So when a pt has Med B and a secondary/supplemental, and then switches to a Med Adv plan, usually they completely lose the secondary/supplemental they used to have. The problem is that if they go on hospice at any time, they are switched back to Med B, and no longer have the secondary/supplemental insurance to pick up the 20% coinsurance after Med B pays.
You cannot bill the Med Adv plan for the coinsurance, they will not pay it, as they are not a secondary or supplemental.
What I would do is look at the ERA/EOB, and see if Med B forwarded the claim to another payer after they paid. If they did, that means that luckily, the secondary/supplemental the patient had before switching to the Med Adv plan is still active, and should pay.
If the claim was NOT forwarded by Med B to another payer, then you bill the pt the 20% coinsurance.
EDITED TO ADD: There are instances when a pt does have a secondary/supplemental but has not had a COB done with Medicare, or it is an insurance that cannot accept crossover claims from Med B. What I personally would do in this situation is if you see no crossover listed on the Med B ERA/EOB, I would contact the pt or their guarantor and inquire as to if there is any secondary/supplemental insurance. And if they try to say it is the Med Adv plan, it is not. Many pts/guarantors do not understand that it is NOT a secondary.