Claim denial codes arrive as a message in three parts: a group code that assigns responsibility, a claim adjustment reason code that says why the payment changed, and often a remark code that names the exact detail the reason code left out. This reference covers the codes a practice actually sees, what each one asks you to do, and the deadlines attached to it.
Claim denial codes are three codes in one message
A denial is not a sentence. It is a short message assembled from parts, and each part answers a different question. The group code says who is financially responsible for the adjustment. The claim adjustment reason code says why the claim or the line was paid differently than it was billed. The remark code fills in the detail the reason code was too general to carry.
Chapter 1 of the Medicare Claims Processing Manual describes the combination plainly: Medicare, like other health payers, uses the codes in combination to create one message, and a remark code is optional unless the reason code itself requires one. Read them in that order. Starting with the reason code is how a front desk problem ends up assigned to a coder.
| Part | The question it answers | Where it sits in the 835 |
|---|---|---|
| Group code | Who owes the money | CAS segment, beside the reason code |
| Claim adjustment reason code | Why the payment changed | CAS segment, one reason per adjustment amount |
| Remittance advice remark code | What detail the reason code left out | LQ segment for a line, MOA or MIA for a whole claim |
| Provider level adjustment code | Adjustments taken at the provider level | PLB segment |
CMS notes that the current 835 structure allows one reason code to explain any single adjustment amount. A claim with three separate problems does not arrive as one line carrying three reasons. It arrives as three lines, or as a claim level adjustment with the detail pushed into remark codes. Sort by line and by dollars before you assign the work.
The group code decides who pays
The group code is the fastest triage signal on a remittance because it settles responsibility before you read the reason. Medicare limits its own remittances to three group codes, defined in Chapter 22 of the Medicare Claims Processing Manual. The 835 standard carries two more that commercial payers use.
| Group code | What it means | What you do with it |
|---|---|---|
| CO | Contractual obligation. A contract or a regulation caused the adjustment, and CMS describes these as a write off for the provider rather than an amount billable to the patient. | Do not bill the patient. Read the reason code, because some CO adjustments are appealable and some are not. |
| PR | Patient responsibility. The amount may be billed to the patient or the insured, and deductible and copay adjustments usually land here. | Send it to patient statements inside the billing cycle, not into the denial queue. |
| OA | Other adjustment. Used when no other group code applies. | Read the reason code. OA often flags a duplicate or a coordination of benefits adjustment. |
| PI | Payer initiated reduction. The payer reduced the payment on its own initiative, without a claim error on your side. | Commercial payers use this one. Ask for the policy that allowed the reduction. |
| CR | Corrections and reversal. The standard marks this value as not to be used with version 005010 and later. | You should not see it on a current 835. If you do, the transmitter is sending an old format. |
Two rows in that table cause most of the avoidable work. CO-45 is a contractual write off: the charge exceeded the fee schedule or the contracted rate, so the difference was never payable and no appeal will recover a dollar of it. Deductible, coinsurance and copay amounts arrive under PR, and they belong in your patient statement process within days rather than in a queue where they age past the point of collection.
Two remark codes exist only to tell you whether an appeal is available. N210 states that you may appeal the decision. N211 states that you may not. MA130 goes further, saying that no appeal rights are afforded because the claim is unprocessable, and asking for a new claim with corrected information. Filing against MA130 is work that cannot pay.
The reason codes behind most denial volume
The reason code set is maintained by the Claim Adjustment Status and Reason Code Maintenance Committee, and a wording cannot change without that committee’s approval. That is why a code means the same thing at every payer. The current X12 list carries roughly 400 codes, most of which a given practice never sees. The list is updated three times a year, and CMS tells its contractors to use only the most current valid codes.
