OBJECTIVE: Radiography is still used worldwide for detection of sacroiliitis in juvenile spondyloarthritis (SpA), despite low sensitivity and reliability. We aimed to define unequivocal evidence of sacroiliitis on pelvic radiography in skeletally immature youth for use in classification criteria when MRI is unavailable.

METHODS: Subjects were a retrospective cohort of juvenile spondyloarthritis (SpA) patients with a radiograph and MRI as part of a diagnostic evaluation for axial disease. Six musculoskeletal imaging experts underwent an iterative consensus process to define "unequivocal sacroiliitis" on radiography in skeletally immature youth. Radiographs were graded using the modified New York (mNY) criteria and the unequivocal sacroiliitis criteria. Interrater agreement was assessed with Fleiss' kappa statistic. Specificity, area under receiver operator characteristic (AUROC), and sensitivity of the two measures were tested using 2 MRI reference standards.

RESULTS: 112 subjects, median age 14.9 years were included. Fleiss' kappa was fair for the mNY [0.51 (95% CI: 0.39-0.64)] and unequivocal sacroiliitis criteria [0.55 (95% CI: 0.43-0.66)]. The unequivocal sacroiliitis criteria achieved >90% specificity using both MRI reference standards. Sensitivity (59.26/57.14 vs 44.83/43.33) and AUROC (0.76/0.76 versus 0.71/0.71) were higher, for both reference standards, for the unequivocal sacroiliitis in youth definition than the mNY criteria, respectively.

CONCLUSION: We propose the first consensus-derived definition of unequivocal sacroiliitis by radiography in skeletally immature youth. This definition achieved excellent specificity and had higher AUROC and sensitivity than the mNY criteria using both MRI reference standards. This definition has applicability to juvenile SpA axial disease classification imaging criterion when MRI is unavailable.