Abstract The Faurot frailty index (FFI) is a validated algorithm that uses enrollment and International Classification of Diseases, 9th Edition, Clinical Modification (ICD-9-CM)-based billing information from Medicare claims data as a frailty proxy. In October 2015, the US healthcare system transitioned from ICD-9-CM to ICD-10-CM. Applying the Centers for Medicare and Medicaid Services General Equivalence Mappings, we translated diagnosis-based frailty indicator codes from ICD-9-CM to ICD-10-CM, followed by manual review. We used interrupted time series analysis of Medicare data to assess comparability of the pre- and post-transition FFI. In cohorts of beneficiaries enrolled in January 2015-2017 with 8-month frailty lookback periods, we estimated associations between the FFI and 1-year risk of geriatric-relevant outcomes (mortality, hospitalization, skilled nursing facility [SNF] admission). Updated indicators had similar prevalence compared to pre-transition definitions. The median and interquartile range for predicted probability of frailty were similar before and after the ICD transition (pre-transition: 0.034 [0.02-0.07]; post-transition: 0.038 [0.02-0.09]). The updated FFI was associated with increased risks of mortality, hospitalization, and SNF admission, similar to findings from the ICD-9-CM era. Studies of medical interventions in older adults using administrative claims should use validated indices, like the FFI, to mitigate confounding or assess effect measure modification by frailty.