CARC 50 / CO-50: Medical Necessity Denials in Medical Billing

Claim Adjustment Reason Code (CARC) 50 identifies services that a healthcare payer considers non-covered because they do not meet its medical necessity requirements. Medical billers frequently encounter this adjustment when reviewing an Electronic Remittance Advice (ERA) after a payer determines that a service does not satisfy the applicable clinical coverage criteria.

CARC 50 is commonly associated with medical necessity denials and may appear with the CO (Contractual Obligation) adjustment group as CO-50. However, the reason code alone does not explain every aspect of the payer's determination or establish whether the patient is financially responsible.

In this guide, we'll explain what CARC 50 means, how medical necessity denials are evaluated, where the adjustment appears in an X12 835 version 5010 transaction, how CO-50 differs from PR-50, and how medical billers and EDI professionals can investigate and respond to these denials.

What Is Medical Necessity in Healthcare Billing?

Medical necessity generally refers to whether a healthcare service is appropriate and required for the patient's condition under the applicable coverage criteria.

Although medical necessity is a common concept across insurance programs, its exact definition and evaluation can differ among Original Medicare, Medicare Advantage, Medicaid and commercial health plans.

A payer may evaluate several factors when determining whether a service meets its medical necessity requirements, including:

• The patient's diagnosis, symptoms and clinical condition
• The service, procedure, treatment or supply provided
• Applicable clinical guidelines and coverage policies
• Medical records supporting the reason for treatment
• Frequency, duration and intensity of services
• Prior treatment history and response to treatment, when relevant

For example, a diagnostic procedure may be clinically appropriate in one situation but not meet a payer's coverage criteria for a different diagnosis or clinical presentation.

Medical necessity determinations should not be reduced to diagnosis and procedure codes alone. The complete clinical circumstances, applicable policy and supporting medical records may be important to the payer's decision.

What Does CARC 50 Mean in Medical Billing?

CARC 50 is used when a payer determines that a service is non-covered because it is not considered medically necessary under the payer's applicable criteria.

The adjustment identifies the payer's reason for reducing or denying payment. It does not, by itself, identify the exact clinical documentation or policy requirement that resulted in the determination.

Medical billers may need to review additional information, such as:

• Any accompanying Remittance Advice Remark Codes (RARCs)
• The payer's explanation of benefits or denial correspondence
• The applicable coverage or medical policy
• The submitted diagnosis and procedure information
• Relevant patient medical records
• Available reconsideration or appeal procedures

A CARC 50 adjustment can affect an entire claim or an individual service. A payer may also pay other services on the same claim while denying one or more services for medical necessity.

What Is the Difference Between CARC 50, CO-50 and PR-50?

CARC 50 identifies the medical necessity adjustment reason. The accompanying claim adjustment group code identifies the financial adjustment category.

Two possible combinations are CO-50 and PR-50, but they have significantly different implications for reported financial responsibility.

A correctly reported CO adjustment generally represents an amount that is not the patient's financial responsibility because of an applicable contractual obligation or regulatory requirement.

The PR group identifies patient responsibility in the payer's adjudication. However, a PR adjustment does not independently establish that billing or collecting the amount is permitted under every applicable law, coverage rule or provider agreement.

Important: A provider should not transfer a correctly reported CO-50 adjustment to the patient simply because the payer denied payment. If the adjustment group or medical necessity determination appears incorrect, the provider should investigate the adjudication and use the applicable correction, reconsideration or appeal process rather than reassigning the amount to patient responsibility.

Common Reasons for Medical Necessity Denials

Medical necessity denials can arise from several different clinical and coverage circumstances. Identifying the underlying reason is important because the appropriate response may involve correcting claim information, supplying additional documentation or disputing the payer's clinical determination.

Not every issue listed above necessarily results in CARC 50. Depending on the circumstances, a payer may report a different CARC, request additional information or identify a separate billing problem.

For example, a missing or incorrect claim data element may be reported using CARC 16, while an authorization-related issue may involve CARC 197. These adjustments should not be assumed to be medical necessity denials merely because they affect the same service.

Where Is CARC 50 Reported in the X12 835?

In the X12 835 version 5010 transaction, CARC 50 is reported in a CAS (Claim Adjustment) segment with an applicable adjustment group code and monetary amount.

The CAS segment may appear at the claim level in Loop 2100 or at the service level in Loop 2110.