| Code | What the payer is saying | What is usually wrong | Who owns the fix |
|---|---|---|---|
| CO-16 | Claim or service lacks information, or carries a submission or billing error | A required field is missing, wrong or inconsistent, and the remark code names it | Front end |
| CO-22 | This care may be covered by another payer under coordination of benefits | Insurance was not verified, or the coordination of benefits order is wrong | Front end |
| CO-27 | Expenses were incurred after the coverage terminated | The plan ended before the date of service and nobody checked | Front end |
| CO-29 | The time limit for filing has expired | The claim went out after the payer’s window closed | Billing operations |
| CO-109 | Not covered by this payer, and the claim belongs with a different payer | Wrong payer on the claim, with a remark code naming the right one | Front end |
| CO-151 | The information submitted does not support this many or this frequency of services | Units or visits exceed what the documentation shows | Coding and clinical |
| CO-18 | Exact duplicate claim or service | A true duplicate, or a resubmission the payer read as one | Billing operations |
| CO-185 | The rendering provider is not eligible to perform the billed service | An enrollment, taxonomy or credentialing mismatch at the payer | Credentialing |
Four of those are administrative and four are clinical, and mixing them into one worklist is why a denial meeting drifts. A CO-185 is a credentialing problem, which means an enrollment or taxonomy correction tracked through medical credentialing instead of an appeal queue. A CO-29 almost never reverses on its merits: the redetermination window can be extended for good cause under 42 CFR 405.942(b), but the request must be in writing and explain the lateness.
| Code | What the payer is saying | What is usually wrong | Who owns the fix |
|---|---|---|---|
| CO-197 | Precertification, authorization, notification or pre-treatment is absent | No authorization, an expired one, or one that does not match the billed service | Front end and clinical |
| CO-50 | The services are not deemed medically necessary by the payer | The record did not establish necessity against the payer’s own criteria | Coding and clinical |
| CO-97 | The benefit is included in the payment for another service already adjudicated | The payer bundled one procedure into another | Coding |
| CO-234 | This procedure is not paid separately | A bundle that needs a modifier, or a service that genuinely is included | Coding |
| CO-236 | The procedure or modifier combination is not compatible with another on the same day | A National Correct Coding Initiative edit, or a modifier used to force payment | Coding |
| CO-11 | The diagnosis is inconsistent with the procedure | The diagnosis on the claim does not support the code billed | Coding |
| CO-4 | The procedure code is inconsistent with the modifier used | Wrong modifier, or a modifier the code does not allow | Coding |
| CO-8 | The procedure code is inconsistent with the provider type or specialty | A taxonomy or specialty mismatch carried on the claim | Credentialing and coding |
| CO-5 | The procedure code or type of bill is inconsistent with the place of service | Place of service does not match where the service happened | Front end and coding |
| CO-9 | The diagnosis is inconsistent with the patient’s age | The code chosen does not fit the patient in front of the provider | Coding |
| CO-10 | The diagnosis is inconsistent with the patient’s gender | The code chosen does not fit the patient in front of the provider | Coding |
| CO-55 | The payer deems the procedure, treatment or drug experimental or investigational | Payer policy excludes the service, or the appeal never addressed the policy | Clinical and appeals |
| CO-204 | The service is not covered under the patient’s current benefit plan | A benefit exclusion rather than a coding mistake | Front end |
| CO-96 | Non covered charge, and a remark code must accompany it | A benefit exclusion, with the reason left to the remark code | Front end |
| CO-119 | The benefit maximum for this period or occurrence has been reached | The plan limit is exhausted, so the balance belongs to the patient | Front end and patient accounts |
| CO-272 | Coverage or program guidelines were not met | A payer program rule, often tied to frequency or screening | Front end |
| CO-288 | Referral absent | No referral on file where the plan requires one | Front end |
One detail changes how a coder reads this table. Several of these codes appear alone but cannot be worked alone. CMS requires at least one remark code beside CO-16, CO-96, CO-234 and CO-252, because the reason code on its own does not say what is missing. When one of those arrives bare, the payer owes you detail before you owe the payer a correction.
