Table 1. Summary of the methodological constructs of the four frailty indexes compared.

Frailty index	Evaluated parameters	Frailty determination	Conceptual approach
FI-SOF (11, 12)	3 items: 1) unintentional weight loss; 2) inability to rise from a chair five times without the use of arms; 3) low energy level	Robust: 0 component; prefrail: 1 component; frail 2 or 3 components. Only grading available	Phenotypic
FI-CD (14)	32 items: difficulty with eating, dressing, walk around, getting in/out bed, getting bath, toileting, using telephone, going out, shopping, cooking, light house work, taking medicine, managing money, arthritis, Parkinson's disease, glaucoma, diabetes, stomach problems, history of heart attack, hypertension, history of stroke, flu, broken hip, broken bones, trouble with bladder/bowels, dementia, self-rated health, as well as problems with vision, hearing, ear, teeth, and feet	The sum of the presence of deficits divided by the total number of all potential deficits. No grading available	Accumulation of deficits
FI-CGA (13)	10 domains: 1) cognitive status; 2) mood and motivation; 3) communication; 4) mobility; 5) balance; 6) bowel function; 7) bladder function; 8) basal ADL and IADL); 9) nutrition; 10) social resources	Problems in each domain were scored 0 (no problem), 1 (minor problem) and 2 (major problem). The sum determine the index. The cut-off for mild, moderate and severe frailty were respectively 0–7, 7–13 and >13	Multidimensional
MPI (14)	8 domains: 1) basal ADL; 2) IADL; 3) cognitive (SPMSQ); 4) comorbidity (CIRS); 5) nutrition (MNA); 6) risk of developing pressure sores (ESS); 7) the number of drugs taken by patients at admission; 8) co-habitation status	For each domain a tripartite hierarchy was used: 0 = no problems, 0.5 = minor problems and 1 = major problems. The sum was divided by the total number of the domains. The final MPI was expressed as three grades of risk of all-cause mortality: low risk (value ≤0.33), moderate risk (value between 0.34 and 0.66) and severe risk (value >0.66)	Multidimensional

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.