Medicare is the single largest payer in the U.S. healthcare system, and for most practices, getting Medicare billing wrong is expensive in ways that are rarely visible until the damage is done. According to the American Medical Association, up to 12% of medical claims are submitted with inaccurate codes, and poor billing practices cost U.S. providers an estimated $125 billion annually. For practices where Medicare makes up a significant portion of payer mix, that number is not abstract. It shows up in delayed reimbursements, unrecoverable write-offs, and audit exposure.
Understanding how medicare medical billing works, the rules that govern it, and the coding structures behind it is foundational to running a financially healthy practice.
Understanding Medicare Coverage
Medicare is a federal health insurance program administered by the Centers for Medicare and Medicaid Services (CMS). It covers individuals aged 65 and older, certain younger people with qualifying disabilities, and individuals with End-Stage Renal Disease (ESRD). As of 2024, more than 67 million Americans are enrolled in Medicare, representing close to 20% of the U.S. population.
Medicare is divided into four parts, and each one carries distinct billing implications:
Part A covers inpatient hospital care, skilled nursing facility stays, hospice, and home health services. Institutional claims under Part A are submitted using the UB-04 form (CMS-1450), or its electronic equivalent, the 837I.
Part B covers outpatient and physician services, diagnostic tests, durable medical equipment, and preventive care. Professional claims under Part B are submitted using the CMS-1500 form, or its electronic equivalent, the 837P. Part B is where the majority of physician and specialist billing occurs.
Part C (Medicare Advantage) allows beneficiaries to receive Medicare benefits through CMS-approved private insurance plans. This distinction matters enormously for billing: providers cannot submit Part C claims to CMS directly. Claims go to the individual Medicare Advantage plan, which sets its own rules, prior authorization requirements, and filing deadlines.
Part D covers prescription drug costs through approved private plans. Only licensed Part D providers can bill for drugs and vaccines under this part.
How Medicare Insurance Works in Medical Billing
When a provider renders a service to a Medicare beneficiary, the claim does not go to CMS directly. It is submitted to a Medicare Administrative Contractor (MAC), a regional entity designated by CMS to process and adjudicate Medicare claims. The MAC evaluates each claim and typically processes it within 30 days.
For traditional Medicare (Parts A and B), the MAC determines reimbursement based on the Medicare fee schedule, the provider’s participation status, and whether the claim meets medical necessity requirements.
Provider participation status affects reimbursement in a direct way:
- Participating providers accept Medicare’s approved amount as payment in full. Medicare pays 80% and the beneficiary is responsible for the 20% coinsurance.
- Non-participating providers may charge up to 115% of the Medicare fee schedule, but Medicare reimburses at a lower rate, and the patient pays the difference.
- Opted-out providers have formally withdrawn from Medicare. They operate under private contracts with patients, and Medicare does not reimburse for their services at all.
Understanding a practice’s participation status, and ensuring patients are informed accordingly, is one of the first steps in clean medicare claim submission.
Medicare Billing Rules
Medicare operates under a distinct set of medicare billing rules that go beyond what commercial payers typically require. The key rules every billing team must know:
Provider enrollment via PECOS. Before billing Medicare, providers must be enrolled through the Provider Enrollment, Chain, and Ownership System (PECOS). Claims submitted under a provider not yet active in PECOS are automatically denied with a CO-B7 code. This is one of the more avoidable and costly enrollment gaps in practices that hire new clinicians.
National Provider Identifier (NPI). Every claim must include a valid 10-digit NPI. This is the standardized identifier CMS uses to link all billing activity to a specific provider or organization.
Medicare as primary vs. secondary payer. CMS requires providers to determine at every visit whether Medicare is the primary or secondary payer. Medicare Secondary Payer (MSP) rules apply when a beneficiary has other coverage, such as an employer group health plan or workers’ compensation. Failure to maintain a system for identifying other payers is considered a violation of the Medicare provider agreement.
Advance Beneficiary Notice (ABN). When a provider believes Medicare may not cover a service, the patient must be notified in advance using a valid ABN. Without a signed ABN on file, providers cannot bill the patient for a denied claim. The modifier GA is appended to the claim to indicate an ABN is on file; GY indicates the service is categorically non-covered.
Timely filing. Under 42 CFR § 424.44, Medicare Part A and Part B claims must be submitted within 12 months of the date of service. Missing this window results in a CO-29 denial, and in most cases, the revenue is permanently unrecoverable. Exceptions exist for administrative error by a CMS contractor or retroactive Medicare entitlement, but these are narrow and require documentation.
