Frailty is increasingly relevant for clinicians to improve care for vulnerable older adults. Prominent frailty measures include the frailty phenotype and the frailty index. The frailty phenotype is grounded in a theoretical construct hypothesized to have an underlying biological basis. The frailty index describes frailty as a nonspecific age-associated vulnerability, reflected in an accumulation of medical, social, and functional deficits. Building on this model, Minitski et al. describe the development of a biological index that proves to be a reasonable method to predict mortality when compared to other frailty measurements. Strengths include its ability to import clinical measures, interchangeable components, and its potential ability to identify latent risk factors. Obstacles include the lack of a unifying biological theory related to aging, inclusion of costly research measures, and its inability to provide specific clues to the etiology of frailty according to the frailty index definition. Refinement in measures focused on aging-related biological changes rather than using measures that result from chronic disease states could help provide important biological insights and aid in the development of future treatment and preventive modalities.Please see related article: http://www.biomedcentral.com/1741-7015/13/161 .