LEFT-TRUNCATED DATA WITH AGE AS TIME-SCALE - AN ALTERNATIVE FOR SURVIVAL ANALYSIS IN THE ELDERLY POPULATION

Background. The standard approach for survival analysis of the elderly population is to define the survival time as the elapsed time from en try into the study until death, and to adjust by age using stratificat ion and regression procedures. However, the interest is in the study o f the aging process and the risk factors related to it, not in the use of time-on-study as the time scale. Here, we present methods to use a ge as the time scale and compare inferences and interpretations with t hose obtained using the standard approach. Methods. A total of 1,315 i ndividuals aged 65 years or older from the city of Barcelona, Spain, w ere interviewed in 1986 (baseline). The vital status of the cohort was assessed in October 1994. To illustrate the usefulness of age as time scale (alternative approach) instead of time-on-study in the survival analysis of the elderly population, both methods were used to assess the relationship between baseline functional capacity and mortality. R esults, Using the alternative approach, we observed that 50% of the sa mple died at age 80.6 years; this information could not be estimated w ith the standard approach. Using age as a covariate in the standard an alysis with time-on-study as the time scale and using age as the time scale in the alternative analysis, the association of functional capac ity at baseline and mortality was of similar magnitude under both anal yses. Nevertheless, using the alternative approach, relative risks wer e slightly lower, and the adjustment by age was tight and was not subj ect to the inherent assumptions in regression models of the functional relationship of independent variables with outcome. We illustrated th e methods with fixed covariates (i.e., gender) and baseline values of time-dependent covariates (i.e., functional capacity), but we discusse d the extension of our methods for the analysis of time-dependent cova riates measured at several visits in a cohort study. Methods proposed here are easily implemented with widely available statistical software packages. Conclusions. Although the use of standard survival analysis generally produces correct estimates, the use of age as time scale is deemed more appropriate for survival analysis of the elderly: Inferen ces are easier to interpret and final models are simpler. We therefore recommend the use of age as time scale for survival analysis of the e lderly population.