Summary of the methodological constructs of the four frailty indexes compared.

FI-SOF: Frailty Index derived from the Study of Osteoporotic Fractures; FI-CD: Frailty Index based on cumulative deficits; FI-CGA: Frailty Index based on a Comprehensive Geriatric Assessment; ADL: activities of daily living; IADL: instrumental activities of daily living; SPMSQ: Short Portable Mental Status Questionnaire; CIRS: Cumulative Illness Rating Scale; MNA: Mini Nutritional Assessment; ESS: Exton-Smith Scale; MPI: Multidimensional Prognostic Index.