Table 1.

Characteristics of included studies

Reference	Study design	Inclusion criteria	Exclusion criteria	Cohort size	Age Illness severity (SD or IQR)	Frailty scale(s) used and how determined	Modification of scale(s) (Y/N) training for assessors (Y/N)	Frailty definition	Number frail/non-frail	Outcomes assessed	Outcomes reported by frailty severity (Y/N)
				n					n (%)
Bagshaw [26]	Prospective observational multicenter cohort study in 6 Canadian ICUs	Age >50 years Consent obtained	ICU stay <24 h Previous study enrollment	421	67 (10) years APACHE II: 20 (7)	CFS (9 point) Interview with pt., surrogate, or caregiver	No Yes	CFS > 4	Frail: 138 (33) Non-frail 283 (67)	Mortality, morbidity, health service utilization, AEs, QoL, discharge disposition	Yes
Brummel [32]	Prospective observational multicenter study in 5 ICUs in the USA	Age ≥18 Treated for respiratory failure or shock	Organ dysfunction for >72 h Recent ICU admission Severe cognitive impairment Inability to speak in English Substance abuse, Homeless or too far from study site	1040	62 (53–72) years APACHE II: 24 (18–30)	CFS (7 point) Interview with pt. or proxy and review of medical record	No Yes	CFS > 4	Frail: 307 (30) Non-frail 733 (70)	Mortality, morbidity, health service utilization, AEs, QoL, discharge disposition	Yes
Fisher [31]	Prospective observational feasibility study in an Australian ICU	All patients admitted to ICU over 3 months	Anticipated death within 24 h Admission for palliation or for organ donation	205	60 (17.4) years APACHE III: 24 (26)	CFS (9 point) Interview with next of kin or from chart review	No Yes	CFS > 4	Frail: 28 (14) Non-frail: 177 (86)	Mortality, health service utilization, discharge to rehab	Yes
Heyland [35, 36]	Prospective observational multicenter study in 24 Canadian ICUs	Age ≥80 years ICU stay >24 h	Non-Canadian residents Lack of family members	610	84 (3) years APACHE II: 22 (7)	CFS (7 point) FI (43 item) from CGA Interview with surrogate or caregiver	Yes Yes	CFS > 4 FI > 0.2	Frail:   CFS: 193   FI: 360 Non-frail:   CFS: 416 FI: 250	Mortality, morbidity, health service utilization, QoL, discharge disposition	Yes
Hope [30, 37]	Prospective observational single-center study in the USA	Critically ill adult patients	NR	84	57.4 (18.6) years APACHE IV: 60.1 (24.7)	CFS FAT ICU Interview with surrogate	Yes NR	CFS N/R FAT-ICU > 3	Frail:   CFS: 29 (35)   FAT-ICU: 23 (27) Non-frail:   CFS: 55 (65)   FAT-ICU: 61 (73)	Mortality, health service utilization	No
Hope [33]	Prospective observational cohort study in two centers in the USA	Age ≥18 years ICU admission within 30 days of ER admit ICU admit for non-elective procedure	ICU stay expected <24 h Did not speak English or Spanish No surrogates available	95	57.1 (17.5) years APACHE IV: 60.2 (22)	CFS (9 point) Interview with patient or surrogate	No Yes	CFS > 4	Frail: CFS: 34 (36) Non-frail: 61 (64)	Mortality, morbidity, health service utilization	Yes
Kizilarslanoglu [27]	Prospective observational single-center study of patients in a medical ICU in Turkey	Age >60 years	Inability to evaluate functional, physical, and mental status prior to ICU	122	71 (range 60–101) years N/R	FI (55 item) from CGA Interview with next of kin	No N/R	FI > 0.4	Frail: 26 (21) Non-frail: 96 (79)	Mortality, morbidity, health service utilization	Yes
Le Maguet [28]	Prospective observational multicenter study in 4 French ICUs	Age >65 years ICU stay >24 h	No surrogates and patient could not be interviewed	196	75 (6) years SAPS II: 48 (17)	FP CFS (9 point) Interview with patient or relative	No N/R	FP > 2 CFS > 4	Frail:   FP: 80 (41)   CFS: 46 (24) Non-frail:   FP: 116 (59)   CFS: 150 (76)	Mortality, morbidity, health service utilization	Yes
Mueller [29]	Prospective observational study in two surgical ICUs at 1 center in the USA	Age >18 years ICU stay >24 h	Patients from nursing homes Pre-existing paralysis	102	61.9 (15.8) years APACHE II: 10 (7–15)	FI (50 item) Interview with patient or proxy	No N/R	FI > 0.25	Frail: 39 (38) Non-frail: 63 (62)	Mortality, morbidity, health service utilization	No
Zeng [34]	Prospective observational study in a geriatric ICU in China	Age >65 years	None reported	155	82.7 (7.1) years APACHE II: 14.6 (5.8)	FI (52 items) ICU admission records	Yes N/R	FI > 0.22	Frail: 93 (60) Non-frail: 62 (40)	Mortality	Yes

ICU intensive care unit, SD standard deviation, IQR interquartile range, QoL quality of life, CFS clinical frailty scale, FI frailty index, pt. patient, LOS length of stay, FAT-ICU frailty assessment tool for intensive care unit (FAT-ICU), APACHE acute and chronic health evaluation, SAPS simplified acute physiology score, FP frailty phenotype