Dear community,

I have a question regarding the standard error of "marginal hazard ratio"
estimates from a cox proportional hazard model with an interaction term.

I have to dichotomous variables ADMA and ALA (both low/high) where I have
divided the continuous variables at a certain level.
I am aware about the contra arguments categorizing a continous variable and
I could as well use the variables continuously.
Just to keep my question simple.

This is the function call and output:

> coxph( ami ~ ADMA2 * ALA2 , data = d2)
Call:
coxph(formula = ami ~ ADMA2 * ALA2, data = d2)

                           coef exp(coef) se(coef)      z       p
ADMA2high                  1.137     3.118    0.302  3.759 0.00017
ALA2high                   0.148     1.160    0.196  0.756 0.45000
ADMA2high:ALA2high        -0.671     0.511    0.448 -1.496 0.13000

Likelihood ratio test=13.1  on 3 df, p=0.00437  n= 1364, number of events=
129


For publication I would like to present the marginal estimates like


|          | ADMA low         | ADMA high        |
|----------+------------------+------------------|
| ALA low  | Reference        | 3.12 (1.72,5.63) |
|----------+------------------+------------------|
| ALA high | 1.16 (0.79,1.70) | 1.85 ( ? , ? )   |

Now, the HR for high AMDA, high ALA would be exp(1.137 + 0.148 - 0.671) =
1.85 adding up the betas from output,
but how would I calculate a 95% CI for this estimate?

Regards,
Reinhard

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