Sometimes patient enrolled in Managed care plan or HMO in order to receive more benefits and for low premium or low copays.
If the patient is working after 65 years then the employer provide the EGHP plan to cover the employee medical expenses.
In the above case Medicare will be inactive and HMO will be active for Medical benefits.
The basic idea behind the HMO or Managed care plan is PCP and in-network. All the special treatment required authorization from PCP.
As a Medical biller we have to understand the above scenario and have to

work logically.
If Medicare denied as CO – 24 (Patient covered under Managed care/Capitation ) then find the HMO details and file the claims.
If patient enrolled in HMO then always get the approval from PCP and insurance in order to avoid the denials.