Differences in the Measurement of Comorbidities Based on ICD-10-CM Coding Definitions Using Medicare Advantage Encounter Data and Fee-For-Service Claims

BACKGROUND: Medicare Advantage (MA) encounter data became available for research in 2019; data quality concerns remain. OBJECTIVES: We evaluated the consistency of ICD-10-CM comorbidity coding between MA and Fee-For-Service (FFS) data. METHODS: We used round 7 (2017) of the National Health and Aging Trends Study (NHATS) linked to Medicare enrollment, MA encounter, and FFS claims (2016-2017). We included participants continuously enrolled in MA or FFS for 1 year before round 7. Comorbidities were identified using ICD-10-CM codes from the Gagne combined comorbidity index. Demographic, socioeconomic, and clinical covariates from NHATS for FFS beneficiaries were standardized to resemble those for MA beneficiaries. We estimated crude and standardized comorbidity prevalence differences (PDs) between MA and FFS beneficiaries. RESULTS: Among 5158 beneficiaries (MA: 40%, FFS: 60%), MA beneficiaries were more likely to be Black, Hispanic, and socioeconomically disadvantaged. After standardization, comorbidity prevalence was similar between groups. Peripheral vascular disorder (PD=7.2%, 95% CI: 3.8%-10.6%) and renal failure (PD=3.7%, 95% CI: 0.9%-6.5%) were more common in MA beneficiaries; fluid/electrolyte disorders (PD=-3.2%, 95% CI: -5.5 to -1.0%) and deficiency anemias (PD=-5.0%, 95% CI: -7.6 to -2.3%) were more common in FFS beneficiaries. Other PDs were less than 3 percentage points. CONCLUSIONS: Discrepancies in comorbidity prevalence may reflect true differences or coding variations influenced by provider incentives, documentation standards, or diagnostic priorities. Comorbidity prevalence was largely consistent between MA encounters and FFS claims, supporting the reliability of MA encounter data for aging research. Additional validation studies should address remaining discrepancies.