A Tale of Many Canadas: Associations of Ancestry With Juvenile Idiopathic Arthritis Categories and Disease Severity at Presentation to Care

Objective To assess associations of ancestry with juvenile idiopathic arthritis (JIA) categories and clinical Juvenile Arthritis Disease Activity Scores (cJADAS10) at presentation to pediatric rheumatology care in a multicultural country with universal health care. Methods Parents reported their child's ancestry in the Research in Arthritis in Canadian Children Emphasizing Outcomes (ReACCh‐Out) cohort. For each ancestry reported for ≥30 children, we compared JIA category distribution and median cJADAS10 scores among three groups: only that ancestry, with that and other ancestries, and without that ancestry. Chi‐square, Fisher's exact, and Kruskal‐Wallis tests compared the three groups and multivariable linear regression assessed factors associated with cJADAS10 scores. Results Among 1,407 participants, 629 (44.7%) reported more than one ancestry. British ancestry was associated with higher median cJADAS10 scores (7.5) and higher frequency of enthesitis‐related arthritis (18.7%) and psoriatic arthritis (10.0%), French ancestry was associated with lower cJADAS10 scores (5.8) and higher oligoarthritis (51.2%), Indigenous ancestry was associated with higher cJADAS10 scores (11.0) and higher rheumatoid factor–positive polyarthritis (21.9%), Black ancestry was associated with higher rheumatoid factor–positive polyarthritis (16.0%), and Eastern European ancestry was associated with lower cJADAS10 scores (3.6). Associations of ancestry with cJADAS10 scores were largely explained by differences in JIA categories (total R 2 = 0.28, with R 2 = 0.25 for JIA category alone). Black ancestry was associated with longer time from symptom onset to diagnosis (27 vs 18.9 weeks). Conclusions British and French ancestries had distinct associations with JIA categories and cJADAS10 scores, and many children had multiple ancestries, questioning the use of a single “European” reference group. Higher cJADAS10 scores were largely explained by differences in JIA categories across ancestries.