How to incorporate sampling sites as a covariate?

Dear NMusers: I have a question regarding covariates in pop PK analysis. Has anyone used sampling sites from subjects as a covariate for PK parameters? In my dataset, each subject had samples from carotid arterial and jugular vein. And the PK profiles from different sites appeared differently. I was wondering if it is possible to incorporate sampling sites as a covariate. Thank you very much for any suggestions! Best regards, Huali