“Collagenous gastritis (CG) is a rare gastrointestinal disorder with fewer than 300 cases reported in the English-language literature.”  If you have to manage one of these rare cases, here is a useful reference:

• The prevalence of CG was 2.1/100,000 in children aged younger than 18 years
• The endoscopic and histologic findings remained pathologic in all the examined patients during a median follow-up of 4.4 years
• The serum levels of calprotectin and amyloid A were increased in 10/15 (67%) and 5/15 (33%) of the patients, respectively