The term 'gluten-free' implies no gluten, but in practice it is not possible to test for a zero level of gluten. Research has shown that people with coeliac disease are able to safely tolerate a very small amount of gluten. As a result low levels of gluten are allowed in products that are labelled gluten-free. When you see the term gluten-free this means that the food contains no more than 20 parts per million (ppm) gluten.