CO-50 Denial in Behavioral Health: How to Appeal (Template)

Denial code CO-50 means the payer decided the service was not medically necessary, and the CO group code means the provider, not the patient, carries the loss unless the denial is overturned. The official X12 description reads: "These are non-covered services because this is not deemed a 'medical necessity' by the payer." To reverse it, get the criteria the payer applied, find the gap between those criteria and your record, and appeal within the deadline for that plan type with documentation that answers each criterion.

In behavioral health (BH) and substance use disorder (SUD) treatment, CO-50 can hit residential days, PHP and IOP weeks, and long runs of outpatient psychotherapy. This guide covers the remark codes that come with it, why BH claims draw it, appeal routes and deadlines by plan type, a prevention checklist by service, and a full appeal letter template. For the documentation standards themselves, see our medical necessity documentation guide; this article is about the denial and the appeal.

The short version

CO-50 is a medical necessity denial with the loss assigned to the provider. Under Medicare, liability shifts to the beneficiary when a valid ABN was given and reported on the claim, for example with the GA modifier.
Read the remark code first. N661, N115, N386, N10 and M127 point to different problems: the record, a coverage policy, a reviewer's judgment, or records that never arrived.
Ask for the criteria. ERISA group health plans must disclose the rule or guideline relied on, or provide it free on request, and mental health and SUD medical necessity criteria must be available to contracting providers on request.
Deadlines depend on plan type: at least 180 days for ERISA group health plans, 60 days for Medicaid managed care and Medicare Advantage, 120 days for a Medicare redetermination.
Win by mapping the record to each criterion: diagnosis, functional impairment, treatment plan, response to treatment, and why this level of care and not a lower one.

What CO-50 means, and when it becomes PR-50

CARC 50 is the payer's judgment that the service, as documented, did not meet its medical necessity standard. X12 adds a usage note: refer to the 835 Healthcare Policy Identification Segment (loop 2110 REF), if present. When the payer fills that segment, it names the policy behind the denial, which is the first thing to pull.

The group code decides who pays. CO means a contract or regulatory requirement assigns the amount to the provider; PR assigns it to the patient. Under Medicare, when a provider gives a valid written notice before the service (an Advance Beneficiary Notice) and reports it on the claim, typically with the GA modifier, the contractor holds the beneficiary liable. If the notice was invalid, the contractor overrides the GA modifier and the provider is liable.

Do not confuse CO-50 with neighbors that need a different response: 151 (frequency not supported), 197 (authorization absent), 96 (non-covered charge) and 204 (not covered under the benefit plan).

Remark codes that come with CO-50

RARC	X12 text (short)	What it tells you
N661	Documentation does not support that the services were medically necessary	Records were reviewed and found short. Answer the specific gap.
N115	Decision based on a Local Coverage Determination	Medicare LCD applies. Pull the LCD and its billing article.
N386	Decision based on a National Coverage Determination	A national Medicare policy applies.
N10	Based on findings of a review organization, medical advisor or peer review	A person made the call. Ask for the rationale and reviewer credentials.
M127 or N706	Missing patient medical record; missing documentation	The payer never got the records. Send them.
N188	Approved level of care does not match the procedure code	Billed level and authorized level differ.
N42 or N731	Missing, or incomplete or invalid, mental health assessment	The assessment is absent or does not support the service.
N130	Consult plan benefit documents for restrictions	A plan-level rule applies. Read the plan document.

Why behavioral health and SUD claims get CO-50

Level of care not justified. The record names a level but does not show why it is needed, or why a less intensive level would not work.
Continued stay without new information. Concurrent review notes copied from admission, with no change in symptoms, risk or plan.
Frequency without a reason. More sessions than the treatment plan calls for, with no documented trigger.
Missing functional impairment. A diagnosis alone. One Medicare psychotherapy LCD (L33252, First Coast Service Options) expects each session's record to show a psychiatric illness or symptoms sufficient to alter baseline functioning.
Treatment plan gaps. The same LCD expects measurable goals, session frequency and estimated duration, updated generally at least every three months, and a documented rationale for any extension.
Generic notes. L33252 says statements such as "supportive psychotherapy given" are not adequate.
No expected benefit. L33252 requires a reasonable expectation of improvement, or of maintaining function where decline would otherwise be expected.

Before you appeal: a quick triage

Confirm the code at line level. Note the RARC and any policy ID in the remittance.
Identify the plan type. Employer plan under ERISA, state-regulated insured plan, Medicaid managed care, Medicare or Medicare Advantage. It sets the deadline and the levels.
Calendar the deadline from the date on the notice.
Request the criteria and claim file. For ERISA group health plans, the denial must include any internal rule or guideline relied on, or say one was used and offer a free copy, and, for medical necessity denials, explain the clinical judgment or offer that explanation free. Claimants can get copies of all relevant documents free of charge, and group health plans must make MH and SUD medical necessity criteria available to contracting providers on request.
Choose the route. Corrected claim if the claim was wrong, records submission if the payer never got them, appeal if the payer disagrees on necessity.
Use peer-to-peer if offered, and log the date, reviewer name, credentials and outcome.
Get written authorization from the patient when you appeal on their behalf.

