Glossary · Reimbursement

ASC (ambulatory surgical center)

An ambulatory surgical center (ASC) is a Medicare-certified, freestanding or hospital-operated facility that furnishes outpatient surgical services exclusively—meaning patients are treated and discharged the same day without an overnight stay.

Verified May 8, 2026 · 7 sources ↓

Drawn from CMSAAPCAaomsBristolhcsBlog

Definition

Source · Editorial summary grounded in 7 cited references ↓

Under Medicare's definition (42 CFR 416, CMS Claims Processing Manual Chapter 14), an ASC is a distinct legal entity operating solely to provide outpatient surgical services. It must hold a participation agreement with CMS and may be either independent or operated under a hospital's ownership or control. Unlike a hospital outpatient department (HOPD), an ASC is reimbursed under its own prospective payment system and bills only facility fees—not room-and-board or inpatient charges.

ASCs submit claims on the CMS-1500 form (or its 837P electronic equivalent) using CPT and HCPCS Level II codes to identify procedures, and ICD-10-CM codes to establish medical necessity. Place of Service (POS) code 24 must appear on the claim. The facility fee covers the ASC's space, equipment, supplies, and clinical staff; the operating surgeon and anesthesiologist bill separately under their own NPIs. Medicare pays ASC facility services under Part B, and the amount is determined by the ASC payment group assigned to each approved HCPCS/CPT code—rates are published annually by CMS.

For orthopedic ASCs specifically, covered procedures span arthroscopy, joint arthroplasty, fracture care, and implant-intensive surgeries. CMS maintains an approved procedures list; any procedure not on that list cannot be billed as an ASC facility service to Medicare. Commercial payers maintain their own approved lists and fee schedules, which frequently differ from Medicare's, requiring payer-specific billing protocols.

Why it matters

Reimbursement in an ASC setting is systematically lower than in a hospital outpatient department for most procedures—sometimes 50–60% of the HOPD rate—so mis-identifying the place of service (e.g., billing POS 22 instead of POS 24) triggers incorrect payment, potential overpayment recoupment, and audit exposure. Conversely, failing to confirm that a procedure appears on CMS's ASC Covered Procedures List before scheduling a Medicare patient can result in a full claim denial with no path to collect the facility fee, leaving the ASC uncompensated for the case.

Common mistakes

Where people most often go wrong with this concept.

Source · Editorial brief grounded in cited references ↓

Billing POS 22 (outpatient hospital) instead of POS 24 (ASC) on the CMS-1500, which misroutes the claim to HOPD payment logic and triggers overpayment reviews.
Submitting ancillary services (pass-through devices, separately payable drugs, radiology) on a separate claim rather than on the same claim as the triggering surgical procedure—CMS will return the ancillary claim as unprocessable if no approved ASC surgical procedure appears in history for the same beneficiary, provider, and date.
Assuming a procedure covered by a commercial payer is also on Medicare's ASC Covered Procedures List; the two lists are not identical, and performing a Medicare-excluded procedure in the ASC shifts the financial risk entirely to the facility.
Confusing the facility fee with the professional fee—the ASC bills only the facility component; the surgeon must bill independently under their own NPI, and bundling both on one ASC claim causes a denial.
Applying hospital UB-04 billing logic to an ASC Medicare claim; Medicare Part B requires the CMS-1500 for ASC facility services, and revenue code classification used for implant reimbursement applies only when a specific commercial payer explicitly allows it.
Neglecting to update the internal ASC approved-procedure list after CMS's annual payment rule takes effect January 1, leading to claims for newly excluded or reclassified procedures.

Related codes

Codes commonly involved when this concept appears in practice.

CPT

ICD-10

M17.11 M17.0

Modifiers

50 Bilateral procedure LT Left side RT Right side 59 Distinct procedural service XS Separate structure

Frequently asked questions

Source · Generated from the editorial pipeline, verified against 7 cited references ↓

01What makes an ASC different from a hospital outpatient department for billing purposes?

An ASC bills only a facility fee on the CMS-1500 under Part B using POS 24, is reimbursed under the ASC prospective payment system, and can only bill for procedures on CMS's approved list. A hospital outpatient department bills on the UB-04 under the OPPS with POS 22 and generally receives higher payment rates for the same procedure.

02Does the ASC bill for the surgeon's work?

No. The ASC facility fee covers the center's overhead—space, equipment, supplies, and clinical staff. The operating surgeon and anesthesiologist each submit their own professional fee claims under their individual NPIs, completely separate from the ASC facility claim.

03How does Medicare determine the ASC payment rate for a procedure?

CMS assigns each approved CPT or HCPCS code to an ASC payment group. The payment rate for that group, adjusted for local wage differences, determines what Medicare pays the ASC for facility services. Rates and group assignments are updated annually in the Medicare Physician Fee Schedule and ASC final rule, typically effective January 1.

04Can any outpatient surgical procedure be performed in a Medicare-participating ASC?

No. Medicare limits ASC facility payment to procedures on its ASC Covered Procedures List—those CMS has determined pose no significant patient safety risk and are not expected to require an overnight stay. Procedures not on the list cannot be billed as ASC facility services to Medicare, even if clinically appropriate for outpatient care.

05What claim form does an ASC use for Medicare?

An ASC bills Medicare Part B using the CMS-1500 claim form (or its 837P electronic equivalent) with POS code 24. Some commercial payers allow the UB-04, and some permit revenue code-based implant billing—always confirm payer-specific requirements before submitting.

06How should a separately payable implant or drug be billed in the ASC setting?

Per CMS Chapter 14, pass-through devices, separately payable drugs, and other covered ancillary items must be billed on the same claim as the related ASC surgical procedure, not on a separate claim. A standalone ancillary claim with no matching approved surgical procedure in history for the same date will be returned as unprocessable.

Sources & references

Editorial content was developed using the following public sources. Last verified May 8, 2026.

Mira AI Scribe

When Mira detects a surgical encounter documented in an ASC setting, it automatically applies POS 24 to the CMS-1500 claim shell and flags the primary CPT code against the current CMS ASC Covered Procedures List before the claim is finalized. If the procedure is not on that list for the patient's payer (Medicare or mapped commercial equivalent), Mira surfaces a pre-submission alert so the coding team can verify coverage or obtain an ABN before the case is performed. For orthopedic cases involving implants or separately payable devices, Mira prompts the coder to attach the corresponding HCPCS Level II device code to the same claim as the surgical CPT—not on a standalone claim—consistent with CMS Chapter 14 requirements. When bilateral procedures are documented, Mira suggests Modifier 50 (or LT/RT pair per payer preference) and checks NCCI edits for any column 1/column 2 bundling conflicts between the primary procedure and any add-on codes. Mira does not auto-populate the surgeon's professional fee; that remains a separate workflow under the physician's NPI.

See Mira's approach

A hospital outpatient department (HOPD) is a facility owned and operated by a hospital that provides outpatient services billed under Medicare's Hospital Outpatient Prospective Payment System (OPPS), typically reimbursed at higher rates than the same service performed in a freestanding physician office.

HCPCS Level II Coding

HCPCS Level II is the CMS-maintained alphanumeric code set used to bill products, supplies, and services—such as DME, orthotics, prosthetics, and injectable drugs—that CPT codes do not adequately describe. Each code consists of one letter (A–V) followed by four digits.