A cardiology office billed a 72-year-old patient's stress test to Medicare, received a CO-109 denial (claim not covered by this payer), and discovered the patient had switched to a Medicare Advantage HMO during open enrollment. By the time they found the right plan, the plan denied for no prior authorization. By the time they appealed that, the plan's 90-day timely filing limit had passed. Three denials, one test, no payment. Every one of those was a Medicare Advantage vs Original Medicare billing difference the front desk could have caught with one eligibility check in January.
Original Medicare (Parts A and B, sometimes called traditional or fee-for-service Medicare) is administered by the federal government through Medicare Administrative Contractors, pays a published fee schedule, requires almost no prior authorization for office services, and has uniform rules everywhere. Medicare Advantage (Part C) is Medicare coverage delivered by a private insurer under contract with CMS. The insurer receives a monthly payment per member, and in exchange it manages the care, which means networks, authorizations, its own claim system, its own timely filing rules and its own appeals process. More than half of Medicare beneficiaries are now in Medicare Advantage plans, so a practice's "Medicare" patients are, on average, more likely to be commercial-style patients than not.
A glossary line: when a patient enrolls in Medicare Advantage, Original Medicare stops paying for their care; claims sent to the MAC deny, and the plan becomes the only payer. The patient keeps their Medicare card, which is why the front desk sees it, but the card that matters for billing is the plan's.
Key takeaways
- A Medicare Advantage patient's claims go to the plan, never to the MAC; billing Medicare produces CO-109 and wastes weeks.
- Plans may require prior authorization, referrals and network status that Original Medicare never asks for.
- Plan payment is contractual, often expressed as a percentage of the Medicare fee schedule, and must be loaded and checked like any commercial contract.
- Timely filing, appeal deadlines and appeal levels are the plan's, not Medicare's, and are usually shorter.
- The MIPS adjustment, sequestration mechanics and Medicare secondary payer rules all behave differently or not at all under Medicare Advantage.
Medicare Advantage vs Original Medicare billing, side by side
| Area | Original Medicare | Medicare Advantage |
|---|---|---|
| Where the claim goes | The MAC for your jurisdiction | The plan's payer ID; wrong routing denies CO-109 |
| Network | Any enrolled provider who accepts assignment | HMO: in-network only with referrals; PPO: out-of-network allowed at higher patient cost; some plans require contracting to be paid at all |
| Prior authorization | Rare for office services (with limited exceptions such as the WISeR model in six states) | Common for imaging, procedures, specialty drugs, DME and some visits; plan-specific lists |
| Payment amount | Medicare Physician Fee Schedule, uniform by locality | Contracted rate, often a percentage of the fee schedule; non-contracted providers generally receive the Medicare rate |
| Patient cost-share | Part B deductible, then 20 percent coinsurance | Copays and coinsurance set by the plan; often $0 for primary care, fixed copays for specialists |
| Timely filing | 12 months from date of service | Contractual, commonly 90 to 180 days; some plans 365 |
| Appeals | Five levels starting with redetermination at the MAC, 120 days to file | Plan reconsideration first, with plan-set deadlines (often 60 days); then independent review |
| Remittance | Standard 835 from the MAC with Medicare codes | Plan's 835 with its own CARC and RARC mapping |
| MIPS adjustment | Applied on every Part B remit | Not applied |
The table is the training document. Every front desk and billing staff member should be able to say, for any Medicare patient in front of them, which column applies. The way to know is the eligibility check, not the card.
Finding the right payer
Run Medicare eligibility through the MAC portal or the clearinghouse for every patient with a Medicare card. The response for a Medicare Advantage enrollee shows the plan name, the plan contract number and often the plan's contact information in the "other payer" or "Medicare Advantage" segment, and it shows the enrollment effective date. That one response tells you where to bill and when the enrollment started, which matters in January, when plan changes take effect and patients arrive with last year's card.
Register the plan as the primary payer with the correct payer ID for that specific product. Large insurers have separate payer IDs and addresses for their Medicare Advantage line, and sending the claim to the commercial payer ID is the second most common routing error after sending it to Medicare. Do not enter Medicare as secondary; Original Medicare pays nothing for a Medicare Advantage enrollee and the secondary claim simply denies.
