Multimorbidity in Medicare Beneficiaries: Performance of an ICD‐Coded Multimorbidity‐Weighted Index

Abstract

OBJECTIVES Most older adults have multimorbidity that impairs physical functioning, but it is difficult to quantify using claims data. We previously developed and validated a multimorbidity‐weighted index (MWI) that embeds physical functioning through disease weightings. We mapped these conditions to International Classification of Diseases, Ninth Revision, Clinical Modification (ICD‐9‐CM) codes and compared them with existing indices. DESIGN Population‐based prospective cohort. SETTING Respondents to the 2006‐2016 waves of the Health and Retirement Study (HRS) with linked Medicare claims data and continuous enrollment in 2006. PARTICIPANTS Community‐dwelling Medicare‐eligible HRS participants (N = 9923; mean age = 75.5 ± 8.5 y). MEASUREMENTS Individuals were followed for future physical functioning (2006‐2014) and mortality (2007‐2016). MWI conditions were mapped to ICD‐9‐CM codes to produce an ICD‐coded MWI (MWI‐ICD). We compared MWI‐ICD, simple disease count, Charlson, Elixhauser, and the health‐related quality of life comorbidity index (HRQOL‐CI) through distributions, hazard ratios for mortality, and relationships with future physical functioning. RESULTS MWI‐ICD exhibited the broadest distribution and most unique values (5891). Left censoring was most pronounced for Charlson (34.3% score = 0) and Elixhauser (13.1% score = 0) vs MWI (5.0% score = 0). Hazard ratios and concordance (C)‐statistics for mortality across extreme quartiles were similar for MWI‐ICD, Elixhauser, and Charlson but lower for disease count and the HRQOL‐CI. For physical functioning, MWI‐ICD yielded the greatest contrast across extreme quartiles and overall coefficient of determination (R 2 ). CONCLUSION MWI‐ICD was significantly associated with mortality and future physical functioning and comparable with established metrics for mortality prediction although not weighted to mortality. MWI‐ICD successfully captures diseases accumulation and functioning in claims data. J Am Geriatr Soc 68:999–1006, 2020

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