The remark codes worth memorizing
Remark codes are maintained by CMS, and any health plan may use them where they apply. Two kinds exist. Supplemental codes explain an adjustment and imply an action, usually naming the exact field or document to fix. Informational codes begin with the word Alert and stand alone with no reason code attached, because they describe remittance processing rather than a denial.
| Code | What it tells you | The fix |
|---|---|---|
| M76 | Diagnosis or condition is missing, incomplete or invalid | Correct the diagnosis and submit a new claim |
| M79 | The charge is missing, incomplete or invalid | Correct the charge on the line |
| M62 | The treatment authorization code is missing, incomplete or invalid | Add the authorization number, or request a retroactive one |
| N265 | Ordering provider identifier is missing, incomplete or invalid | Add the ordering provider identifier the payer expects |
| N286 | Referring provider identifier is missing, incomplete or invalid | Add the referring provider identifier |
| N290 | Rendering provider identifier is missing, incomplete or invalid | Add the rendering provider identifier |
| N822 | A procedure modifier is missing | Add the modifier the documentation supports |
| N823 | A procedure modifier is incomplete or invalid | Correct the modifier |
| N56 | The procedure code is not correct or valid for the service or the date billed | Check the code against the date of service in effect |
| N54 | Claim information is inconsistent with the pre-certified or authorized services | Match the claim to the authorization on file |
| N362 | Days or units exceed the payer’s acceptable maximum | Reduce the units to what the documentation supports |
| N130 | Consult the plan benefit documents for restrictions on this service | Read the benefit, then decide who owes the balance |
| N4 | The prior insurance carrier explanation of benefits is missing or invalid | Get the primary payer’s explanation of benefits and resubmit |
| N479 | The coordination of benefits or Medicare secondary payer explanation of benefits is missing | Obtain the document that establishes the primary payer |
| MA04 | Secondary payment cannot be considered without the primary payer’s identity or payment information | Submit the primary payer’s payment detail |
| N20 | The service is not payable with another service on the same date | Compare the lines and the modifiers |
| M15 | Separately billed services are bundled as components of the same procedure | Check whether the edit allows a modifier |
| N657 | The service should be billed with the appropriate code | Correct the code to the service provided |
| N674 | Not covered unless a prerequisite procedure or service was provided | Send the record that shows the prerequisite |
| N706 | Documentation is missing | Send the specific record the payer asked for |
| N347 | Payment has already been made to another provider for the same service | Check for a duplicate or a purchased service |
| N522 | Duplicate of a claim processed, or to be processed, as a crossover claim | Do not resubmit. Confirm the crossover |
| N179 | Additional information has been requested from the member | Wait for the member response, then track the reconsideration |
| MA130 | The claim is unprocessable, and no appeal rights are afforded | Correct it and submit a new claim |
| N210 | Alert: you may appeal this decision | Appeal inside the payer’s window |
| N211 | Alert: you may not appeal this decision | Stop, and work the cause the reason code names |
When a payer sends CO-16 beside N286, the denial is not a mystery. It is a missing referring provider identifier. Fix the field, resubmit, then put a rule into charge review so the field cannot go out blank again. A practice that works the reason code without the remark code fixes nothing and rebills the same error.
Documentation codes carry the largest balances
Codes 16, 11, 50, 96 and 252 all point at the record rather than at the payer. That matters because documentation is where Medicare loses the most money. MedPAC’s June 2026 report to Congress found that insufficient or missing documentation accounted for 68 percent of fee for service improper payments in fiscal 2025.
Medicare improper payment rate by claim type, fiscal 2025
Source: MedPAC, Report to the Congress, June 2026, Chapter 3, citing CMS Comprehensive Error Rate Testing data.
The CMS fact sheet for fiscal 2025 puts the Medicare fee for service improper payment rate at 6.55 percent, or $28.83 billion, down from 7.66 percent a year earlier and below the 10 percent compliance threshold for a ninth straight year. An improper payment rate is not a denial rate. It measures claims that should not have been paid as submitted, including underpayments and claims with no documentation to review, and it is the number post-payment auditors work from.
The response is the same for every code in that family. The note has to establish the element the payer’s policy asks about: severity, a failed conservative treatment, the functional limitation, the order, or the face to face encounter. If the record cannot carry that element, no modifier recovers the claim and no appeal letter wins. That is why medical coding and the clinical note, not the appeal desk, are where these codes get fixed.
The deadlines that arrive with the denial
Medicare allows one calendar year from the date of service for a claim to be filed, under 42 CFR 424.44. Once a determination has been made, the clock changes shape. A redetermination request has to be filed within 120 calendar days of receiving the notice, and the regulation presumes the notice was received five calendar days after its date unless evidence shows otherwise.