For the first adjustment in a CAS segment, CAS01 identifies the adjustment group, CAS02 identifies the CARC and CAS03 identifies the associated adjustment amount.

Additional adjustments may be reported within the same CAS segment when they share the same group code, using the next available adjustment positions.

The official X12 CARC 50 definition also directs readers to the Healthcare Policy Identification REF segment in Loop 2110, if present. This segment uses REF01 = 0K (numeral zero followed by the letter K), with REF02 containing the healthcare policy identifier supplied by the payer. The identifier may help providers locate an applicable coverage policy, but it does not independently explain which medical necessity criteria were not met.

Important: The Healthcare Policy Identification REF segment is not exclusive to CARC 50 and may accompany other adjustment reasons, including CARC 16. Its presence does not automatically indicate a medical necessity denial, and not every CARC 50 adjustment includes this segment. Providers should interpret the policy identifier alongside the reported CARCs, RARCs and other available payer information.

Example 1: Reading a CO-50 Medical Necessity Denial

Consider a hypothetical professional medical claim with one service billed at $180. The payer determines that the service does not meet its applicable medical necessity criteria and reports a full service-level adjustment under CO-50.

Hypothetical educational excerpt using fictional billing information. This is not a complete X12 835 transaction. The procedure code and amounts are illustrative and do not establish that a particular payer would deny this service.

Understanding the Example

The payer reported an adjustment equal to the full submitted charge and issued no payment for the service.

$180 submitted charge - $180 CO-50 adjustment = $0 payer payment

As reported, the $180 CO adjustment is not patient responsibility. The provider should investigate the medical necessity determination and applicable review options rather than simply transferring that amount to the patient.

The remittance identifies the adjustment reason but does not explain exactly which clinical criterion was not met. Additional payer information, supporting medical records and applicable coverage policies may be needed to investigate the denial.

Example 2: A Claim With Both Paid and Denied Services

A medical necessity denial does not necessarily mean every service on a claim was denied.

In this hypothetical example, a professional claim contains two services totaling $300. The payer pays the first service after applying a separate contractual adjustment but denies the second service under CO-50.

Hypothetical educational excerpt showing two differently adjudicated services. The claim identifiers, procedure codes, charges and payment amounts are fictional. No actual medical necessity or coverage determination is implied for either procedure.

How the Amounts Balance

The claim balances as follows:

$300 submitted charges - $30 CO-45 - $150 CO-50 = $120 payer payment

Although the payer denied the second service in full, it paid $120 toward the first service. The medical necessity adjustment therefore applies to one service rather than the entire claim.

Important: A payer may report multiple adjustment reasons within the same claim, and those adjustments may involve different services or amounts. Medical billers should review the actual CLP, SVC and CAS information rather than assuming a CARC 50 adjustment describes every service on the claim.

How RARCs and Healthcare Policy Information Help Explain CARC 50

CARC 50 explains the general adjustment reason, but a payer may provide additional information about its determination.

Remittance Advice Remark Codes (RARCs) can supplement the CARC, while payer correspondence and applicable coverage policies may provide further details about the denial.

For example, RARC N115 identifies a decision based on a Local Coverage Determination (LCD). When reported, it can help a provider identify a relevant Medicare contractor coverage policy.

Illustrative excerpt demonstrating how a service-level RARC may accompany a CARC 50 adjustment. The example does not represent an actual Medicare claim or establish that a specific LCD applies to the procedure shown.

In this example:

• CAS01 = CO identifies the adjustment group
• CAS02 = 50 identifies the medical necessity reason
• CAS03 = 150 identifies the adjustment amount
• LQ01 = HE identifies the healthcare remittance remark code qualifier
• LQ02 = N115 identifies the RARC associated with an LCD-based determination

The LQ segment provides supplemental explanatory information and does not change the financial balancing amounts reported through the payment and adjustment segments.

The official CARC 50 definition also directs readers to the 835 Healthcare Policy Identification REF segment in Loop 2110, if present. Providers should review that information along with any RARCs, payer correspondence and relevant policy documents.

Not every CARC 50 adjustment will include N115 or a Healthcare Policy Identification segment. The available information depends on the specific payer determination and reporting requirements.

Medical Necessity Denial vs. Missing Documentation

One of the most important distinctions when investigating CARC 50 is determining whether the payer actually concluded that the service failed medical necessity criteria or whether it lacked sufficient information to complete its review.

