Eating a 100-percent gluten-free diet may be theoretically possible, "but in reality, it's nearly an impossible feat," National Foundation for Celiac Awareness president Alice Bast told the Mayo Clinic. The Food and Drug Administration actually allows products to be labeled gluten-free, even if they contain extremely small amounts of up to 20 parts per million, because it says that there aren't reliable laboratory tests to detect gluten at lower levels than that.
Further, the FDA doesn't actually require food manufacturers to test their finished products for gluten content. Instead, they can meet the requirements simply by obtaining certificates from their ingredient suppliers, attesting that the ingredients are gluten-free [source: FDA]. As Consumer Reports points out, if a manufacturer uses the same equipment for manufacturing food with gluten in it, it's entirely possible for cross-contamination to occur. Additionally, recent research has shown that some supposedly gluten-free products turn out to contain malt, malt extract or malt syrup as minor ingredients, which usually are made from barley with gluten in it.
The FDA reassures people with celiac disease that all this probably doesn't put them at too much risk, since research shows that most of them can tolerate extremely small amounts of gluten. But for those who believe that gluten is an intolerable poison that must be avoided in any amount, the reality is going to be difficult to swallow.