Constructing a quality frailty index: you get out what you put in.

The number of papers on frailty has risen exponentially over the last decade, with an ever-increasing diversity of researchers investigating the role of frailty in their area of interest. This rise has been matched by heterogeneity in how and when frailty is assessed. Over this period, the frailty index has become a stalwart method for enabling quantification of frailty, where its robust yet flexible design allows a frailty index to be applied in diverse settings and across a range of datasets. However, this variation and flexibility has also meant that the use of the frailty index has varied considerably in the application of the principles set out in Searle et al’s 2008 description of the standard approach [1]. ; Peter Hanlon is funded through a Clinical Research Training Fellowship from the Medical Research Council (Grant reference: MR/S021949/1). Nicholas Evans is funded through a Stroke Association Senior Clinical Lectureship (SA SCL MED 22\100006).