Dear forum,

I have encountered a problem in that for my Cox regression my output gives enourmous Hazard Ratios for my outcome (disease recurrece), such as 1.33e+10.


A) First I would like to give you the specifics:
I have a project, where I assess the impact of reponse to chemotherapy (i.e. "pres" a variable with 3 levels: complete, partial, no response) on disease recurrence within my given follow up. Displayed graphically (Kaplan Meier plot), the outcome is quite impressive:

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However, when I run a Cox regression, adjusted for other variables (age, smoking status, etc), my ouput displays grotesque Hazard ratios:

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The problem remains, even if a run a univariable model. I believe the issue here is collinearity in that for example "No response" predicts my outcome (disease recurrence) almost perfectly and therefore has a very large HR.

B) My questions are be the following:

-Do you find my explanation plausible (s. above)?
-Is there a solution, i.e. a way to run the cox model (uni- or multivariable; as I only have 37 events I´d fear an overfit) and have more approachible HRs?
-Lastly, if I run the cox regression without a prefix/factorial (by that I mean omitting "i." for categorial) for my independent variable of choice ("pres"), I get a HR of approximately 6. I do however not know, how STATA runs that specific regression, if "pres" is not specified:
Does it treat the first level of the variable as reference against the other two levels which would be "partial" and "no response"?

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Thank you very much for your help and taking time to read this !