Why it matters in orthopedics
Medical necessity decides payment for many high-cost orthopedic services, from total joint replacement to knee arthroscopy, injections and bracing. Kodiak Solutions reported that medical-necessity initial denials rose 5% in 2024, and that denials for authorization and medical necessity drove virtually all of the increase in denial rates in 2025. Our guide to common orthopedic claim denials puts CO-50 in context with other denial types.
How it works
Payers use CARC 50 when they won’t cover a service because they judge it not medically necessary (X12 wording last modified July 1, 2017). If the payer fills in the 835’s healthcare policy identification segment, it names the policy it applied.
Remark codes often point to the rule behind the decision: N115 (based on a local coverage determination), N386 (based on a national coverage determination), N130 (check plan benefit documents) and N180 (the service doesn’t qualify for the category it was billed under). Noridian’s JF Part B list pairs CO-50 with N115.
Group code CO versus PR is the key difference. In Original Medicare, if you expect a medical-necessity denial and give the patient a valid Advance Beneficiary Notice, you report modifier GA, and the denial can be assigned to the patient. Without a valid notice, CMS may hold the practice liable, and Medicare lets providers bill beneficiaries only for adjustments under PR.
Medicare Advantage plans and their contracted providers don’t use the ABN. If you think a service may not be covered, the patient or the practice should request a pre-service decision from the plan.
Orthopedic example
A 61-year-old Original Medicare patient has knee pain and osteoarthritis but no mechanical symptoms, and the surgeon performs arthroscopic debridement. National coverage determination 150.9 doesn’t cover arthroscopic debridement for an osteoarthritic knee when the patient presents with knee pain only, or for severe osteoarthritis (Outerbridge grade III or IV). The claim can return CO-50, with remark code N386 signaling the NCD basis, and without a valid ABN the practice absorbs it. A total knee replacement denied because notes don’t show 12 weeks of failed nonoperative care is the commercial-payer version of the same problem.
Common denial triggers
Notes that don’t document the payer’s criteria, such as imaging grade, functional limits or the duration of failed conservative care.
A diagnosis outside the covered list in an LCD or payer policy.
Records not sent, or not sent on time, when the payer asks for documentation.
For durable medical equipment, a missing order or a missing KX modifier where the policy requires one.
No prior authorization, or a lapsed one, when the payer reviews necessity before service; see CO-197.
How to prevent or fix
Build note templates around the payer’s published criteria for your highest-volume procedures.
Issue an ABN before the service when you expect Original Medicare to deny it, and bill with modifier GA.
If the remark code shows the diagnosis was the problem, check for a coding error first; see CO-11.
Appeal with records mapped to each criterion and cite the policy and version in effect on the date of service. Our knee and hip replacement appeal guide includes a documentation checklist and deadlines.
Request a peer-to-peer review when the payer offers one, but keep the written appeal on schedule.
Frequently asked questions
Can you bill the patient for a CO-50 denial?
Not while the group code is CO. Medicare lets providers bill beneficiaries only for amounts assigned to group code PR. In Original Medicare, a valid Advance Beneficiary Notice given before the service, reported with modifier GA, lets Medicare assign liability to the patient. Without it, CMS may hold the practice liable. For commercial plans, your participation agreement and state law decide whether the patient can be billed.
What is the difference between CO-50 and PR-50?
Both mean the payer judged the service not medically necessary. The group code decides who pays. CO-50 leaves the balance with the provider, while PR-50 assigns it to the patient, which in Original Medicare typically follows a valid Advance Beneficiary Notice and modifier GA. If you see CO-50 when you had a valid ABN on file, check whether the modifier was reported and correct the claim.
How do you appeal a CO-50 denial?
Find the specific criterion the payer says wasn’t met, then send records that prove it with dates: imaging reports, notes describing functional limits, and a timeline of failed treatments. Cite the payer’s policy number and version. For Original Medicare, file a redetermination within 120 days of receiving the initial determination. Medicare Advantage plans allow 65 days from the notice date for a reconsideration request.
Guides that cover this term
How to Appeal a Medical-Necessity Denial for a Knee or Hip Replacement
Payer criteria for total knee and hip replacement, the records to send, peer-to-peer tips, and Medicare, MA and commercial appeal deadlines.
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The Most Common Orthopedic Claim Denials in 2026, and How to Prevent Them
Ten orthopedic denial types, the CARCs behind them, their root causes and how to prevent them, plus when to appeal or correct and resubmit.
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Sources
Claim Adjustment Reason Codes — X12, accessed October 14, 2026
Remittance Advice Remark Codes — X12, accessed October 14, 2026
Medicare Advance Written Notices of Non-coverage (MLN006266) — CMS Medicare Learning Network, May 2025
Remittance Advice Resources and FAQs (ICN 905367) — CMS Medicare Learning Network, accessed October 14, 2026
Medicare National Coverage Determinations Manual, Chapter 1, Part 2 (section 150.9) — CMS, NCD effective June 11, 2004
Denial Code Resolution, JF Part B — Noridian Healthcare Solutions, last updated July 9, 2025
Reason Code 50, JD DME — Noridian Healthcare Solutions, last updated June 6, 2025
Parts C & D Enrollee Grievances, Organization/Coverage Determinations, and Appeals Guidance — CMS, effective July 6, 2026
Rate of initial denials of medical insurance claims continued to rise in 2024, Kodiak Solutions’ proprietary data show — Kodiak Solutions via Business Wire, May 21, 2025
Healthcare Provider Organizations Saw Net Revenue Losses From Final Denials and Bad Debt Grow by 25% in 2025, According to Kodiak Solutions’ Proprietary Data — Kodiak Solutions via Business Wire, March 31, 2026
This entry is educational and is not legal, coding-compliance or billing advice. Payer rules change often, so confirm requirements with the payer before you bill. CPT® is a registered trademark of the American Medical Association; code descriptions here are Rette’s plain-language summaries.