These situations may require different follow-up actions and may be reported using different CARCs.

For example, if the diagnosis was submitted incorrectly and the correct diagnosis is supported by the medical record, the provider may need to review the claim correction process.

However, if the payer reviewed the clinical facts and determined that the service did not meet its coverage criteria, merely resubmitting the same claim with unchanged information is unlikely to resolve the medical necessity determination.

Providers should not change diagnosis codes solely to obtain payment. Any coding correction must accurately reflect the clinical documentation and applicable coding requirements.

Understanding Medicare NCDs and LCDs

For Original Medicare, medical necessity review may involve National Coverage Determinations (NCDs), Local Coverage Determinations (LCDs) and other applicable Medicare coverage requirements.

National Coverage Determinations (NCDs)

An NCD establishes a national Medicare coverage determination for a particular item or service. Medicare contractors must follow applicable national coverage requirements.

When reviewing a CARC 50 adjustment, the provider should determine whether an NCD applies and whether the service meets the relevant coverage conditions for the date of service.

Local Coverage Determinations (LCDs)

LCDs are issued by Medicare Administrative Contractors and address coverage under the applicable reasonable-and-necessary standard within the contractor's jurisdiction.

An LCD may identify clinical indications, limitations or documentation expectations related to a service. Associated billing and coding articles may provide additional operational details.

Providers should review the policy effective for the date of service, including relevant revisions, rather than assuming the current policy text always reflects the requirements that applied when the claim was processed.

Diagnosis Codes and Medical Necessity

Some coverage policies identify diagnosis codes or clinical indications relevant to coverage. However, the presence of a particular ICD-10-CM code on a claim does not automatically prove that medical necessity requirements have been satisfied.

Likewise, a denial involving diagnosis information does not necessarily mean the underlying service was clinically unnecessary. The provider must determine whether the problem concerns coding accuracy, documented clinical circumstances or the payer's coverage determination.

Can Patients Be Billed for Medical Necessity Denials?

Patient financial responsibility depends on the reported adjustment group and the applicable laws, provider agreements, coverage rules and notice requirements.

A correctly reported CO-50 adjustment generally represents an amount that is not the patient's responsibility. The provider should not transfer that amount to the patient simply because the payer did not pay for the service.

A PR-50 adjustment indicates that the payer reported patient responsibility, but that designation does not independently establish whether collection is permitted under all applicable requirements.

For certain Original Medicare services, advance-notice rules may affect whether financial liability can be assigned to the beneficiary when Medicare is expected to deny payment for lack of medical necessity.

Original Medicare and Advance Beneficiary Notices

An Advance Beneficiary Notice of Noncoverage (ABN) is used in certain Original Medicare fee-for-service situations when a provider expects Medicare may deny payment for an item or service that is ordinarily covered but may not be considered reasonable and necessary under the specific circumstances. When a mandatory ABN is required, it generally must be provided before the item or service is furnished so the beneficiary can make an informed decision about receiving the care and potentially accepting financial responsibility.

A valid ABN must satisfy Medicare's applicable timing, content and notice requirements. Simply obtaining a signature does not automatically establish beneficiary liability, and an ABN generally cannot be issued after the service to retroactively transfer responsibility for a denial.

Medicare uses claim modifiers, including GA and GZ, in particular advance-notice circumstances:

For applicable Original Medicare medical necessity denials, the presence or absence of a valid advance notice can affect whether the beneficiary or provider is financially responsible for the denied service. Modifier GA generally indicates that an applicable mandatory ABN was issued, while modifier GZ indicates that the provider expected a reasonable-and-necessary denial but did not issue an ABN. In applicable circumstances, a missing required ABN may result in provider liability rather than beneficiary liability.

Important: Reporting a modifier does not independently establish the final financial responsibility. The applicable Medicare notice requirements, coverage determination, limitation-on-liability rules and resulting remittance information must be considered together. Providers should not assume that every CARC 50 denial has the same patient billing outcome.

Important: These Original Medicare ABN requirements should not be automatically applied to Medicare Advantage, Medicaid or commercial insurance. Each program may have separate notice, coverage and patient billing requirements.

