Summary

Background and objectives The subclinical pathogenesis of granulomatosis with polyangiitis (GPA) has not been completely elucidated. Proteinase 3 (PR3) antibodies are strongly associated with GPA, but have not been evaluated before disease presentation. Design, setting, participants, & measurements This was a retrospective case-control serum bank study in which PR3 antibodies andC-reactive protein (CRP) in up to three longitudinal serum samples for 27GPApatients before diagnosis (1 day–19 years) were comparedwith 27 controls whose serum sampleswerematched for age, sex, and race. This study analyzed all patients with American College of Rheumatology criteria–confirmed disease identified in the Department of Defense electronic medical records between 1990 and 2008. ResultsAgreater percentage of GPA patients had at least one elevated PR3 antibody level ($6 U/ml) as well as at least one detectable PR3 antibody level (.1U/ml) before diagnosis comparedwithmatching controls (63 % [17 o