ROC Analysis Help-Prevalence

I have a sample of patients all diagnosed with disease X. I have measured several biomarkers and am running ROC curves to see ones that can predict disease activity (categorical variable present vs not present).
In calculating PPV/NPV I need to determine the prevalence.
- am i supposed to use a set prevalence rate of the disease x in the general population obtained from the literature?
- or prevalence of disease in my cohort (they all ave disease X)
- or prevalence of disease activity in my cohort (eg 40/70 patients have disease activity)--this is what I have been using but I would like to clarify.