There are approximately 300 active Medicare denial codes. Ten of them account for 70 to 80 percent of your denial volume. Four of them (CO-16, CO-45, CO-50, CO-197) eat most of the recoverable dollars. This is the operator reference for each of those four, plus the appeal templates, the workflow buckets, and the automation opportunities that turn them from a chronic backlog into a solved problem.
1. Medicare denial code lookup
Type a code (for example CO-16, 16, or PR 1) or a word like duplicate or hospice. This table covers the codes Medicare billers see most. For every payer, see the full CARC and RARC codes list. Descriptions are plain-English summaries; check the current X12 and CMS code lists before quoting a code in an appeal.
| Code | What it means | Usual next step | Guide |
|---|---|---|---|
| CO-4 | Procedure code and modifier do not match, or a required modifier is missing | Correct the modifier and send a corrected claim | CO-4 guide |
| CO-11 | Diagnosis is inconsistent with the procedure | Review the coding against the record | CARC list |
| CO-16 | Claim lacks information or has a billing error (read the remark code, often MA130) | Fix the field the remark code names and resubmit | CO-16 guide |
| CO-18 | Exact duplicate claim | Check the original claim status. Do not resubmit | CARC list |
| CO-22 | Another payer may be primary (Medicare Secondary Payer) | Bill the primary payer first, then Medicare | CO-22 guide |
| OA-23 | Shows what the primary payer already paid, on a secondary or crossover claim | Usually informational. Post the remaining patient amount | OA-23 guide |
| CO-24 | Covered under a capitation or managed care plan, often a Medicare Advantage member billed to Original Medicare | Bill the Medicare Advantage plan | CARC list |
| CO-27 | Service after coverage ended | Check eligibility dates and find current coverage | CARC list |
| CO-29 | Time limit for filing has expired (Original Medicare: 12 months from the date of service) | Appeal only with proof of timely filing | CO-29 guide |
| CO-31 | Patient cannot be identified as a Medicare beneficiary | Check the name, date of birth, and Medicare Beneficiary Identifier (MBI) | CARC list |
| CO-45 | Charge is above the Medicare fee schedule amount | Contractual write-off. Confirm the allowed amount is right | CO-45 guide |
| CO-50 | Not medically necessary under an LCD or NCD (remark N115 or N386) | Fix the diagnosis coding, or appeal with documentation | CO-50 guide |
| PR-50 | Not medically necessary, and a valid ABN was signed (GA modifier) | Bill the patient | CO-50 guide |
| CO-96 / PR-96 | Non-covered charge, including statutorily excluded services (GY modifier) | Check the reason, then appeal or bill the patient as allowed | CO-96 guide |
| CO-97 | Bundled into another service already paid (often an NCCI edit) | Check the edit and modifier indicator; correct or appeal | CO-97 guide |
| CO-109 | Sent to the wrong payer or contractor (for example a DME MAC claim sent to the Part B MAC) | Submit to the correct contractor or plan | CO-109 guide |
| CO-119 | Benefit maximum reached for the period | Check benefits; bill secondary or patient if allowed | CARC list |
| CO-151 | Frequency or number of services not supported | Review units and frequency limits; appeal with documentation | CARC list |
| CO-167 | Diagnosis not covered | Review diagnosis coding and the coverage policy | CARC list |
| CO-181 | Procedure code not valid on the date of service | Use a code valid for that date | CARC list |
| CO-185 | Rendering provider not eligible to perform the service billed | Check enrollment and scope | CARC list |
| CO-197 | Required authorization or precertification missing | Find the authorization; if none, appeal where allowed | CO-197 guide |
| CO-204 / PR-204 | Not covered under the patient's current benefit plan | Confirm the plan and coding; bill the patient if allowed | PR-204 guide |
| CO-252 | Attachment or other documentation required | Send the requested documentation | CARC list |
| CO-B7 | Provider not certified or eligible for the service on the date of service | Check Medicare enrollment dates | CARC list |
| CO-B9 | Patient is enrolled in hospice | Check whether the service is related to the terminal condition and bill accordingly | CARC list |
| PR-1 | Deductible | Bill the patient or their supplemental plan | PR-1 guide |
| PR-2 | Coinsurance | Bill the patient or their supplemental plan | PR-1 guide |
| PR-3 | Copayment | Bill the patient or their supplemental plan | PR-1 guide |
No match in this table. Try the full CARC and RARC list.
Related: timely filing limits by payer and days in AR.
