How is celiac disease diagnosed?
Celiac disease is suspected in people who have signs or symptoms of recurrent diarrhea, abdominal bloating, and malabsorption or malnutrition. Other diseases, however, can produce malabsorption and malnutrition, such as pancreatic insufficiency (a pancreas that is not able to produce digestive enzymes), Crohn's disease of the small intestine, and small intestinal overgrowth of bacteria. It is important, therefore, to confirm suspected celiac disease with appropriate testing.
Specific antibody tests for celiac disease
Antibodies are proteins that are produced by the immune system to fight viruses, bacteria, and other organisms that infect the body. Sometimes, however, the body produces antibodies against non-infectious substances in the environment (for example, in hay fever) and even against its own tissues (autoimmunity).
Certain blood tests should be performed initially to look for antibodies specific to celiac disease. Immunoglobulin A (IgA) anti-tissue transglutaminase antibody is the single preferred test for detection of celiac disease. If abnormally elevated levels of IgA endomysial and anti-tissue transglutaminase antibodies are found, a person almost certainly has celiac disease. High levels of another antibody, anti-gliadin antibody (AGA), may also be detected, but they don't necessarily mean a person has celiac disease. However, anti-gliadin antibody levels are useful in monitoring response to treatment, because they will usually begin to fall within several months of successful treatment of celiac disease with a gluten-free diet. Similarly, anti-endomysial antibody levels decrease on a gluten-free diet and often become negative in treated patients.
Who should undergo antibody blood tests for celiac disease?
Experts in the U.S. recommend antibody blood tests for people who have a higher likelihood than normal of having celiac disease. They include those with:
- Chronic diarrhea (diarrhea that does not resolve in three weeks), increased amount of fat in the stool (steatorrhea), and weight loss
- Excess gas, bloating, and abdominal distension
- Parents or siblings with celiac disease
- Growth retardation
- Unexplained iron deficiency anemia, or deficiency of folate and vitamin B12
- Skin blisters that are itchy (dermatitis herpetiformis)
- Recurrent, painful mouth sores (aphthous stomatitis)
- Diseases associated with celiac disease, such as type I (insulin-dependent) diabetes, autoimmune thyroid disease, rheumatoid arthritis, systemic lupus, ulcerative colitis, etc.
Small intestinal biopsy
People who test positive for celiac disease antibodies, or who have a high probability of celiac disease regardless of the results of the blood tests, should have a small intestinal biopsy to confirm the diagnosis. A small intestinal biopsy is performed with an esophagogastroduodenoscopy (EGD). During an EGD, the doctor inserts a long, flexible viewing endoscope through the mouth and into the duodenum (the first part of the small intestine, which is connected to the stomach). A long, flexible biopsy instrument then can be passed through a small channel in the endoscope to obtain samples of the intestinal lining of the duodenum. A pathologist evaluates the tissue samples for loss of villi and other characteristics of celiac disease, such as an increased number of lymphocytes.
Why is it important to accurately diagnose celiac disease?
Diagnosis of celiac disease should be firmly established before starting a person on a gluten-free diet. Here's why:
- The gluten-free diet requires avoiding wheat, barley, and rye -- products that are dietary staples, at least in the U.S.
- Patients with irritable bowel syndrome (IBS) may experience improvements in bloating, abdominal pain, and diarrhea with a gluten-free diet. These patients may be misdiagnosed as having celiac disease. Without confirmation of celiac disease by small intestinal biopsy, they may be unnecessarily committed to lifelong gluten restriction.
- A gluten-free diet can lower blood antibody levels and allow the microscopic appearance of the small intestine to lose the typical appearance of celiac disease, complicating subsequent efforts at making a firm diagnosis of celiac disease.
How are malabsorption and malnutrition evaluated in celiac disease?
Celiac disease causes malabsorption of nutrients and leads to malnutrition. Tests are available that help in the evaluation of malabsorption and malnutrition; however, because other diseases can cause both malabsorption and malnutrition, these tests cannot be used to diagnose celiac disease.
Stool examination for malabsorption
Stool from patients with celiac disease often contains many globules of fat -- a condition called steatorrhea -- that can be viewed under a microscope using a dye to make them visible. To conclusively diagnose steatorrhea, however, stool is collected over a 72-hour period, and the fat in the stool is chemically measured and quantified.
Because malabsorption and steatorrhea can occur with other intestinal diseases (such as small intestinal bacteria overgrowth, prior small intestinal resection, extensive Crohn's disease of the small intestine, and chronic pancreatitis), stools with large amounts of fat raise the suspicion of celiac disease but cannot be used to diagnose celiac disease.
Blood tests for malabsorption and vitamin deficiencies
Malabsorption reduces the absorption of protein and causes a reduction in blood protein levels. This can be seen commonly as a reduced blood level of albumin, the most concentrated protein in blood. Other proteins in blood -- pre-albumin and transferrin -- also may be reduced.
Intestinal malabsorption can lead to deficiencies and low blood levels of iron, calcium, vitamin B12, folate, and the fat-soluble vitamins (A, D, E, and K). These deficiencies, in turn, can lead to other blood test abnormalities, such as:
- Iron deficiency anemia: Iron is an important component of hemoglobin in red blood cells. When iron is deficient, production of red blood cells is impaired, and anemia develops. Iron-deficiency anemia can occur through loss of blood (with its iron-containing red blood cells) or lack of intestinal iron absorption. Heavy menstrual bleeding, GI tract bleeding, and cancer of the colon that bleeds into the intestine are three common causes of iron-deficiency anemia due to blood loss. Celiac disease causes iron-deficiency anemia by reducing intestinal iron absorption. In fact, iron-deficiency anemia can be an important clue to the presence of celiac disease.
- Abnormally prolonged prothrombin time (PT): PT is a blood test that measures how quickly blood clots. Clotting of blood requires special proteins or clotting factors, many of which are made by the liver. Formation and activation of clotting factors by the liver requires vitamin K. When vitamin K absorption from the intestine is reduced, as in celiac disease, the production of clotting factors by the liver is inadequate, and blood clotting is delayed. Delayed clotting is reflected in an abnormal PT, and individuals with an abnormal PT have a higher risk of abnormal or excessive bleeding.
Iron deficiency anemia, abnormal PT, steatorrhea, and low iron and vitamin levels can occur in diseases other than celiac disease. Therefore, the presence of these abnormalities only raises the suspicion of celiac disease, but does not specifically diagnose celiac disease.