Adaptation of the Charlson Comorbidity Index for Register-Based Research in Sweden

PURPOSE: Comorbidity indices are often used to measure comorbidities in register-based research. We aimed to adapt the Charlson comorbidity index (CCI) to a Swedish setting. METHODS: Four versions of the CCI were compared and evaluated by disease-specific experts. RESULTS: We created a cohesive coding system for CCI to 1) harmonize the content between different international classification of disease codes (ICD-7,8,9,10), 2) delete incorrect codes, 3) enhance the distinction between mild, moderate or severe disease (and between diabetes with and without end-organ damage), 4) minimize duplication of codes, and 5) briefly explain the meaning of individual codes in writing. CONCLUSION: This work may provide an integrated and efficient coding algorithm for CCI to be used in medical register-based research in Sweden.