The authors of this paper argue for a greater use of more traditional measures of age-related frailty as the research community expands efforts to find biological biomarkers to measure the progression of aging. It is true that some groups have used weighted combinations of existing clinical tests, such as grip strength, walking speed, and so forth, to produce biomarkers of aging that are not that different in accuracy when compared to epigenetic clocks and the like. It seems likely that there is more room for long term improvement in terms of accuracy and utility on the biomarker side of the house, however.
Biological age is the concept of using biophysiological measures to more accurately determine an individual's age-related risk of adverse outcomes. Grading of the degree of frailty and measuring biomarkers are distinct methods of measuring biological age. Chronological age is rigid and fails to account for the variable effects of time on individuals. The construct of "biological age" aims to give a more ordered relation between an individual's current health state and their proximity to death. This in turn can enable a novel approach to individualizing care and potentially yield ways in which aging might be modified.
A quantitative approach to frailty yields a proxy measure of biological age that can be formulated from deficit accumulation indices and geriatric assessment. The great advantage of this approach is that it offers information that is immediately relevant to guiding treatment strategies and estimating prognosis. Biomarkers derived from the mechanistic underpinnings of aging hold out the promise of measuring processes of aging before clinically recognizable deficits ensue. So doing, they can complement the information about individuals' health that is required for appropriate care planning and that can readily be summarized in a frailty index. These clinically derived frailty indices can outperform measures of biological age in predicting proximity to death without incorporating chronological age into their measurement. Furthermore, the cost of routinely performing sophisticated biomarker-based measures of biological age is prohibitive.
Complementarity of biomarkers and frailty indices can be demonstrated through both measures used simultaneously or by being combined. Further, frailty indices can use biomarkers as deficits, even when normal ranges have not been established for that purpose. In ways like this, the siloed approach to biological biomarkers can be overcome, and perhaps enhanced, through their combination with frailty measures that are likely already to be in the clinical record or that can feasibly be derived from it.