2. The four denial codes that eat most of the appeal pool
Every billing operation runs into hundreds of distinct Claim Adjustment Reason Codes (CARCs) in production. In practice, four of them account for the majority of the recoverable pool: CO-16, CO-45, CO-50, and CO-197. Master these four workflows and you have captured most of the appeal ROI available to your team.
Each of the four has a different workflow, a different success rate, a different appeal template, and a different prevention lever. Treating them all the same is the single most common operational failure in denial management. Below is what each actually means, what triggers it, and how to work it.
3. CO-16: Claim/service lacks information
What it means
The claim is missing required information or has a submission/billing error. Payer cannot adjudicate until you fix and resubmit. Almost always technical, not clinical.
Top 5 triggers
- Missing modifier. Most common on E/M with procedure combos (missing 25 modifier), bilateral procedures (missing 50), and split professional/technical (missing 26 or TC).
- Missing referring provider NPI. Required for diagnostics ordered by another provider; missing NPI kills the whole claim.
- Missing prior authorization number. Auth was obtained but the number was never placed on the claim, or was placed in the wrong loop/segment.
- Missing or invalid diagnosis code. ICD-10 truncation, missing 4th/5th/6th character specificity, or unspecified codes where LCD requires specificity.
- Place-of-service mismatch. CPT and POS combo not valid (e.g., 99215 with POS 21 inpatient).
The fix
CO-16 is rarely an appeal. It is a correct-and-resubmit. Pull the RARC (N-code) attached to the denial for the specific error, correct the missing element, resubmit within the payer's timely filing window (typically 90 days from date of service, some payers up to 365 days). Success rate on corrected resubmission: 70 to 85 percent.
4. CO-45: Charge exceeds fee schedule
What it means
The charge exceeds the contracted or legislated fee schedule allowed amount. In most cases this is a routine contractual adjustment, the payer paid at their contracted rate and the excess is a write-off.
When CO-45 is actually a denial
CO-45 becomes an actionable denial in three scenarios:
- Payer applied the wrong fee schedule. Your contract has a specific rate schedule for your specialty or provider type; payer applied a lower one.
- Silent rate change. Payer updated the fee schedule and your contract store is stale, so what looks like an in-contract payment is actually below the contracted rate.
- Coding downgrade disguised as CO-45. Payer downgraded the CPT to a lower-paying code and marked the difference as CO-45. This is a coding dispute, not a contractual adjustment.
The fix
Every CO-45 needs to be reconciled against the contracted rate for that CPT, that payer, that date of service. If the payment matches the contract, close as expected write-off. If the payment is below the contract, file a contract dispute (not a clinical appeal). Success rate on properly documented underpayment disputes: 70 to 90 percent. See our contract underpayment recovery pillar for the full workflow.
5. CO-50: Non-covered because not deemed medically necessary
What it means
The payer reviewed the claim and determined the service was not medically necessary based on their coverage criteria. Requires clinical documentation to overturn.
Top triggers by category
- Frequency limits exceeded. Service billed more frequently than payer's coverage policy allows in a given period.
- Diagnosis not on covered list. The diagnosis code(s) on the claim do not match the LCD/NCD covered indications for that CPT.
- Thin documentation. Note supports the service was performed but does not connect it to a documented clinical rationale.
- Step therapy failure. Payer requires trial of lower-tier treatment before covering the billed service.
The fix
CO-50 requires a true appeal with clinical documentation. Pull the LCD/NCD or payer coverage policy that applies, identify which specific criterion the payer flagged, and write an appeal letter that connects the clinical documentation to that specific criterion. Attach the relevant chart notes, results, and prior treatment history. Success rate: 40 to 60 percent depending on service category and documentation quality.
6. CO-197: Precertification/authorization absent
What it means
Precertification or authorization is absent, invalid, or does not cover the service billed. One of the fastest-growing denial categories under CMS-0057-F.
Three flavors of CO-197 (each with different overturn odds)
- Auth never obtained. Retroactive auth request required. Success rate: 20 to 40 percent. Payer discretion. Best chance is emergent care with documented clinical urgency.
- Auth obtained but expired. Common when auth was for a series (physical therapy, chemotherapy) and clinical extended past the auth window. Success rate: 40 to 60 percent if you can document continued medical necessity.
- Auth obtained but not attached to claim. Auth number was never placed in the correct segment of the 837, or was for a different CPT. Success rate: 80 to 95 percent, this is almost always overturnable with proof of auth.
The fix
Determine the flavor first. If auth was obtained (flavors 2 or 3), documentation is the key: pull the original auth from the payer portal, attach to the appeal, cite the auth number, dates, and CPT range. If auth was never obtained (flavor 1), the appeal argument is clinical urgency or benefit-coverage exception. See our prior authorization automation pillar for the prevention workflow.
