There's a persistent idea that McCann's Oats are safe for people with celiac sprue, but McCann's itself doesn't make any guarantee. Its website currently states
All McCann’s oat products are processed in a dedicated oatmeal mill that handles only oatmeal. In the supply chain between farm and mill, there is a possible cross contamination with other grains, such as wheat and barley but we reckon that the level of non-oat grains to be less than 0.05%. Cleaning equipment within the milling process would remove the vast majority of these grains along with other elements such as stones, straw etc.A study by Tricia Thompson, published in the New England Journal of Medicine in 2004, actually tested samples of McCann's as well as Quaker and Country Choice, finding that each brand had levels of gluten above 200 ppm. In fact, as noted at celiac.com, Country Choice—not McCann's—was the best of the three in this very limited sampling. The Country Choice website states
But we cannot guarantee that McCann’s oats are totally gluten free and we recommend that consumers use their own judgement as to whether they wish to use our oatmeal or not. Many celiacs can tolerate our oatmeal products without any adverse effect but they may not suit those who are particularly sensitive.
Some of our products do not contain wheat. However, all of our products are processed on shared equipment that uses wheat. Please read the ingredient list carefully before purchasing.In recent years, some pure oats vendors finally emerged. They include Gluten Free Oats, Gifts of Nature, and Cream Hill Estates, which offers this overview of the issues.
While research has indicated that many people with celiac sprue appear able to eat oats safely, it has not excluded the possibility that some people may have a serious adverse reaction to oats. More studies should be done on this subject, to clarify the situation. In the meantime, if you are on a medical gluten-free diet and you want to try oats, it is a good idea to heed the advice of the Gluten-Free Indy Celiac Support Group and wait until your blood tests indicate that you have successfully gone gluten-free, stick with pure oats, introduce them slowly into your diet, restrict the quantity you eat, and do so in coordination with your celiac sprue specialist and, perhaps, a nutritionist. The monitoring of your serum levels and your digestive health can help you and your medical team determine whether pure oats are appropriate for your particular diet.
Thanks to members of the New York City Celiac Disease Meetup Group for providing information used in this post.