The first step to be able to accurately screen for coeliac disease is to ensure a gluten containing diet has been consumed for enough time prior to a test. A gluten containing diet is specified as 2 slices of gluten containing bread per day for 6-8 weeks prior to the test, without this symptoms may be masked and the screening inaccurate.
The test most commonly issued by a GP in Australia screens for tissue transglutaminase Antibodies (tTG-IgA). The tTG-IgA test will be positive in about 98% of patients with coeliac disease who are on a gluten-containing diet. This is called the tests sensitivity (the accuracy of the test for picking up coeliac disease). The same test will come back negative in about 95% of healthy people without celiac disease (so about 5% of people won’t screen accurately). This is called the test’s specificity. Though rare, this means patients with coeliac disease could have a negative antibody test result (even though they have coeliac disease).
There is also the possibility of having a false positive test (a person screens as having coeliac disease when they don’t). This is uncommon and most often found in individuals that have an autoimmune disease like type 1 diabetes, autoimmune liver disease, Hashimoto’s thyroiditis, psoriatic or rheumatoid arthritis, and heart failure.
The upside of the antibody screening tests is that they are easy to administer (referral from your GP for a blood test at your local collection centre) but the down side is a small percentage of the population may return false positive or negative result. Therefore, the only way to clinically diagnose coeliac disease is through a tissue biopsy of the small intestine (colonoscopy).
While it is very rare, the possibility for someone with coeliac disease to have negative antibody test results does exist, therefore if your tests were negative, but you continue to experience symptoms, consult your GP and dietitian about a tissue biopsy.