I am on Medicare with plan G supplemental (Blue Cross). My supplemental will only pay the difference between bill and what Medicare paid. It will not pay anything if Medicare does not pay.

In other words, if it is not a Medicare approved treatment / tests, my supplemental will not pay anything. I have not had to many problems,

Not sure if all supplemental policies are like that.

Also, make sure you know difference between Original Medicare with supplemental vs Medicare Advantage. And if your clinic / hospital accepts the policy you plan on taking.

Someone on Breast Cancer group asked similar question in last few days, you make want to check out the responses.
https://connect.mayoclinic.org/discussion/going-on-medicare-soon-any-advice/?pg=1#chv4-comment-stream-header
Laurie