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How to Bill Medicare: Claim Submission
Clean medicare claim submission follows a consistent structure regardless of specialty. Key required data elements include:
- Medicare Beneficiary Identifier (MBI): The 11-character alphanumeric identifier that replaced the old Social Security-based HICN. All claims must use the MBI, regardless of service date.
- NPI: Both the billing and rendering provider NPIs are typically required.
- ICD-10-CM diagnosis codes: Must support medical necessity for the service billed.
- CPT or HCPCS procedure codes: Must accurately reflect the service rendered.
- Place of Service (POS) code: Identifies where the service was delivered. For telehealth, POS 02 applies when the patient is not at home; POS 10 when they are.
- Modifiers: Applied where required to modify the meaning of a procedure code without changing its definition.
Electronic submission to the designated MAC is standard practice. Unlike commercial payers, Medicare does not require a clearinghouse, but most billing teams use one for tracking and error-checking purposes.
Medicare Advantage requires a different approach. Claims go to the individual MA plan, not CMS. Each plan has its own formularies, prior authorization protocols, and timely filing windows, which typically run 90 to 120 days rather than the 12-month federal window for traditional Medicare. Treating a Medicare Advantage plan like traditional Medicare is one of the most consistent denial generators in the billing cycle.
For a deeper look at how the billing process works end to end, Neolytix’s article on what is medical billing covers the full revenue cycle from encounter to payment.
Medicare Fee Schedule: 2026 Updates
The medicare fee schedule determines how much Medicare pays for a given service. Payment is calculated using Relative Value Units (RVUs) across three components: physician work, practice expense, and malpractice expense. These are multiplied by a conversion factor and adjusted for geography using Geographic Practice Cost Indices (GPCIs).
For 2026, CMS finalized two separate conversion factors under the Physician Fee Schedule (PFS) final rule (CMS-1832-F), effective January 1, 2026:
- Non-APM Qualifying Participant (QP) Conversion Factor: $33.4009, representing a 3.26% increase from 2025.
- APM Qualifying Participant Conversion Factor: $33.5675, representing a 3.77% increase from 2025.
This marks the first increase in Medicare physician pay in several years, driven in part by a 2.5% statutory bump passed through the One Big Beautiful Bill Act.
However, the increase is partially offset by significant structural changes:
- Efficiency adjustment: CMS applied a -2.5% adjustment to work RVUs for non-time-based services, reflecting productivity gains over time. CMS intends to revisit this adjustment every three years.
- Site-of-service differential: Office-based payments increased approximately 5%, while payments for services delivered in facility settings decreased by approximately 7%. This affects hospital-based specialists more significantly than office-based primary care.
Practices with substantial hospital-based billing volume should model the net impact on their specific CPT code mix, as the conversion factor increase and efficiency adjustment do not cancel out evenly across all specialties.
Common Medicare Billing Codes
Medicare uses CPT, ICD-10-CM, and HCPCS Level II codes within the same broader coding structure as commercial payers, but with several Medicare-specific requirements.
Evaluation and Management (E&M) codes (99202–99215 for outpatient) are the most frequently billed codes under Part B. Since 2021, E&M level selection is based on either Medical Decision Making (MDM) or total time, not documentation of history and exam components. Medicare does not cover consultation codes 99241–99245; when a consult is requested, providers bill the appropriate E&M code instead.
HCPCS Level II G codes are Medicare-specific procedure codes used for services not captured by CPT. Key examples include G0438 and G0439 for the Annual Wellness Visit, and G0556–G0558 for Advanced Primary Care Management (APCM) services, newly established under the 2026 PFS final rule.
Key modifiers in Medicare billing include:
- GA: ABN on file for a service that may be denied
- GY: Service is statutorily excluded or does not meet the definition of a Medicare benefit
- GZ: Provider expects denial and no ABN was obtained
- 95 / 93: Telehealth via synchronous audio/video or audio-only
- POS 02 / POS 10: Telehealth place of service codes
For a full breakdown of how modifiers function in Medicare and commercial claims, see Neolytix’s guide on medical billing modifiers.
Common Medicare Billing Pitfalls
Most Medicare claim failures are not the result of complex fraud scenarios. They are the result of consistent, preventable process gaps.
