Oats and gluten
Gluten is the name given to the protein in wheat, rye, barley and oats. It is a composite name representing gliadin in wheat, hordein in barley, secalin in rye and avenin in oats.
The current laboratory tests can only measure gliadin, hordein and secalin, not avenin as it is a slightly different protein. When gluten-free oats are discussed, it means they are free from wheat contamination (gliadin, hordein and secalin).
Avenin is an essential part of oats and they will never be truly gluten-free even if they are described as being gluten-free.
Research has shown that approximately one in five people with coeliac disease react to pure, uncontaminated oats. Since there is no way of determining who the one in five is who do react to pure oats and gut damage can occur even without symptoms, the advice of Coeliac New Zealand is that oats should not be consumed by those with coeliac disease.
This information is from Coeliac NZ.