The frailty index (FI) is used to measure the health status of older individuals - as a proxy measure of aging. This measure was developed by Dr. Kennether Rockwood and Dr. Arnold Mitnitski at Dalhousie University in Halifax, Nova Scotia, Canada.
It is defined as the proportion of deficits present in an individual out of the total number of age-related health variables considered. An frailty index can be created in most secondary data sources related to health by utilizing health deficits that are routinely collected in health assessments. These deficits include diseases, signs, symptoms, laboratory abnormalities, cognitive impairments, and disabilities in activities of daily living.
Frailty Index (FI) = (number of health deficits present) รท (number of health deficits measured)
For example, a person with 20 of 40 deficits collected has an FI score of 20/40 = 0.5, for someone with 10, the FI score is 10/40 = 0.25. The FI takes advantage of the high redundancy in the human organism; this is why it is replicable across different databases, even when different items, and different numbers of items, are used. The standard procedure for creating a frailty can be found in an open-access publication.
There are more than one type of fraility index - a clinical deficits frailty index (FI-CD) and the biomarker-based frailty index (FI-B) amongst them.