What is the denial code for not medically necessary?

CO-50 is the main medical necessity code: the payer determined the service does not meet its criteria for the documented condition. Related codes are CO-11 when the diagnosis does not support the procedure, CO-55 when the service is considered experimental, and CO-58 when the payer disputes the treatment setting rather than the care itself.

How do I appeal a medical necessity denial?

Get the payer's coverage policy for the service, then build the appeal around its criteria point by point, citing where the medical record shows each one is met. A letter of medical necessity from the treating clinician helps, but it persuades far more when it addresses the policy's actual criteria rather than restating the diagnosis.

Medical necessity denial codes

A payer can say care was not necessary in four distinct ways, and each one is answered differently. CO-50 is the direct version: the service does not meet the payer's medical necessity criteria for this patient's documented condition. CO-11 means the diagnosis codes on the claim do not support the procedure, which is often a coding gap rather than a clinical one. CO-55 means the payer classifies the service as experimental or investigational under its policy. CO-58 accepts that the care was needed but says it belonged in a different, usually less costly, setting.

Also described as: does not meet medical necessity criteria; diagnosis does not support the service; experimental or investigational; wrong level of care; should have been outpatient.

The 4 codes and what separates them

CO-50 - Not deemed medically necessary by the payer
The payer reviewed the service against its medical necessity criteria and found the documented condition does not meet them.
Who pays: Provider
CO-11 - The diagnosis does not support the procedure billed
The diagnosis codes on the claim do not establish a reason the payer accepts for the procedure. A more specific or omitted diagnosis frequently resolves it without an appeal.
Who pays: Provider
CO-55 - The service is considered experimental or investigational
The payer's policy treats the procedure, device, or drug as experimental or investigational and excludes it.
Who pays: Provider
CO-58 - The payer considers the treatment setting inappropriate
The service was appropriate, but the payer decided it belonged in a different setting - outpatient rather than inpatient, or an office rather than a hospital department.
Who pays: Provider

How to tell which one you have

Look at what the payer is actually disputing. If the clinical picture is sound but the claim's diagnosis codes do not reflect it, CO-11 is a coding fix: review the record for a diagnosis that was documented but not coded, and send a corrected claim. CO-50 and CO-55 are policy decisions, so the appeal has to engage the payer's own coverage policy - quote its criteria and show, from the record, where the patient meets each one, or for CO-55 show that current evidence or the payer's updated policy no longer treats the service as investigational. CO-58 is argued on the setting alone: why this patient, specifically, needed the higher level of care.

Frequently asked questions

What is the denial code for not medically necessary?: CO-50 is the main medical necessity code: the payer determined the service does not meet its criteria for the documented condition. Related codes are CO-11 when the diagnosis does not support the procedure, CO-55 when the service is considered experimental, and CO-58 when the payer disputes the treatment setting rather than the care itself.
How do I appeal a medical necessity denial?: Get the payer's coverage policy for the service, then build the appeal around its criteria point by point, citing where the medical record shows each one is met. A letter of medical necessity from the treating clinician helps, but it persuades far more when it addresses the policy's actual criteria rather than restating the diagnosis.

Other denial reasons

Out of network No prior authorization Service not covered Bundled or not paid separately Coding mismatch or invalid code Patient not eligible or coverage ended Another payer is responsible Missing information, duplicate, or late filing Provider not eligible or not credentialed Benefit or unit limit reached Deductible, coinsurance, or copay Paid less than billed

Every denial code in one table

Last reviewed October 3, 2026.

These are original plain-English summaries written for reference, not the official X12 code descriptions. Payer handling of any code varies by contract - always verify against the remittance advice and your payer agreement before adjusting a claim.