Medicare Billing Fundamentals for Providers
Medicare is the largest payer in the United States, covering approximately 67 million beneficiaries through Part A (hospital insurance), Part B (medical insurance), Part C (Medicare Advantage), and Part D (prescription drugs). Traditional Medicare (Parts A and B) is administered by Medicare Administrative Contractors (MACs) appointed by CMS, while Medicare Advantage is managed by private insurers like UnitedHealthcare, Humana, and Aetna. This guide focuses on traditional Medicare (fee-for-service) billing, which follows standardized rules set by CMS.
Every practice that treats patients over 65, individuals with disabilities, or those with end-stage renal disease needs a solid Medicare billing foundation. Medicare accounts for 20% to 80% of revenue depending on your specialty, and errors in Medicare billing carry higher stakes than commercial payer mistakes due to False Claims Act exposure, Recovery Audit Contractor (RAC) audits, and CERT (Comprehensive Error Rate Testing) reviews.
Enrollment and Fee Schedule Basics
Before billing Medicare, providers must be enrolled in the Medicare program through PECOS (Provider Enrollment, Chain, and Ownership System). Enrollment processing takes 60 to 120 days, and providers cannot backdate billing to before their effective enrollment date. Keep your PECOS record current, as changes in practice location, ownership, or specialty require updated enrollment within 90 days.
Medicare reimbursement follows the Medicare Physician Fee Schedule (MPFS) for professional services and various prospective payment systems for facilities. The MPFS is based on Relative Value Units (RVUs) that account for physician work, practice expense, and malpractice cost, multiplied by a geographic adjustment factor and the national conversion factor. The 2026 conversion factor is $32.35. You can look up expected payment for any CPT code using the CMS Physician Fee Schedule Search tool.
Medicare participation status affects reimbursement. Participating providers accept assignment on all claims and receive the full Medicare-approved amount. Non-participating providers can choose to accept or decline assignment on a claim-by-claim basis. Non-par providers who accept assignment receive 95% of the Medicare-approved amount. Those who do not accept assignment can bill up to the limiting charge (115% of the non-par allowed amount).
Timely Filing and Claim Submission
Medicare’s timely filing deadline is 365 days from the date of service. This is a statutory deadline under Section 1848(g)(4) of the Social Security Act and cannot be extended by MACs or CMS except in very limited circumstances (natural disasters, retroactive Medicare entitlement, or system outages documented by CMS). Claims submitted after the deadline are denied and cannot be billed to the beneficiary.
Submit electronic claims through your MAC using the 837P (professional) or 837I (institutional) format. Medicare requires electronic submission for all providers except those with fewer than 25 claims per year, who may submit paper claims on CMS-1500 or UB-04 forms. Each MAC has a specific electronic payer ID. Common MAC payer IDs include Novitas (12502 for JL, 12501 for JH), Palmetto GBA (12401 for JJ, 12402 for JM), and First Coast (09102).
Medicare processes clean electronic claims within 14 to 30 calendar days. The MAC must pay or deny clean claims within 30 days of receipt for electronic claims and 45 days for paper claims. Interest accrues on clean claims not paid within these timeframes. Monitor your remittance advice (ERA/835) carefully to identify underpayments, as Medicare’s automated systems occasionally apply incorrect fee schedules or deductible calculations.
Medical Necessity and Coverage Determination
Medicare covers services that are “reasonable and necessary for the diagnosis or treatment of illness or injury or to improve the functioning of a malformed body member.” This standard is applied through National Coverage Determinations (NCDs), which are binding nationwide, and Local Coverage Determinations (LCDs), which are developed by individual MACs and may vary by jurisdiction.
Before billing a service that is frequently denied for medical necessity, check the applicable NCD and LCD. The CMS Medicare Coverage Database contains all active NCDs and LCDs searchable by CPT code, ICD-10 code, or keyword. LCDs specify the covered diagnosis codes, documentation requirements, and frequency limitations. Billing a service that does not meet LCD criteria results in a denial that the beneficiary cannot be held responsible for, unless you obtained an ABN before the service.
The Advance Beneficiary Notice of Noncoverage (ABN, form CMS-R-131) is a critical tool for Medicare billing. Issue an ABN before providing a service when you reasonably expect Medicare will deny it. The beneficiary signs the ABN acknowledging they may be financially responsible if Medicare does not pay. Without a valid ABN, you must write off denied services. ABNs are not required for services Medicare never covers (excluded services), as beneficiaries are always responsible for those.
Coding Rules and NCCI Edits
Medicare coding follows the National Correct Coding Initiative (NCCI), which bundles certain procedure codes together and restricts unbundling through modifier use. NCCI edit pairs are published quarterly and available on the CMS NCCI website. When two codes have an NCCI edit, the lower-valued code is typically bundled into the higher-valued code unless a modifier is applied to indicate a separate and distinct service.
Modifier usage on Medicare claims is scrutinized more closely than on commercial claims. Modifier 25 (significant, separately identifiable E/M) is the most frequently used and audited modifier. Medicare expects documentation to clearly support a separate E/M service beyond the standard pre- and post-service work included in a procedure’s global period. RAC auditors regularly target claims with modifier 25 for medical record review.
HCPCS Level II codes (alphanumeric codes starting with a letter) are used for Medicare billing of supplies, DME, drugs, and services not covered by CPT codes. Medicare requires HCPCS Level II codes when they exist, even if a CPT code could technically apply. For example, injectable drugs administered in the office must use the specific J-code rather than a generic administration code.
The Medicare Appeal Process
Medicare provides a 5-level appeal process that offers more robust protections than any commercial payer. Level 1 is a Redetermination by the MAC, filed within 120 days of the initial determination. The MAC reviews the claim and issues a decision within 60 days. Approximately 40% of Level 1 appeals result in a fully or partially favorable outcome.
Level 2 is a Reconsideration by a Qualified Independent Contractor (QIC), filed within 180 days of the Level 1 decision. The QIC conducts an independent review and decides within 60 days. Level 3 is a hearing before an Administrative Law Judge (ALJ), available for claims with an amount in controversy of at least $180 (2026 threshold). ALJ hearings must be requested within 60 days and decided within 90 days.
Level 4 is review by the Medicare Appeals Council, and Level 5 is judicial review in federal district court (for amounts over $1,760 in 2026). Most providers resolve Medicare disputes at Levels 1 or 2. The ALJ level has historically been the most favorable for providers, with overturn rates exceeding 70%, though recent reforms have streamlined the process and reduced some of that advantage.
For each appeal level, submit all supporting documentation upfront. Medical records, physician orders, clinical notes, and relevant literature should accompany every appeal. Do not hold back documentation for later levels, as building the strongest case at Level 1 gives you the best chance of an early favorable decision and avoids the months or years that higher-level appeals can take.