The distinction between Original Medicare (fee-for-service Parts A and B) and Medicare Advantage (MCO/Part C) is one of the most operationally important things to understand when building a Medicare billing practice. They share a patient population but operate very differently from the provider side.
This page is a direct, practical comparison.
Enrollment and credentialing
| Original Medicare | Medicare Advantage (MCO) | |
|---|---|---|
| Where you enroll | CMS via PECOS | Each plan individually |
| Form | CMS-855I / 855B | CMS-10114 + plan credentialing |
| One enrollment covers | All Original Medicare patients nationwide | Only that plan’s members |
| Network | None — any enrolled provider | Plan-specific network |
| Credentialing time | 60–90 days (PECOS) | 30–90 days per plan |
| CAQH required | No | Usually yes |
| Annual re-credentialing | Revalidation every 5 years | Many plans annually |
Reimbursement rates
| Original Medicare | Medicare Advantage (MCO) | |
|---|---|---|
| Who sets rates | CMS national fee schedule | Plan-negotiated rates |
| How rates are set | Medicare Physician Fee Schedule (updated annually) | Contract negotiation |
| Rate variation | Some geographic adjustment | Varies widely plan to plan |
| Contract modification | Not negotiable | Negotiable |
| Rate transparency | Public (Medicare fee schedule is published) | Confidential contract |
In practice: MCO rates can be above or below Original Medicare rates. High-demand specialties in competitive markets sometimes negotiate above Medicare rates. Lower-leverage specialties or providers in MCO-dominant markets may accept below-Medicare rates.
Prior authorization
| Original Medicare | Medicare Advantage (MCO) | |
|---|---|---|
| Prior auth prevalence | Required for limited services | Required for many more services |
| Prior auth for therapy | No (KX modifier for threshold) | Often yes, per plan |
| Prior auth for imaging | No (for most services) | Often yes |
| Prior auth for surgery | No (for most procedures) | Plan-specific |
| Approval timeline | Not applicable | 1–14 business days typically |
| Denial rate | Not applicable | Varies by plan and service type |
| Appeal rights | Not applicable | Required by law |
⚠️ Prior authorization is the biggest operational challenge for high-MCO practices. A practice with 60% Medicare Advantage patients and no robust prior auth workflow will see significant denials and write-offs. Build the workflow before joining the plan.
Claims submission
| Original Medicare | Medicare Advantage (MCO) | |
|---|---|---|
| Submitted to | MAC (CMS’s contractor) | The plan directly |
| Format | CMS-1500 or UB-04 | Usually same, but plan may have variations |
| Timely filing limit | 1 year from date of service | Varies by plan (often 90–180 days) |
| Claims system | PECOS / MAC system | Plan’s claims portal or clearinghouse |
| ERA/EOB format | CMS standard | Plan-specific |
| Payment speed | 14–30 days after approval | Varies by plan |
Documentation requirements
| Original Medicare | Medicare Advantage (MCO) | |
|---|---|---|
| Medical necessity standard | CMS LCD/NCD standards | Plan may add requirements |
| Therapy documentation | CMS functional reporting requirements | Plan may require additional |
| Progress notes | Per CMS frequency requirements | Plan may require more frequent |
| Audit exposure | MAC ADR / OIG audit | Plan audits AND MAC audits |
Important: Original Medicare’s documentation requirements apply as a floor. MCOs can add requirements on top. Always document to Medicare standards as your baseline.
Referrals and coordination
| Original Medicare | Medicare Advantage (MCO) | |
|---|---|---|
| PCP referral for specialist | Not required | Required for HMO plans |
| Care coordination | Optional | Central to HMO model |
| Out-of-network coverage | Any enrolled Medicare provider | Limited (PPO) or none (HMO) |
Appeals and disputes
| Original Medicare | Medicare Advantage (MCO) | |
|---|---|---|
| Claim denial appeal | CMS administrative appeal process (5 levels) | Plan-level appeal, then external review |
| Prior auth denial appeal | Not applicable | Required internal/external review |
| Provider dispute | MAC process | Plan dispute process + state insurance department |
| CMS involvement | Yes (MAC is CMS contractor) | CMS oversight, but plan is primary |
The dual-track reality
Most practices that accept Medicare see both Original Medicare and Medicare Advantage patients. This means:
- Dual enrollment: Enrolled with CMS for Original Medicare AND contracted with MCO networks
- Dual billing systems: Claims submitted to MAC for fee-for-service, submitted to plans for MCO patients
- Dual documentation standards: CMS standards apply everywhere; plans add requirements on top
- Dual accounts receivable: Two sets of payers, two sets of rules, two sets of timely filing limits
Staff training, billing system configuration, and prior auth workflows need to accommodate both tracks simultaneously.
Making the right choice about MCO participation
You don’t have to join every MCO in your market. Consider:
- Patient panel analysis — which plans are your existing Medicare patients enrolled in?
- New patient referrals — which plans do referring providers and health systems use?
- Administrative capacity — do you have the staff to manage MCO prior auth and credentialing for each plan?
- Rate analysis — what does the plan’s fee schedule look like for your primary service codes?
Joining too many plans with insufficient billing infrastructure creates more problems than selective participation.
Related pages
- What Are Medicare MCOs?
- MCO Overview
- Contracting with Medicare Advantage Plans
- CMS-10114 — Medicare Advantage Provider Enrollment
Questions about navigating Original Medicare vs. MCO billing in your practice? Ask Mae → Free Metolius Health enrollment audit →