Appeal routes and deadlines by plan type

Plan type	Route	Filing deadline	Decision timeframe
Employer group health plan (ERISA)	Internal appeal; no more than two mandatory levels	At least 180 days after the denial notice	Post-service: 60 days (one level) or 30 per level (two). Pre-service: 30 or 15 per level. Urgent: 72 hours
Non-grandfathered plans, after internal appeal	External review by an independent review organization	4 months under the federal process; at least 4 months under state processes	45 days standard; 72 hours expedited
Medicaid managed care	One plan appeal, then State fair hearing	60 calendar days from the notice date	30 calendar days standard; 72 hours expedited; extensions up to 14 days
Medicare Parts A and B	MAC redetermination, QIC reconsideration, ALJ hearing, Medicare Appeals Council, federal court	120 days, then 180 days, then 60 days at each later level	MAC and QIC generally 60 days. 2026 amount in controversy: $200 for an ALJ hearing, $1,960 for federal court
Medicare Advantage	Reconsideration by the plan	60 calendar days after receiving the decision	Per the plan's notice

A few details matter in BH. The federal external review process explicitly covers decisions involving medical judgment, including medical necessity, health care setting and level of care, as well as whether the plan complies with parity rules for nonquantitative treatment limits. Expedited external review is available when the standard timeframe would seriously jeopardize the patient's life or health, and in certain admission and continued-stay situations. In ERISA appeals involving medical judgment, the plan must consult a health care professional with appropriate training and experience in that field who was not consulted on the original denial. And in Medicare, evidence not submitted at the QIC reconsideration may be excluded at later levels unless you show good cause, so send the full record by the second level. Fully insured plans also follow state insurance law, government and church plans generally fall outside ERISA, Medicaid fee-for-service provider appeals follow state rules, and in-network providers should check their contract's dispute process. This is general information, not legal advice.

What to put in the appeal packet

The appeal letter (template below) and a copy of the denial notice or remittance.
The payer's criteria, with a one-page crosswalk showing where the record meets each element.
The assessment that supports the service and level of care, with standardized scores where used.
The treatment plan in effect on the dates of service, signed and dated.
Progress notes for the denied dates, plus enough before and after to show the trajectory.
The authorization and utilization review record: request times, approvals, peer-to-peer notes.
A letter of medical necessity from the treating clinician or medical director.
The patient's authorized representative form, and confirmation that disclosure of SUD records meets 42 CFR Part 2 consent requirements.

CO-50 appeal letter template

Adapt this to the plan type and level of appeal, replace every [bracketed] field, and send it on letterhead. Keep it factual and specific to the criteria the payer cited.

Appeal letter template: CO-50 medical necessity denial

[Program or practice name]
[Address] | [Phone] | NPI [NPI] | Tax ID [Tax ID]
[Date]

[Payer name]
Attn: [Appeals unit named on the denial notice]
[Appeals address or fax number from the notice]

Re: Request for [appeal / reconsideration / redetermination] of medical necessity denial (CARC 50)
Patient: [Patient name] | Member ID: [Member ID] | Date of birth: [DOB]
Claim number: [Claim number] | Date(s) of service: [Dates]
Service(s): [CPT or HCPCS codes and description]
Denial notice date: [Date] | Remark code(s): [RARC, for example N661]

Dear [Reviewer name or Appeals Committee]:

I am writing on behalf of [patient name], [as the patient's authorized representative (signed authorization enclosed)], to appeal the denial of [service] for [dates of service], reported with claim adjustment reason code 50. We ask that you reverse the denial and pay the claim.

Clinical summary. [Patient name] is a [age]-year-old with [diagnoses and ICD-10-CM codes], assessed on [date]. [Two to four sentences on current symptoms, risk and functional impairment at work, school, home, self-care or safety, with specific examples and the scores from any standardized measures used.]

Why this service and this level of care. [For SUD care, summarize the multidimensional assessment and the level of care selected.] [Explain why a less intensive level would not safely meet the need, citing prior treatment and its outcome.] [Explain why a more intensive level was not required.]

How the record meets your criteria. Your notice cites [criteria name and edition, or policy number]. The enclosed records show each element:
1. [Criterion]: [where the record meets it, with page reference]
2. [Criterion]: [where the record meets it, with page reference]
3. [Criterion]: [where the record meets it, with page reference]

Treatment plan and progress. The treatment plan dated [date] sets [measurable goals] at [frequency] for [expected duration]. Progress notes from [dates] document [response to treatment, or why continued treatment is needed to prevent deterioration].