Days allowed to file, Medicare fee for service
Sources: 42 CFR 424.44 for the claim filing limit; 42 CFR 405.942 and CMS Medicare Claims Processing Manual Chapter 29 for the appeal levels.
| Level | Who decides it | Time to file the request |
|---|---|---|
| Redetermination | Medicare Administrative Contractor | 120 days from receipt of the initial determination |
| Reconsideration | Qualified Independent Contractor | 180 days from receipt of the redetermination |
| Administrative Law Judge hearing | HHS Office of Medicare Hearings and Appeals | 60 days from receipt of the reconsideration |
| Departmental Appeals Board review | Appeals Council | 60 days from receipt of the ALJ decision |
| Federal court review | Federal district court | 60 days from receipt of the Appeals Council decision |
Commercial payers set their own windows, and the only reliable source is the payer’s provider manual and your contract, not a code sheet handed out at onboarding. What generalizes is the sequence. Post the remittance, note the deadline on the day the denial lands, and work backward with an internal buffer. A denial that goes untouched for a week is already competing with its own deadline.
What changes for out of network claims in 2027
CMS published guidance on 17 July 2026 implementing the Federal Independent Dispute Resolution Operations final rules, which appeared at 91 FR 33900 on 4 June 2026. For items and services furnished on or after 1 January 2027, plans and issuers must use nine specified remark codes on remittance advice sent to providers with no contractual relationship with the plan, so the provider can tell whether the payment is subject to the No Surprises Act and whether a dispute is eligible for the federal independent dispute resolution process.
| Code | The circumstance it describes | Federal dispute resolution path |
|---|---|---|
| N871 | Initial payment calculated under a specified state law | Not eligible. The state process applies |
| N877 | Initial payment reflects the lesser of the qualifying payment amount or the billed charge | Eligible after the open negotiation period |
| N876 | Notice of denial of payment, with the amount based on the qualifying payment amount or the billed charge | Eligible after the open negotiation period |
| N944 | Notice of denial of payment, with the amount based on a specified state law or an all payer model agreement | Not eligible. The state process applies |
| N872 | Final payment calculated under a specified state law | Not eligible |
| N873 | Final payment calculated under an all payer model agreement | Not eligible. The state process applies |
| N874 | Final payment reached through open negotiation | Not eligible |
| N875 | Final payment equal to the certified dispute resolution entity’s determination | Not eligible |
| N943 | The item or service is not subject to the surprise billing provisions | Not eligible |
The code descriptions take effect on 1 November 2026. On an electronic remittance the code belongs in the LQ segment within loop 2110, once per line item, and on a paper remittance it has to sit with the line it explains. If your out of network volume runs through the disputes process, out of network billing and the federal independent dispute resolution process is covered in more depth, and the codes above are how you will tell which pile a claim belongs in.
Keeping the code map current
Both code lists are refreshed on or around the first of March, July and November, and the remark code list moves at the same time as the reason code list. CMS requires its contractors to use only the current valid codes, which means a mapping built two years ago is a mapping built on retired codes. The failure is quiet: a stale code still maps to a bucket in your denial report, and the bucket is now wrong.
- Read every unfamiliar code on the live X12 lists rather than a clearinghouse crosswalk, and check the row for a stop date
- Map denials by owner as well as by code, so a front end failure cannot sit in a coding queue for a month
- Note the date each denial was posted, because the redetermination clock runs from receipt of the notice
- Retest your top ten codes after each March, July and November update, since a wording can change without a code changing
- Keep payer-specific crosswalks separate from the national code list, and date them, because payers reuse codes for their own purposes
One retired code does more damage than ten unfamiliar ones. CO-15, the old authorization code, was deactivated on 1 May 2018, and it still turns up in denial analyses as though it were current. A denial dashboard reporting CO-15 volume is describing remittances nobody has received for years. If the work above is happening in a spreadsheet, the denial workflow behind these codes shows how the codes turn into a monthly process, and denial management is how we run it inside a practice.
Questions practices ask us about this
What is the difference between a CARC and a RARC?