How to Investigate a CARC 50 Medical Necessity Denial

A structured review can help determine whether the denial resulted from incorrect claim information, incomplete clinical support, an applicable coverage restriction or a disputed medical necessity determination.

Step 1: Review the Complete Remittance

Identify the affected claim or service, the adjustment group, CARC 50 amount, any accompanying RARCs and any relevant payer policy identification or denial correspondence.

Determine whether the entire claim was denied or whether other services were processed and paid.

Step 2: Identify the Applicable Coverage Policy

Review the payer's medical necessity criteria for the service and the relevant date of service.

For Original Medicare, determine whether an applicable NCD, LCD or other Medicare coverage requirement governs the service. For other payers, review the relevant medical policy, benefit terms and program requirements.

Step 3: Review the Submitted Claim Information

Compare the reported procedure, diagnosis codes, modifiers, units, dates and other relevant claim details against the available medical record and the payer's denial explanation.

Determine whether the claim contains an actual coding or data error. Do not change diagnosis codes unless the corrected values accurately reflect the clinical documentation.

Step 4: Review the Supporting Medical Documentation

Work with the appropriate clinical staff to determine whether the medical record supports the service under the payer's applicable requirements.

Depending on the service, relevant information may include examination findings, diagnostic results, treatment history, clinical progress and the documented rationale for the service.

Step 5: Determine the Appropriate Resolution Process

If the denial resulted from an actual claim coding error, determine whether a corrected claim is permitted and appropriate.

If the payer has made a medical necessity determination that the provider disputes, investigate the available reconsideration, peer-to-peer review, medical review or formal appeal procedures, as applicable.

The availability and timing of these options depend on the payer, the program and the type of determination.

Step 6: Prepare the Supporting Explanation

When pursuing an appropriate review or appeal, provide the information required by the payer and explain how the documented clinical circumstances satisfy the applicable medical necessity criteria.

Clinical evidence should be tied directly to the disputed requirement rather than relying only on a general statement that the service was necessary.

Step 7: Track the Outcome and Financial Responsibility

Retain relevant submission records, claim identifiers, denial correspondence, supporting documentation and appeal decisions.

Review any subsequent remittance advice to confirm whether the payer upheld, revised or reversed the determination and whether payment or adjustment amounts changed.

Do not reassign a CO adjustment to the patient while a determination is under review unless a properly supported change in adjudication and applicable billing requirements permit the resulting financial treatment.

Corrected Claim vs. Medical Necessity Appeal

Not every medical necessity denial should be handled through a corrected claim. The appropriate response depends on whether the issue involves the submitted claim information or the payer's substantive coverage determination.

Submitting an unchanged claim repeatedly generally does not resolve a medical necessity determination. It may instead create duplicate-claim processing problems.

Additionally, submitting a corrected claim does not necessarily preserve an appeal deadline. Providers should verify the applicable review and filing requirements before selecting a response.

Medicare, Medicaid and Commercial Payer Considerations

Medical necessity review and claim resolution requirements vary by healthcare program and insurer. A standardized CARC does not establish identical coverage policies or appeal procedures across payers.

Original Medicare

Original Medicare may evaluate medical necessity under applicable national coverage requirements, NCDs, LCDs and other Medicare rules.

Providers should review the relevant Medicare Administrative Contractor instructions and the coverage policy applicable to the date of service.

When an appealable Original Medicare initial claim determination is disputed, a first-level redetermination may be available through the Medicare Administrative Contractor. The standard deadline for requesting redetermination is generally 120 days from receipt of the initial determination, subject to applicable rules and exceptions.

Original Medicare beneficiary liability for certain medical necessity denials may also depend on valid ABN and limitation-on-liability requirements.

Medicare Advantage

Medicare Advantage plans must follow applicable Original Medicare National Coverage Determinations (NCDs), Local Coverage Determinations (LCDs) and general Medicare coverage requirements when evaluating basic Medicare benefits. In certain circumstances where Medicare coverage criteria are not fully established, plans may use additional internal coverage criteria consistent with applicable CMS requirements.

When investigating a Medicare Advantage medical necessity denial, providers should determine whether the decision involves an applicable Medicare coverage policy, permitted plan coverage criteria or another coverage requirement. The applicable policy and clinical documentation should be reviewed together with the plan's explanation of the denial.

