Unstable disability and the fluctuations of frailty.

Despite increasing use in the literature of old age, 'frailty' remains an ill-defined term. The components of frailty have not been sufficiently defined to be used to identify a research population or a group requiring a public health intervention. It is currently too indefinite a state to be used to identify particular needs in an individual, indeed being 'too frail' may be used to justify investigative and therapeutic nihilism. However, we contend that, properly defined, with the major contributing factors to frailty teased out, it is a useful concept and is the essential component of that important condition of old age—unstable disability.