Cox regression results interpretation

#1
Hi!

I am currently working on a statistical analysis on a prospective cohort population. We are looking at a specific protein concentration at baseline and relate this to a certain outcome. The outcome group have been matched with twice as many control individuals from the same cohort. The statistical analysis we are using is a cox regression. We've received HRs and p-values that show a significant correlation between this protein and outcome but here comes my question: we've gotten lower HRs and better (lower) p-values after adjustment for risk factors connected to this exact outcome which to us is surprisning. Does anyone have a clue how this came to be? We've looked at all risk factors in both case and control groups and they are as you would except. Higher smoking prevalence in case group for example (we're looking at CVD as outcome). From what I understand, the HRs should rise a bit and p-values become less strong when you adjust for risk factors (i.e only the protein concentration is taken into consideration).

I'm using SPSS and here's my syntax (with protein, variables and status/time variables changed to just y, x and z respectivley:

COXREG z
/STATUS=z
/METHOD=ENTER y
/PRINT=CI(95)
/CRITERIA=PIN(.05) POUT(.10) ITERATE(20).


COXREG z
/STATUS=z
/CONTRAST (x)=Indicator
/CONTRAST (x)=Indicator
/CONTRAST (x)=Indicator
/CONTRAST (x)=Indicator
/CONTRAST (x)=Indicator
/METHOD=ENTER y x x x x x x x x x x x x
/PRINT=CI(95)
/CRITERIA=PIN(.05) POUT(.10) ITERATE(20).

Grateful if somebody could help me wtih this one. Am I doing something wrong in SPSS? Am I doing things right but need to interperate it some other way? Have anyone heard about this phenomenae, i.e HRs/P-values going "the wrong way" after adjustment for risk factors ?

Best regards, Filip
 

hlsmith

Not a robit
#2
Please provide output and confidence intervals not p-values for both models. HRs becoming less could mean 0.99 to 0.84 or 1.23 to 1.1, which are totally different.
 
#3
Hi hlsmith!

With unadjusted model the HR is 0.693 (0.555-0.865, p=0.001) and with adjusted model 0.55 (0.405-0.747, p=0.0001).

Best regards, Filip