Depending on the service and determination, medical necessity disputes may involve organization determinations, reconsiderations, provider disputes or other review processes. Providers should follow the applicable plan's instructions and should not assume that Original Medicare fee-for-service appeal or ABN procedures apply unchanged to Medicare Advantage.

Medicaid

State Medicaid fee-for-service programs and Medicaid managed care organizations may apply different clinical coverage standards, documentation requirements and claims review procedures, subject to applicable federal and state requirements.

Providers should verify the coverage requirements, relevant state guidance and permitted provider or member review procedures for the particular Medicaid program.

Commercial Payers

Commercial insurers, including Blue Cross Blue Shield plans, UnitedHealthcare and other health plans, may evaluate medical necessity using plan terms, clinical policies and applicable regulatory requirements.

Some medical necessity determinations may be reconsidered or appealed with additional clinical information. The availability of a peer-to-peer review, the required supporting materials and the applicable deadlines can vary by plan and determination type.

Providers should distinguish a contractual provider dispute from an appeal or review right available to the covered member when those processes differ.

Clearinghouses and Billing Applications

Clearinghouses and billing applications can help route claims, retain acknowledgments, process ERAs and identify reported adjustment codes.

However, an accepted electronic claim does not establish that the billed service meets the payer's medical necessity criteria. A claim can successfully pass electronic validation and subsequently be denied during adjudication.

For EDI developers, the adjustment group, CARC, RARCs and any applicable policy-identification information should be preserved so billing staff can investigate the denial without losing important remittance details.

How to Help Prevent Medical Necessity Denials

Not every medical necessity denial can be prevented, but consistent documentation, coding and coverage review practices can reduce avoidable claim problems.

Recommended practices include:

• Review applicable medical necessity policies before billing services that are subject to coverage criteria
• Use diagnosis and procedure codes that accurately reflect the documented services and clinical circumstances
• Verify relevant coverage policy effective dates and revisions
• Maintain appropriate clinical records supporting the service
• Review frequency, duration and treatment limitations when applicable
• Follow program-specific advance-notice requirements when applicable
• Track medical necessity denials by payer, service and underlying reason
• Retain denial correspondence and supporting records for permitted reviews and appeals

When recurring denials involve a particular service or payer policy, identifying the underlying pattern may help medical billing and clinical teams address avoidable documentation or coverage issues.

Official CARC 50 Definition

X12 maintains the official Claim Adjustment Reason Code list, including CARC 50.

For the current code definition and usage guidance, refer to X12 Claim Adjustment Reason Codes.

CARC 50 should be interpreted together with the applicable adjustment group, any accompanying RARCs, available payer policy references and the complete adjudication information.

Frequently Asked Questions About CARC 50

Key Takeaway

CARC 50 indicates that a healthcare payer has determined a service to be non-covered because it does not meet the payer's applicable medical necessity criteria. When reported with the CO adjustment group, it appears as CO-50.

In the X12 835 version 5010 transaction, CARC 50 is reported through a CAS segment at the claim level in Loop 2100 or the service level in Loop 2110. Additional RARCs or healthcare policy references may help explain the payer's determination.

A medical necessity adjustment does not necessarily mean every service on a claim was denied, and it does not independently determine patient financial responsibility. The reported group code and applicable liability requirements must also be reviewed.

For medical billers and EDI professionals, the appropriate response is to verify the actual denial reason, review the applicable coverage criteria and supporting clinical documentation, and determine whether correction, reconsideration or formal appeal is appropriate.

Most importantly, a medical necessity denial should not be treated as an automatic coding error, documentation failure or patient-billable balance. Each determination requires review of the complete adjudication and applicable payer requirements.

Related Eclaims Articles and Resources

• What Are Claim Adjustment Reason Codes (CARCs)?
• How to Read an X12 835 ERA: Claim Payments, Adjustments & Denials
• What Are Claim Adjustment Group Codes (CO, PR, OA & PI)?
• CARC 16 / CO-16: Missing or Incorrect Claim Information
• Medical Claim Denials: Common Reasons Claims Are Denied
• 277CA Claim Status Lookup

Disclaimer: This article provides general educational information about medical necessity denials, CARC 50, X12 remittance transactions and healthcare claim resolution. It does not replace official X12 implementation guides, clinical coverage policies, medical advice, provider agreements or applicable laws and regulations. Medical necessity and liability requirements vary by payer and may change. Always verify the requirements applicable to the specific service, coverage and date of service.