Incorrect or missing MBI. Using outdated HICN numbers or transposing characters in the MBI will cause claim rejection before it even reaches adjudication.
Failure to identify the primary payer. Not querying patients at every visit about other coverage is a CMS compliance issue, not just a billing one. Claims sent to Medicare when another payer is primary result in overpayments that CMS will pursue for recovery.
Treating Medicare Advantage like traditional Medicare. Part C plans operate independently of CMS for billing purposes. Their prior authorization requirements, covered service lists, and timely filing windows all differ, often significantly. A claim that would sail through traditional Medicare may require pre-authorization under a beneficiary’s MA plan.
No ABN on file for non-covered services. Providers who render services they expect Medicare to deny, without obtaining a signed ABN, cannot bill the patient. This is not a technicality. It is a direct revenue loss with no pathway to recovery.
Timely filing lapses. The 12-month window is longer than most commercial payers, which creates a false sense of margin. Claims that are held internally for review, delayed by documentation gaps, or overlooked in the accounts receivable cycle can quietly age past the filing deadline. Unlike most denials, CO-29 timely filing denials are not appealable through standard redetermination.
Incorrect place-of-service coding for telehealth. Post-pandemic telehealth policy updates under the 2026 PFS final rule continue to require precise POS coding. Using the wrong code between POS 02 (patient not at home) and POS 10 (patient at home) will generate a denial or incorrect reimbursement.
Billing consultation codes. Medicare discontinued payment for consultation codes (99241–99245) in 2010. Billing them generates an automatic denial.
Credentialing and enrollment gaps. Submitting claims under a provider whose PECOS enrollment is pending or inactive results in a CO-B7 denial. The only protection is tracking enrollment effective dates before new providers begin seeing Medicare patients.
Conclusion
Medicare billing operates by a consistent logic: the rules are defined, the forms are standardized, and the fee schedule is published. What makes it difficult is the volume of requirements, the specificity of each one, and the cost of getting any of them wrong. A missed timely filing deadline, an incorrect POS code, or an ABN that was never obtained can each represent unrecoverable revenue.
For practices billing a significant Medicare population, the operational question is not whether to understand these rules but who is accountable for applying them consistently. Whether that accountability sits in-house or with an outsourced billing partner, it requires current knowledge of CMS requirements, clean enrollment records, and a denial management workflow that catches issues before they age past the point of recovery.
Neolytix’s medical billing services support the full Medicare billing cycle, from enrollment and eligibility verification through clean claim submission, denial resolution, and reporting, for practices across specialties and care settings.
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Frequently Asked Questions
What is the difference between Medicare Part B and Medicare Advantage billing?
Part B (traditional Medicare) claims are submitted directly to a Medicare Administrative Contractor. Medicare Advantage (Part C) claims go to the private plan the beneficiary is enrolled in. Each MA plan sets its own coverage rules, prior authorization requirements, and timely filing deadlines, which are typically 90 to 120 days rather than the 12-month federal window.
What modifiers are specific to Medicare billing?
Medicare uses several HCPCS Level II modifiers not used in commercial billing. GA indicates an ABN is on file for a potentially non-covered service. GY indicates the service is categorically excluded from Medicare coverage. GZ indicates the provider expects denial but did not obtain an ABN. For telehealth, modifier 95 is used for synchronous audio/video visits, and modifier 93 for audio-only.
What is the Medicare conversion factor for 2026?
The 2026 Medicare Physician Fee Schedule established two conversion factors: $33.4009 for non-APM Qualifying Participants (a 3.26% increase from 2025) and $33.5675 for APM Qualifying Participants (a 3.77% increase). These figures include a statutory 2.5% increase passed by Congress and a positive budget neutrality adjustment, partially offset by a -2.5% efficiency adjustment applied to non-time-based service work RVUs.
Can Medicare consultation codes be billed in 2026?
No. Medicare discontinued payment for consultation codes (CPT 99241–99245) in 2010. Providers should bill the appropriate outpatient E&M code (99202–99215) based on medical decision making or total time, regardless of whether the visit originated as a referral or consult.
What is a MAC in Medicare billing?
A Medicare Administrative Contractor (MAC) is a regional entity contracted by CMS to process and adjudicate Medicare claims. MACs handle both Part A and Part B claims within their designated jurisdictions, typically within 30 days of submission. Providers work with their assigned MAC for enrollment, claim status inquiries, and reopening requests.
