What is Medicare Part B? Definition, Formula, and Benchmark

Medicare Part B is the Medical Insurance component of Original Medicare, covering physician services, outpatient hospital services, durable medical equipment, preventive services, home health (non-Part-A-eligible), clinical laboratory services, and most other medically necessary outpatient services. Part B complements Part A (hospital insurance) to form Original Medicare.

Part B enrollment is voluntary; beneficiaries must actively enroll or decline. Enrollment typically occurs at age 65 during the Initial Enrollment Period; automatic enrollment happens for beneficiaries already receiving Social Security. The standard monthly premium for 2025 is $185, deducted from Social Security checks for most beneficiaries. Higher-income beneficiaries pay Income-Related Monthly Adjustment Amount (IRMAA) surcharges scaling from ~$74 to $443+ additional monthly premium based on modified adjusted gross income. Low-income beneficiaries may qualify for Medicaid-paid Part B premium through Medicare Savings Programs.

Part B cost-sharing: annual deductible $257 (2025); 20% coinsurance on most allowed amounts after deductible. Certain preventive services (annual wellness visit, screening colonoscopy, flu shots, etc.) are covered at 100% with no deductible or coinsurance under ACA preventive-service mandates. Durable medical equipment, mental health services, and certain other categories have service-specific cost-sharing variations.

For provider RCM, Part B billing occurs on the CMS-1500/837P form at the practice-level. Medicare Physician Fee Schedule (MPFS) sets the allowed amount. Providers bill directly to their regional MAC (Medicare Administrative Contractor); the MAC adjudicates and pays. Participating (PAR) providers accept Medicare allowed amount as payment in full; non-participating (non-PAR) providers can charge up to 115% of the allowed amount ("Medicare limiting charge") and bill the patient the difference. Opt-out providers accept no Medicare payment and bill the patient directly at any amount.

Part B billing discipline depends on accurate coding (CPT, HCPCS, ICD-10), correct POS designation, appropriate modifier use, and compliance with NCCI edits. Denial and underpayment management, timely filing (12 months from date of service for Medicare, compared to shorter private-payer windows), and correct rendering/billing provider NPI and taxonomy are core operational requirements.

Quality Payment Program (MIPS and Advanced APM) applies payment adjustments to Part B allowed amounts two years after the performance year. For 2026 payments, 2024 MIPS performance applies; the MIPS adjustment (±9% maximum) modifies the base MPFS-calculated payment. High-performing MIPS clinicians receive positive adjustments; poor-performing or non-reporting clinicians face penalties. Advanced APM participation generally exempts clinicians from MIPS with its own incentive structure.

Medicare Advantage Part C replaces Part A and Part B for enrolled beneficiaries. MA plans receive CMS capitation (risk-adjusted) and manage all Part A and Part B services. Providers serving MA members bill the MA plan, not Medicare directly; MA plan reimbursement rates are typically similar to or slightly higher than Part B rates depending on the provider's MA contract. MA membership has grown steadily; approximately 50%+ of Medicare beneficiaries were in MA as of 2024.