A CARC, the claim adjustment reason code, explains why a claim or a service line was paid differently than it was billed, and it is required with every adjustment. A RARC, the remittance advice remark code, adds the detail the reason code could not carry, such as the exact provider identifier that was missing. The X12 lists carry roughly 400 reason codes and more than 1,200 remark codes, including the deactivated ones kept visible for old remittances.
What does CO-16 mean and how do I clear it?
CARC 16 means the claim or service lacks information, or has a submission or billing error. By definition it has to arrive with at least one remark code naming the missing or wrong element. Find that remark code first, correct the field it names, and resubmit. A bare CO-16 with no remark code is a payer error you cannot work blind, so call the payer and ask for the detail before you touch the claim.
Is a contractual obligation adjustment appealable?
Usually not, and the answer depends on the reason code rather than the group code. CO-45 means the charge exceeded the fee schedule or the contracted rate, so the difference was never payable and no appeal recovers it. Other CO codes describe real denials, including missing authorization and medical necessity, and those can be appealed. Read the reason code and the remark code before you decide which pile the claim lands in.
How long do I have to file a Medicare redetermination?
120 calendar days from the date you receive the notice of the initial determination, under 42 CFR 405.942. The regulation presumes receipt five calendar days after the date on the notice unless there is evidence to the contrary. If the window has already closed, the contractor may extend it for good cause, but the request has to be in writing, state why it was late, and include the evidence supporting the extension.
What does a deactivated code mean on my remittance?
A deactivated code has been retired from the reason code set, and the current list keeps it visible with a stop date so that older remittances stay readable. CO-15, the old authorization code, was deactivated on 1 May 2018. A payer still reporting CO-15 on a current claim is running a mapping that predates the retirement. Check the status of the row before you build any rule on a code.
Why does one denial show more than one reason code?
Because a claim is made of lines, and each line can be adjusted for its own reason. Within a single adjustment amount the 835 permits one reason code only, so a claim with several distinct problems arrives as several lines, or as a claim level adjustment with the detail carried in remark codes. Sort the denial by line and by dollars before you assign it to a person.
Do the remark codes on my remittances change in 2027?
For out of network claims, yes. CMS guidance published on 17 July 2026 requires nine specific remark codes on remittance advice sent to providers with no contract with the plan, for items and services furnished on or after 1 January 2027. The code descriptions take effect on 1 November 2026. Those codes are how you tell whether a payment dispute belongs in the federal dispute resolution process.
Put the code work on a schedule
We read the remittance, sort denials by group code and by owner, and work appeals inside the filing windows. Denial management sits on top of full cycle medical billing and medical coding.
Book a free billing review- X12, Claim Adjustment Reason Codes (current list). x12.org/codes/claim-adjustment-reason-codes
- X12, Remittance Advice Remark Codes (current list). x12.org/codes/remittance-advice-remark-codes
- CMS, Medicare Claims Processing Manual, Chapter 1, section 01.1, Remittance Advice Coding Used in this Manual. cms.gov/files/document/clm104c01.pdf
- CMS, Medicare Claims Processing Manual, Chapter 22, Remittance Advice. cms.gov/regulations-and-guidance/guidance/manuals/downloads/clm104c22.pdf
- CMS, Medicare Claims Processing Manual, Chapter 29, Appeals of Claims Decisions. cms.gov/regulations-and-guidance/guidance/manuals/downloads/clm104c29.pdf
- 42 CFR 405.942, Time frame for filing a request for a redetermination. ecfr.gov/current/title-42/section-405.942
- 42 CFR 424.44, Time limits for filing claims. ecfr.gov/current/title-42/section-424.44
- CMS, Guidance on Required Remittance Advice Remark Codes Related to the No Surprises Act, 17 July 2026. cms.gov/cciio/programs-and-initiatives/other-insurance-protections/caa-nsa-rarc-codes.pdf
- CMS, Fiscal Year 2025 Improper Payments Fact Sheet, 15 January 2026. cms.gov/newsroom/fact-sheets/fiscal-year-2025-improper-payments-fact-sheet
- MedPAC, Report to the Congress, June 2026, Chapter 3, Medicare payment operations and their role in identifying improper payments. medpac.gov/wp-content/uploads/2026/06/Jun26_Ch3_MedPAC_Report_To_Congress_SEC.pdf
