Validation of International Classification of Diseases Code–Based Case Definitions of Immune Checkpoint Inhibitor–Associated Inflammatory Arthritis From Administrative Health Data

Objective Immune checkpoint inhibitors (ICIs) for cancer can lead to immune-related adverse events, including ICI-associated inflammatory arthritis (ICI-IA). There are no validated International Classification of Diseases (ICD) code–based case definitions for ICI-IA. Methods We conducted a validation study using the Canadian Research Group of Rheumatology in Immuno-Oncology (CanRIO) database. Rheumatologist-confirmed ICI-IA was the reference standard, defined as new onset of at least one swollen joint on examination or synovitis on imaging following ICI exposure, without another cause or pre-existing IA. We compared ICD-9 (714.x and 696.0) and ICD-10 (M05.x, M06.x, and M07.x) IA diagnostic codes against the reference standard. Seven core case definitions of different combinations of ICD codes from the Physician Claims Database (PCD) and the Discharge Abstract Database were tested. Results were stratified by sex. Sensitivity testing with additional criteria was also evaluated. Results We included 228 patients in the final analysis: 100 with ICI-IA and 128 without ICI-IA. Sensitivity of the tested case definitions ranged from 1.0% to 88.0%, whereas specificity ranged from 86.7% to 100.0%. The case definition with at least one PCD IA code achieved the best balance of sensitivity (88.0%, 95% confidence interval [CI] 81.6%–94.4%) and specificity (86.7%, 95% CI 80.8%–92.6%). Case definition performances were similar between sexes. Additional criteria minimally improved specificity but sacrificed sensitivity. Conclusion ICD code–based case definitions of ICI-IA can accurately detect ICI-IA and can be used to support ICI-IA surveillance and research with administrative health data.