In patients using NuvaRing for contraception, is the 1mg dexamethasone suppression test reliable to screen for cortisol excess?   

NuvaRing should have minimal effect on cortisol-binding globulin (see Duijkers et al., PMID 15697102). Therefore, I think you can perform the DST. However, should it be abnormal, I would repeat it after 1 month off NuvaRing, and if it returned significantly different, you should post the info here.