KANSAS CITY — When ordering gluten-free food in a restaurant, is it truly free of that allergen, especially if it’s served in an establishment that already serves baked foods or similar grain-based products?

Maybe not because it’s so easy for these products to be cross contaminated by flour or other gluten-containing residues during the preparation process, in ovens, through dishwashing or even in the air in such facilities. That’s why the web sites of so many pizza chains and other restaurants suggest that people with celiac or severe gluten-intolerance disorders avoid ordering — or use caution when ordering — gluten-free foods.

There’s no way to guarantee those free-from foods do not have the allergen in or on them. Such challenges with serving gluten-free foods in food service establishments were highlighted in a study published recently in The American Journal of Gastroenterology that showed gluten was detected in 32% of restaurant items promoted as gluten-free. More than half of gluten-free pizza and pasta items tested positive. The study reportedly involved 804 people performing 5,624 tests by using a commercially available gluten-detection device that has been shown to detect gluten at levels as low as 5 parts per million (p.p.m.).

However, the study was unable to quantify the specific percentage of items with more than 20 p.p.m. of gluten. That’s the threshold in a Food and Drug Administration rule for packaged foods labeled as gluten-free. No similar rule applies to restaurant foods. However, when marketing any food with allergens, it’s always a good rule of thumb to play it safe.

Food service establishments must be transparent with consumers to make sure that they are not playing with fire — and potentially with consumers’ health — when jumping on the gluten-free bandwagon.