Several major schools of thought support different approaches to the measurement of frailty:
Physical Frailty: As introduced by Fried and colleagues considers frailty to arise because of biological problems that disrupt the body’s production and use of energy, its stress response systems, and its ability to maintain homeostasis. This sort of disruption has downstream effects—notably, muscle wasting and difficulties processing nutrients. In this model, molecular and cellular changes associated with aging result in declines in the health of the body’s systems that prevent a frail person from adapting to stressors. To assess frailty from this perspective, weakness, slowness, exhaustion, decreased activity, and unintentional weight loss are measured and summarized as present or absent, with the number of factors present then added to give a score. Those with scores of 0 are non-frail, 1-2 criteria met are to be considered pre-frail, and 3-5 criteria met constitutes frailty. Scores from this Physical frailty tool predict adverse outcomes in many different patient populations —that is, worsening chronic disease, loss of ability to bounce back from physical stressors such as injuries or surgeries, new or worsening disability, health crises such as falls or hospitalizations, entering a nursing home, and early mortality (Fried et al., 2001; Bandeen-Roche et al., 2006).
Deficit Accumulation: Developed by Rockwood and colleagues, this assessment characterizes frailty through the lens of many illnesses and health problems--“deficits”--that by themselves may not strongly predict health declines but sum to something larger--an increased rate of aging, or an increased risk of adverse outcomes (Mitnitski et al., 2001; Rockwood et al., 2007). This model includes both direct measures of physical function and a history of disability and comorbidities to identify vulnerable older adults. To assess frailty from this perspective, a frailty index score is calculated as the proportion of potential deficits an individual has. The more problems accumulated, the more frail an individual is considered to be. The scales vary in content and number of items but generally include 30-70 items representing multiple domains of health. They can include laboratory findings, physical function problems or disabilities, diseases, symptoms, sensory or cognition difficulties, and other health problems. Longer scales such as the Frailty Index can include over 70 items. Other scales have been designed for ease of use or screening with more limited items (Subramaniam et al., 2018). Like physical frailty, deficit accumulation frailty predicts adverse outcomes.
Pre-Disability: This assessment model has emerged as a definition of frailty that posits frailty as a precursor to overt disability in older persons. This definition has been operationalized using the Short Physical Performance Battery (SPPB) as a SPPB score of 3-9 among persons with no mobility disability (Cesari et al., 2017).
It is important to note that the sub-groups deemed frail by different methods often have minimal overlap (Cigolle et al., 2009; Xue et al., 2019). Therefore it is increasingly widely recognized that a frailty measure should be chosen with care to address the research question most directly (Buta et al., 2016).