7. Same-day triage matrix
Every 835 remittance should be triaged the day it lands. Route each denial to the correct workflow bucket by CARC before anyone touches the substance:
8. Appeal templates by code
CO-16 correction packet
- Original claim number and denial date
- RARC identifying missing element
- Corrected claim (837 with the fix applied)
- Timely filing verification (date of service within window)
CO-45 contract dispute packet
- Claim number, DOS, CPT, paid amount, contracted amount, variance
- Contract page showing correct rate and effective date
- Batch reference if multiple claims share the same underpayment pattern
- Request for corrected remittance with retroactive adjustment
CO-50 clinical appeal packet
- Original claim, denial, and specific LCD/NCD citation
- Chart notes supporting medical necessity
- Diagnostic results and prior treatment history
- Provider narrative connecting documentation to LCD criteria
CO-197 auth appeal packet
- Auth number, date obtained, CPT range, expiration
- Payer portal screenshot proving auth existence
- If auth expired: continued medical necessity documentation
- If never obtained: clinical urgency justification
9. Medicare's five appeal levels and deadlines
Original Medicare (Parts A and B) appeals move through five levels. Each level has its own deadline, counted from when you receive the previous decision. Medicare presumes you received a notice 5 days after the date printed on it.
| Level | Who decides | Deadline to file |
|---|---|---|
| 1. Redetermination | Your Medicare Administrative Contractor (MAC) | 120 days from receiving the initial determination |
| 2. Reconsideration | A Qualified Independent Contractor (QIC) | 180 days from receiving the redetermination |
| 3. Hearing | An Administrative Law Judge at the Office of Medicare Hearings and Appeals | 60 days from receiving the reconsideration |
| 4. Council review | The Medicare Appeals Council | 60 days from receiving the hearing decision |
| 5. Judicial review | Federal district court | 60 days from receiving the Council decision |
Levels 3 and 5 also require a minimum dollar amount in dispute, which CMS adjusts every year, so check the current figure before filing. Most recoverable denials are won or lost at levels 1 and 2, which is why a clean, complete redetermination packet matters so much.
10. ABNs and modifiers: who pays when Medicare says no
For Original Medicare, an Advance Beneficiary Notice of Noncoverage (ABN, form CMS-R-131) and the modifier on the claim decide whether a denial lands on the provider or the patient.
| Modifier | Meaning | If denied |
|---|---|---|
| GA | Valid ABN signed before the service | PR-50: the patient can be billed |
| GZ | Denial expected, no ABN on file | CO-50: the provider is liable |
| GY | Service is statutorily excluded or not a Medicare benefit | Usually patient responsibility |
An ABN only works if it is given before the service, names the specific item or service, and explains why Medicare may not pay. A blanket ABN signed by every patient does not count. More detail in the CO-50 and CO-96 guides.
11. Medicare Advantage works differently
Medicare Advantage plans are private plans with their own rules. Their timely filing limits are set by your contract, their prior authorization requirements are often broader than Original Medicare's, and denials go through the plan's own appeal process rather than your MAC. If a claim for a Medicare Advantage member goes to Original Medicare by mistake, expect CO-24 or CO-109. Check the member's enrollment at eligibility, especially after open enrollment.
12. Prevention playbook
Every recovered denial contains a signal about a fixable upstream problem. Feed the pattern back into prevention or you run the same audit every quarter.
- CO-16 prevention. Front-end claim scrubbing that catches missing modifiers, NPI, auth numbers, and POS mismatches before the claim ships. See our claim scrubbing content for the mechanics.
- CO-45 prevention. Contract store with rates keyed by CPT, payer, and effective date. Automated reconciliation on every 835. See contract underpayment recovery.
- CO-50 prevention. CDI queries during the encounter, not after. Documentation templates that force LCD criteria capture at time of service.
- CO-197 prevention. Prior auth tracking software that flags expiring auths before the DOS. See prior authorization automation.
13. Five metrics to instrument
- Denial rate by CARC. Track the top 10 CARCs by dollar volume monthly.
- First-pass correction rate. Percent of CO-16 denials corrected and resubmitted within 7 days.
- Appeal overturn rate by CARC. Different targets: CO-16 (70%+), CO-45 (70%+), CO-50 (40%+), CO-197 (60%+ weighted average).
- Time-to-appeal. Days from denial receipt to appeal filed. Target: under 14 days.
- Repeat CARC index. Percent of new denials matching a CARC/payer pattern from the previous quarter. Rising means prevention feedback is broken.
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