From what I've read in the academic medical journals, the level of response for someone with Celiac is somewhere around 50ppm. The international standard for gluten-free certification (known as the Codex) is 20ppm. So I'd think that a beer that comes in at 5ppm would be safe. However, part of the problem is that there's no perfect test to accurately measure gluten in beer. Part of the problem is the fermentation and alcohol, which impacts the tests. The other part of the problem is that most tests (i.e. ELISA) are designed to look for wheat gliadins, not barley hordeins. As a result, some tests underestimate the gluten content, while others overestimate. Coming up with an exact measurement of gluten in beer is a combination of science and educated guesswork.
Cheers, Pete