self” antibodies are anti-endomysial IgA and anti-tissue transglutaminase IgA. The tissue transglutaminase IgA antibody is often abbreviated as “tTG”. Each antibody test varies widely in its sensitivity and specificity for predicting whether the disease is present in any individual. It must be remembered that NO test in medicine is correct 100% of the time in each person!
There are also several conditions which may yield false negative antibody results. A false negative means that the patient actually has the disease, but the test result is negative. One of the conditions that may give a false negative result is Immunoglobulin A or IgA deficiency. If a patient has a low total IgA level, the antibodies may be falsely low. This is why I always recommend that a patient have a total IgA level drawn at the same time the antibody testing is done. Young children may not make the some of the “anti-self” antibodies, as it takes a somewhat mature immune system to make them. So in a young child, antiendomysial antibody, or the TTG antibody, can have false negative results. An inexperienced lab can misread the anti-endomysial IgA test, which requires someone to read a slide through a special microscope. It is possible that a celiac patient could have a positive antibody test at one lab, and a negative test at another. This is because different labs may use different commercial test kits, which vary in their sensitivity and specificity. And lastly, a person has to be ingesting gluten at the time the antibodies are drawn. A gluten-free diet will make the antibody tests negative.
Let’s discuss the different antibodies and what the strengths and weaknesses are for each.
The antigliadin antibodies IgG and IgA recognize a small piece of the gluten protein called gliadin. These antibodies became available during the late 1970′s and were the first step towards recognizing CELIAC DISEASE as an autoimmune disorder. Antigliadin IgG has good sensitivity, while antigliadin IgA has good specificity, and therefore their combined use provided the first reliable screening test for CELIAC DISEASE. Unfortunately, many normal individuals without CELIAC DISEASE will have an elevated antigliadin IgG, causing much confusion among physicians. The antigliadin IgG is useful in screening individuals who are IgA deficient, as the other antibodies used for routine screening are usually of the IgA class. It is thought that 0.2-0.4% of the general population has selective IgA deficiency, while 2 to 3% or more of celiacs are IgA deficient.
If a patient’s celiac panel is only positive for antigliadin IgG, this is not highly suggestive for CELIAC DISEASE if the patient has a normal total IgA level, corrected for age. Younger children make less IgA than older children and adults. A markedly elevated antigliadin IgG, such as greater than three to four times the upper limit of normal for that lab, is highly suggestive of a condition where the gut is leakier to gluten. This can happen in food allergies, cystic fibrosis, parasitic infections, Crohn’s disease, and other types of autoimmune GI diseases. These antibodies may also be slightly elevated in individuals with no obvious disease.
A strength of the antigliadin antibodies is that they are ELISA tests. ELISA is an abbreviation for “enzyme-linked immunosorbent assay”. This is a rapid immunochemical test that involves an enzyme, which a protein that causes a biochemical reaction. An ELISA test also involves an antibody or antigen. ELISA tests are utilized to detect substances that have antigenic properties, primarily proteins, such as gliadin. The importance of an ELISA test is that is it rapid, inexpensive, and run by a machine. Thus the results are independent of observer variability. The TTG test is also an ELISA test. This is in contrast to the antiendomysial IgA, where a slide has to be made, and a person has to look at it through a microscope. These are more prone to human error.
The antiendomysial IgA antibody is an excellent screening test for CELIAC DISEASE, with both a high sensitivity and specificity. It is considered the gold standard of antibodies. However, the subjective nature of this test (someone still needs to look at the slide under a microscope) may lead to false negative values and unacceptable variability between laboratories. This antibody was discovered in the early 1980′s, and rapidly gained use as part of a screening “celiac panel” by commercial labs in combination with antigliadin IgG and IgA. Its major drawbacks are that it may be falsely negative in young children, in patients with IgA deficiency and a lesser degree of villous atrophy, and in the hands of an inexperienced laboratory.
Tissue transglutaminase antibodies or TTG
Since tTG had been first described as the autoantigen of celiac disease in 1997, it has been utilized to develop innovative diagnostic tools. The tTG IgA ELISA test is highly sensitive and specific. The tTG assay correlates well with EMA-IgA and biopsy. However, it represents an improvement over the antiendomysial antibody assay because it inexpensive, rapid, is not a subjective test, and can be performed on a single drop of blood using a dot-blot technique. One negative aspect of the TTG antibody is that it can be falsely positive in a patient who has another autoimmune condition. TTG false positivity has been described in patients with both type I diabetes and autoimmune hepatitis. Theoretically, it can also be falsely positive in other autoimmune disease.