A recent analysis (JA Syage et al.The American Journal of Clinical Nutrition, Volume 107, Issue 2, 1 February 2018, Pages 201–207, https://doi.org/10.1093/ajcn/nqx049) (Thanks to Kipp Ellsworth for this reference) of 259 patients with celiac disease (~75% pediatric) showed that a large number with ongoing gluten ingestion based on urine and stool tests of gluten excretion.
Results: The average inadvertent exposure to gluten by CD individuals on a GFD was estimated to be ∼150–400 (mean) and ∼100–150 (median) mg/d using the stool test and ∼300–400 (mean) and ∼150 (median) mg/d using the urine test. The analyses of the latiglutenase data for CD individuals with moderate to severe symptoms indicate that patients ingested significantly >200 mg/d of gluten.
My take (borrowed from authors): These surrogate biomarkers of gluten ingestion indicate that many individuals following a GFD regularly consume sufficient gluten to trigger symptoms and perpetuate intestinal histologic damage.
Free link to full article: Determination of gluten consumption in celiac disease patients on a gluten-free diet