Requests. Please (1) reverse the denial for [dates or units]; (2) provide, free of charge, copies of all documents, records and other information relevant to this claim, including the specific rule, guideline or criteria relied on; and (3) have this appeal decided in consultation with a health care professional with training and experience in [psychiatry, addiction medicine or behavioral health] who was not involved in the initial decision. [If the plan covers mental health or SUD benefits subject to federal parity law: please also provide the medical necessity criteria used for these benefits.]

Enclosures: [denial notice]; [authorization record]; [assessment dated]; [treatment plan dated]; [progress notes for dates]; [letter of medical necessity from clinician]; [criteria crosswalk]; [authorized representative form].

Please contact [name and title] at [phone or email] with questions.

Sincerely,
[Name, credentials]
[Title], [Program name]

Free templates

Medical necessity appeal letter templates (Word)

Three editable first-level appeals: one for a CO-50 denial, one for a level-of-care downgrade and one for a continued stay denial, with a checklist of what to do before writing and the records to attach.

Get the appeal letter templates Free, with a work email.

Prevention checklist by service type

Outpatient psychotherapy

Each note shows the diagnosis, current symptoms and how they affect functioning.
Time is documented, and the code matches it. Medicare billing article A57520 lists 16 to 37 minutes for 90832, 38 to 52 for 90834 and 53 or more for 90837, with the same ranges for add-on codes 90833, 90836 and 90838, and no psychotherapy under 16 minutes.
Interventions and the patient's response are specific, not boilerplate.
The treatment plan lists measurable goals, frequency and expected duration, and is updated on schedule.
Any increase in frequency is explained in the note.

IOP and PHP

The admission assessment explains why outpatient care is not enough.
Hours delivered match the program billed. H0015 describes a program of at least 3 hours a day and 3 days a week; for Medicare PHP, RARC N787 restates the 42 CFR 410.43 requirement of at least 20 hours of PHP services per week, as evidenced in the plan of care.
Daily attendance is logged and missed sessions are addressed.
Each concurrent review includes new findings and a step-down plan.

Residential

The record explains why a 24-hour setting is needed and why IOP or PHP would not be safe or sufficient.
Every continued-stay review has a fresh multidimensional reassessment, not copied text.
Discharge criteria are set at admission and revisited at each review.
Authorized dates match billed days, so a medical necessity question is not mixed with an authorization gap.

Then log every CO-50 by payer, service, remark code, criteria cited, root cause and appeal outcome. A monthly review of that log shows which template fields or review habits to fix first.

How Adentris helps

Adentris reviews every active chart, not a sample, against payer and regulatory rules before claims go out, including medical necessity, ASAM level of care, and treatment plan and signature timing. Each finding cites the rule that fired and comes with a drafted correction for staff to approve, so the gaps that lead to CO-50 are closed before submission. For denials already on the books, Denial Fightback clusters them by root cause, pushes prevention back to the source and drafts regulation- and parity-cited appeals. It runs on top of any EHR, connecting through its API or FHIR, HL7 feeds, an API our agents build where none exists, or the AI Web Agent, with AI inference inside Microsoft Azure and no patient data sent to external LLM APIs. To see it on your own charts, book a 30-minute call with our team.

Sources

Frequently asked questions

What does denial code CO-50 mean?

CO-50 combines group code CO (Contractual Obligation) with claim adjustment reason code 50, which X12 defines as non-covered services because the payer did not deem them a medical necessity. The provider carries the amount unless the denial is overturned on appeal.

What is the difference between CO-50 and PR-50?

The reason is the same; the group code assigns responsibility. CO puts the amount on the provider, PR on the patient. Under Medicare, the contractor holds the beneficiary liable when the provider gave a valid Advance Beneficiary Notice and reported it on the claim, typically with the GA modifier. Without a valid notice, the provider is liable.

How long do I have to appeal a CO-50 denial?

It depends on the plan. ERISA group health plans must allow at least 180 days after the denial notice. Medicaid managed care enrollees have 60 calendar days from the date on the notice. Medicare Part A and B redeterminations must be filed within 120 days of receiving the initial determination, and Medicare Advantage reconsiderations within 60 calendar days. Your plan or contract may set longer limits.

Can a provider file a medical necessity appeal for the patient?

Usually yes, with the right paperwork. ERISA plans cannot block an authorized representative but may set procedures to confirm the authorization; for urgent care, a health care professional who knows the patient's condition must be allowed to act. Medicaid managed care allows a provider to appeal with the enrollee's written consent where state law permits. In-network providers may also have a separate contract dispute process.

What should a medical necessity appeal letter include?

Patient, claim and service identifiers; the denial date and codes; a short clinical summary with functional impairment; why this service and level of care were needed and why a less intensive option was not enough; a point-by-point match between the payer's criteria and the record; requests for the criteria and claim file; and a list of enclosures.

Should I resubmit or appeal a CO-50 denial?

Appeal when the service was billed correctly and the payer disagrees about necessity. Send a corrected claim only when the claim itself was wrong, for example a code or level that does not match the documented service. If the remark code shows the payer never received records, check whether the payer allows a reopening or a records submission before using an appeal level.