How to handle age at measure-start in Cox regression?

Question

I'm a statistics newbie (medical student) trying my luck with a Cox regression for a survival analysis on the outcome of a specific type of operation. And I'm trying to determine which variables to control for. And how to handle the age of the patient at operation-time (measure-start)...

Currently I've set the following as covariates in my analysis (PASW/SPSS):

Risk (Low/High)
Gender (Female/Male)
Shunt used in operation (False/True)
Artery stenosis (False/True)
Age at operation (Integer value)

I've set the 4 first variables as categories (dummy coded as 0 and 1), and specified the first value (0) as the reference value.

But what about the patients age at the operation? I assume this must be controlled for? But the age at measure-start (operation) isn't time-dependent (is it?), but are always the same value, right?

Does this mean I have to control for the patients real age aswell? And does this mean I have to use the COX with Time-dependent Covariables function instead of the regular COX Regression?

Thanks for any help!

Regards Alex

Answer 1

You are right that you don't need to use a time-dependent model for this situation. If, however, age would be expected to have a dominant effect (as might be expected), you may be safer using an interval-censored model. With R, that is setup with three arguments to the Surv object, but I don't have any experience with SPSS's facilities for survival analysis.