Frailty Phenotype and Frailty Index in Companion Dogs: Are They Identifying the Same Vulnerable Individuals?

Frailty is a multidimensional syndrome of aging increasingly studied in companion dogs. Two main operational models have been developed: the frailty phenotype, based on five clinical criteria, and the frailty index, derived from the accumulation of health deficits. Despite their frequent use in aging research, no comparative analysis of these models has been conducted in canine populations.Baseline data from 566 dogs aged six years and older in the CaniAge cohort were used to assess frailty using both Frailty Phenotype and Frailty Index. Prevalence of frailty was 12% according to the Frailty Index (EARS-FI ≥ 0.49) and 8.4% according to the Frailty Phenotype, with limited overlap between the two classifications. Both measures were positively associated with chronological age and with some adverse health indicators including comorbidity or sleep quality, and reduced quality of life. Multivariable analyses showed that polypharmacy was independently associated with frailty index. The fact of having a daily activity higher than 15 minutes per day was negatively associated with frailty phenotype independently of age and cofounding factors and specific personality traits (low extraversion and motivation) were independently associated with Frailty Phenotype-defined frailty. The Frailty Index showed slightly superior discrimination against poor owner’s canine reported quality of life.Findings indicate that the Frailty Index and Frailty Phenotype identify related but distinct domains. The frailty index captures a broader spectrum of physiological decline, whereas the Frailty Phenotype is more closely aligned with observable physical characteristics and personality. Use of both instruments provides complementary insights into aging trajectories in dogs and supports their combined application in translational geroscience.