2.2. Operation Frailty Risk Score

In the previous study, the HFRS was developed by using cluster analysis in such a way that scores were given for ICD-10 codes that were at least twice as prevalent in the frail group as in the other groups [16]. In this study, we created an Operation Frailty Risk Score (OFRS) by performing univariate and multivariable logistic regression analysis of the mortality within 90 days after surgery according to the operation codes of the 8 groups classified as described above, the HFRS score calculated from the diagnosis codes, age, and sex. A simple scoring system was developed using the penalized maximum likelihood estimates of the covariates in models that followed the method of Sullivan et al. [18]. After selecting a reference group of each variable, we used regression coefficients as weights and the distance from the reference group to generate each point value. Score 1 was defined as the effect of a 10-year increase in age. Through this analysis, we developed a risk scoring system of the predictive model for pre-operative frailty based on the mortality within 90 days after the operation. Based on this defined score, scores were assigned to each of the eight operation risk groups, age, and HFRS by comparing and analyzing the effects of each variable on mortality within 90 days after the operation. Table 1 summarizes the OFRS points for each variable.

Operation frailty risk scoring system for prediction of 90-day mortality.

HFRS—hospital frailty risk score.