Authorizations, referrals and network status
Each plan publishes a prior authorization list, and the lists differ across plans from the same insurer. Build a grid for your top plans: which of your common services require authorization, how to submit, and the turnaround. The 2026 federal rules on prior authorization decision timeframes apply to Medicare Advantage plans, so standard requests have a seven-day decision clock and urgent ones 72 hours, and the plan must give a specific reason for a denial. Track those clocks.
HMO plans generally require a referral from the patient's assigned primary care physician for specialist visits, and the specialist's claim denies without one on file with the plan. Confirm the referral exists before the visit, not after. And check that the rendering provider is in the plan's network for that specific product; a physician contracted with an insurer's commercial PPO is not automatically contracted with its Medicare Advantage HMO. Our credentialing team keeps a plan-by-plan participation grid for exactly this reason.
Rates, remits and the money
Medicare Advantage payment is a contract term. Many contracts pay a percentage of the current Medicare Physician Fee Schedule, which sounds simple and is not: "current" may mean the year of service or a fixed base year, and the contract may or may not follow the fee schedule's locality adjustments, sequestration, or the MIPS adjustment. Read the definition of "Medicare rate" in your contract and load the resulting fee schedule per plan. Then run the same underpayment variance report you run for commercial payers. Plans make mistakes, and the practice that never compares never finds them.
Non-contracted providers who see Medicare Advantage patients in PPO plans, or in emergencies, are generally paid what Original Medicare would have paid, and may collect from the patient only the plan's cost-share, not the Medicare 20 percent. Balance billing a Medicare Advantage patient beyond the plan's cost-share is prohibited, contracted or not.
Remits from plans use standard CARC and RARC codes but the plan's own mapping. Post them to the plan, not to Medicare, in the practice system, so that Medicare and Medicare Advantage denial reports are separate; blending them hides the plan-specific problems that are the whole point of the exercise.
Timely filing and appeals
Original Medicare gives you twelve months. Most Medicare Advantage contracts give you far less, and the clock is the plan's, not CMS's. Load each plan's timely filing limit as a rule and flag claims approaching it. The cardiology office lost the stress test because the plan's 90-day limit ran out while they were still learning where to bill.
Appeals follow the plan's process. A contracted provider files a reconsideration with the plan under the contract's dispute terms. A non-contracted provider has the right to request a reconsideration and, if the plan upholds the denial, an independent review, and must sign a waiver of liability agreeing not to bill the patient. The deadlines are shorter than Medicare's and are printed on the denial notice. Our denial management team treats each Medicare Advantage plan as a separate payer with its own appeal template, because the winning arguments differ: plan policy language for the plan, Medicare coverage rules for the MAC.
Questions we hear
The patient says they have "Medicare" and shows the red, white and blue card. Is that enough?
No. Every Medicare Advantage enrollee keeps that card. Ask specifically "Do you have a Medicare plan through an insurance company, like Humana, UnitedHealthcare or Aetna?" and run eligibility. The eligibility response is the only reliable answer.
Do we need an Advance Beneficiary Notice for Medicare Advantage patients?
The Medicare ABN (form CMS-R-131) is an Original Medicare document. Medicare Advantage plans have their own pre-service denial and notification processes; for a service the plan may not cover, request a pre-service organization determination from the plan, which gives the patient a written decision and appeal rights. Using the ABN with a Medicare Advantage patient does not protect the practice.
Does Medicare Advantage follow the same coverage rules as Original Medicare?
Plans must cover everything Original Medicare covers and must follow Medicare's national and local coverage determinations, but they may apply their own utilization management (authorizations, step therapy for Part B drugs) within federal limits, and they may cover extra benefits. When a plan denies something Medicare would cover, the appeal cites Medicare's coverage rules and the federal requirement that plans follow them.
What to do this week
- Run Medicare eligibility on every patient scheduled next week with a Medicare card and correct the payer on file for any Medicare Advantage enrollee.
- Pull every CO-109 denial from the last quarter and reroute what is still within the plan's timely filing limit.
- Build the plan grid: payer ID, network status per provider, authorization list, timely filing limit, appeal deadline.
- Load each plan's contracted rate as a fee schedule and run the variance report for January.
- Separate Medicare Advantage plans from Medicare in your denial reporting.
