[Age as time-scale: an application to the survival analysis of chronic diseases].

BACKGROUND Cohort study survival analysis does not take into account the age at which an individual dies, but rather the age throughout which the individual has been included in the follow-up. This study is aimed at assessing the resulting differences in the calculation of the relative risk (RR) when the age is used instead of follow-up as a timescale in the chronic diseases survival analysis. METHODS The five-year cohort follow-up data for the Catalan Health Interview Survey (COHESCA) is analyzed. Cox models are adjusted to the follow-up time ("standard" approach) and the age ("alternative" approach) as the timescale. The main covariable used is the comorbility index, which is adjusted by demographics, lifestyles and self-perceived health. RESULTS Among men in the 40-64 age range, using the "standard" approach, no significant increase in the RR of death was found. Among women within the 40-64 age range with a 3-4 score on the index, there was a significant increase in the RR. Among those over age 64, a significant increase in the risk was found for the 3-4 score (RRmen = 2.1 (1.3-2.5); RRwomen = 2.1 (1.2-3.7)) and > = 5 on the index (RRmen = 3.4 (1.7-6.9); RRwomen = 4.0 (2.1-7.7)). Using the "alternative" approach, the RR was 1.0 (0.7-1.4) among men with a score of 1-2, 1.5 (1.0-2.3) for those with a score of 3-4 and 2.6 (1.4-4.9) for those with a score of > = 5. Among women, the RR were respectively 0.8 (0.5-1.2), 1.7 (1.1-2.7) and 2.6 (1.5-4.8). CONCLUSIONS Both approaches showed an increase in the RR according to the comorbility index. However, in the "alternative" approach, it is adjusted by the confounding effect of age.