Comparison of the frailty phenotype and frailty index for identifying vulnerable companion dogs

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 human 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 6.36% according to the Frailty Index binarised (EARS-FI ≥ 0.49) and 8.48% 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, independently of age and cofounding factors showed that polypharmacy and a lower extraversion were independently associated with frailty index. Engaging in daily activity higher than 15 min per day was negatively associated with frailty phenotype and specific personality traits (low extraversion and motivation) were independently associated with Frailty Phenotype-defined frailty. 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 and behavioral characteristics. Use of both instruments provides complementary insights into aging trajectories in dogs and supports their combined application in translational geroscience.