I am running a roportional hazards model on a cohort of ~ 870000 subjects
between 45 and 85 years old. I am looking at the risk of having acute
coronary outcomes function of sex, age class and presence of comorbidities
in the past. The start of follow up is january 1, 1999, and the end of F/U
is december 2002. Co-morbidities are acute coronary outcomes, chronic
coronary outcomes, chronic heart failure, hypertension, diabetes, all
determined on the period 1991, dec 31, 1998.
So far so good. The results considering co-morbidities as 1=Yes, 0=No
only, plus age and sex look as expected, with males having higher risk, an
increase in risk with age and an increase in risk with the presence of co-
morbidities (i.e. HR=3.1 in one example).
However, if I split the subjects with comorbidities in two, (ones that had
co-morbidities less than 3 years prior to the outcome of interest in the
follow-up period and the ones that had co-morbidities earlier than that -
more than 3 years prior to the outcome of interest), I get some
HR=322.7 (co-morbidities < 3 years prior)
HR=101.77 (co-morbidities > 3 years prior);
all the other covariates (sex, age) being the same.
A cross tabulation between co-morbidities and outcome of interest looks
Co-morbid(0)/ACS(0) = 555468
Co-morbid(1)/ACS(0) = 283577
Co-morbid(0)/ACS(1) = 9597
Co-morbid(1)/ACS(1) = 21425 or if split by time 1484(<3yr) & 19941(>3yr);
What can cause such great hazard ratios?