Updating and Validating the Rheumatic Disease Comorbidity Index to Incorporate ICD-10-CM Diagnostic Codes

Objective: To update and validate the Rheumatic Disease Comorbidity Index (RDCI) utilizing International Classification of Diseases, Tenth Revision, Clinical Modification (ICD-10-CM) codes. Methods: We defined ICD-9-CM (n = 1,068) and ICD-10-CM (n = 1,425) era cohorts (n = 862 in both) spanning the ICD-9-CM to ICD-10-CM transition in a multicenter, prospective rheumatoid arthritis registry. Information regarding comorbidities was collected from linked administrative data over 2-year assessment periods. An ICD-10-CM code list was generated from crosswalks and clinical expertise. ICD-9– and ICD-10–derived RDCI scores were compared using intraclass correlation coefficients (ICC). The predictive ability of the RDCI for functional status and death during follow-up was assessed using multivariable regression models and goodness-of-fit statistics (Akaike's information criterion [AIC] and quasi information criterion [QIC]) in both cohorts. Results: Mean ± SD RDCI scores were 2.93 ± 1.72 in the ICD-9-CM cohort and 2.92 ± 1.74 in the ICD-10-CM cohort. RDCI scores had substantial agreement in individuals who were in both cohorts (ICC 0.71 [95% confidence interval 0.68–0.74]). Prevalence of comorbidities was similar between cohorts with absolute differences <6%. Higher RDCI scores were associated with a greater risk of death and poorer functional status during follow-up in both cohorts. Similarly, in both cohorts, models including the RDCI score had the lowest QIC (functional status) and AIC (death) values, indicating better model performance. Conclusion: The newly proposed ICD-10-CM codes for the RDCI-generated comparable RDCI scores to those derived from ICD-9-CM codes and are highly predictive of functional status and death. The proposed ICD-10-CM codes for the RDCI can be used in rheumatic disease outcomes research spanning the ICD-10-CM era.