Key takeaways
- The two letters say who's responsible. CO means you can't bill the client for it. PR means the client owes it.
- Not every code is a denial. CO-45 is the normal discount on an in-network claim, and PR-1, PR-2, and PR-3 are the client's deductible, coinsurance, and copay.
- Fix and resubmit when information was missing or wrong (CO-16, CO-4, CO-11). Appeal when you disagree with a decision (CO-50, CO-197). Don't wait: timely filing (CO-29) is the denial you can't fix.
How to read a denial code
Insurance payments, whether on a paper explanation of benefits (EOB) or an electronic remittance (ERA), explain every unpaid dollar with a pair of codes:
- A group code of two letters says who is responsible for the amount.
- A Claim Adjustment Reason Code (CARC), a number, says why it wasn't paid.
- Often a Remittance Advice Remark Code (RARC), starting with N, M, or MA, adds detail, such as which piece of information was missing.
So “CO-16 with N290” reads as: contractual obligation, claim lacks information, and the missing piece is the rendering provider's identifier. The code lists are standard across insurers and maintained by X12.
The group codes
| Group | Means | Can you bill the client? |
|---|---|---|
| CO | Contractual obligation | No. Under your contract, you absorb it unless the denial is fixed or overturned. |
| PR | Patient responsibility | Yes. This is the client's share: deductible, coinsurance, copay, or a service their plan doesn't cover. |
| OA | Other adjustment | Usually not. Used for things like duplicates and amounts that don't fit the other groups. |
| PI | Payer-initiated reduction | No. The insurer reduced payment for a reason that isn't a contract term or the client's share. |
Codes that aren't really denials
- CO-45: charge exceeds the fee schedule. If you bill $180 and your contract allows $120, the $60 difference appears as CO-45. It's a normal write-off on in-network claims, not something to appeal.
- PR-1, PR-2, PR-3: the client's deductible, coinsurance, and copay. These amounts move to the client's balance. Early in the year, PR-1 is common because deductibles reset.
The denials mental health practices see most
| Code | What it means | What to do |
|---|---|---|
| CO-16 | Claim lacks information or has a billing error. A remark code says what's missing. | Read the remark code, fix the claim, and resubmit it. On Medicare claims, MA130 means there are no appeal rights: send a new, corrected claim. |
| CO-4 | The procedure code doesn't match the modifier used. | Check the modifier, often a telehealth one such as 95, and send a corrected claim. |
| CO-11 | The diagnosis doesn't match the procedure. | Check the diagnosis code and its order on the claim. Some plans won't pay for therapy when the primary diagnosis is a Z code. |
| CO-18 / OA-18 | Exact duplicate claim or service. | Usually the original is still processing or was already paid. Check claim status before you resend anything. |
| CO-22 | Another insurer may be primary (coordination of benefits). | Ask the client about other coverage, and have them update it with the insurer. Then bill the primary plan first. |
| CO-27 | The service was after coverage ended. | Get the client's current insurance. If they have none, the session becomes their balance under your policies. |
| CO-29 | The filing deadline passed. | Rarely fixable. Appeal only with proof you filed on time, such as a clearinghouse acceptance report. |
| CO-50 | Not medically necessary, according to the insurer. | Appeal with the treatment plan and notes showing symptoms, goals, and progress. Look for a 90837 or session-frequency policy behind it. |
| CO-97 | Payment is included in another service already paid. | Common when two codes are billed on the same day, such as 90791 with therapy. Check whether the pair is allowed and whether a modifier applies. |
| CO-109 | Not covered by this insurer. | Usually a wrong payer, such as a Medicaid managed-care plan or a behavioral health carve-out that handles mental health separately. Send it to the right one. |
| CO-197 | Prior authorization or notification was missing. | Check whether the plan required authorization. If you had one, appeal with the authorization number. |
| CO-204 / PR-204 | Service isn't covered under the client's current plan. | If grouped PR, the client may owe it, depending on your financial policy and any notice you gave them. |
A few remark codes that come up often: N30 (client ineligible for the service), M51 (missing or invalid procedure code), and MA04 (a secondary claim was sent without the primary insurer's payment information).
Correct, appeal, or bill the client?
- Correct and resubmit when you sent something wrong or incomplete: a missing identifier, a wrong modifier, a diagnosis mix-up. Use a replacement claim (claim frequency code 7) referencing the original claim number, not a brand-new claim, or it may be denied as a duplicate.
- Appeal when the claim was right and you disagree with the decision: medical necessity, authorization you actually had, a coverage decision. Appeals have their own deadlines, often 60–180 days from the denial.
- Bill the client only for amounts grouped PR, and only under the policy they agreed to.
- Write off the CO amounts you can't overturn, so the client's balance and your books stay accurate.
Watch the deadlines
Every insurer sets a timely filing limit, and many set a separate one for corrected claims. Medicare's is 12 months from the date of service. Commercial plans are often 90 to 180 days. A claim that's rejected before it reaches the insurer, for example by the clearinghouse, hasn't been filed at all. Work denials weekly, oldest first, so a fixable CO-16 doesn't age into an unfixable CO-29.
Where Remitra fits
Remitra reads the codes on every insurance payment and puts each denied line on one list, sorted by age, with the reason in plain English next to the insurer's group and reason codes. CO and PR amounts are kept apart, so the client's balance only includes what they actually owe. Book a demo to see the denial list on sample data, or ask about having our team work denials for you.
Sources
- X12: Claim Adjustment Reason Codes
- X12: Remittance Advice Remark Codes
- CMS MLN booklet: Medicare & Mental Health Coverage (MLN1986542, March 2026)
This article is general information, not legal, coding, or reimbursement advice. Payer rules change and vary by plan and state, so check each payer's current policy. CPT® is a registered trademark of the American Medical Association.