Insurance Denial Codes, Explained in Plain English

A remittance tells you two things about every adjustment: a group code that says who owns the balance, and a reason code that says why the payer did what it did. “CO-50” is group code CO plus reason code 50. This page is the plain-English version of the codes a small practice actually sees, with what to do about each one.

This is general information, not billing or legal advice. Your payer contract and the client's plan documents control, and they vary. Every code below is paraphrased from the official lists kept by X12, the standards body behind the 835 electronic remittance — which is why you sometimes see these called X12 denial codes.¹

Start With the Group Code

The two letters in front matter more than the number, because they decide whether you can send the client a bill. X12 defines four:²

• CO — Contractual Obligation. The adjustment comes out of your contract with the payer. In Medicare this assigns the amount to the provider, and only PR amounts can be billed to the beneficiary.³ On commercial plans, your contract decides, and most network agreements say you write it off.
• PR — Patient Responsibility. The amount belongs to the client: deductible, coinsurance, copay, or a service their plan doesn't cover.
• PI — Payor Initiated Reduction. The payer decided to reduce payment for a reason it considers its own, not a contract term.
• OA — Other Adjustment. Used when neither CO nor PR fits, most often when another payer has already processed the claim.

The Money-Split Codes

These are not denials. They say how the plan divided a covered charge.

• PR-1 — Deductible. The client hasn't met their deductible yet. Bill the client.
• PR-2 — Coinsurance. The client's percentage share. Bill the client.
• PR-3 — Copay. The flat amount per visit. Bill the client, and check it matches what you collected.
• CO-45 — Charge exceeds the fee schedule or contracted rate. The difference between your fee and the allowed amount. On a network claim this is the write-off, not a bill.
• CO-97 — Already included in another service's payment. The payer considers this service bundled into one it has already paid. Check the payer's bundling rules before appealing.
• CO-144 — Incentive adjustment. A contract term, such as a preferred-product or quality incentive, moving the amount rather than denying the service.

Bundling and Coding-Edit Codes

These say the service was fine but shouldn't have been billed separately, or not alongside what else you billed that day.

• CO-234 — Not paid separately. The payer pays for it inside another code. A remark code comes with it and names which.
• CO-231 — Mutually exclusive procedures. Two codes that can't both be done in the same day or setting. One of them has to go, or you need a modifier the payer accepts.
• CO-236 — Not compatible under the NCCI edits. The National Correct Coding Initiative says this procedure or modifier combination doesn't go together. Look the pair up in the NCCI tables before resubmitting, because guessing at a modifier here is how practices end up in an audit.

Coverage Codes: The Plan Doesn't Pay for This

• PR-204 — Not covered under the client's current plan. The single most looked-up code. It isn't a judgment about your work; the benefit simply isn't in the plan. Verify benefits, and if the client wants to continue, you need a private-pay agreement signed before the next session.
• CO-96 / PR-96 — Non-covered charge. Broader than 204. X12 requires a remark code with it, so find the remark code that names what was excluded.
• CO-109 — Not covered by this payer; send it to the right one. Usually the wrong plan, a terminated plan, or a Medicare Advantage member billed to traditional Medicare. Re-verify eligibility and resubmit to the correct payer.
• PR-26 — Expenses incurred before coverage started and PR-27 — after coverage ended. A date problem. Check the effective dates, because clients often don't know their plan lapsed.
• CO-119 — Benefit maximum reached for this period. Visit limits. Worth a parity question if the limit applies to mental health but not to comparable medical care.
• CO-272 — Coverage or program guidelines not met. Vague by design. Ask the payer which guideline, in writing.
• CO-167 — Diagnosis not covered. Check whether the plan excludes that diagnosis, or whether the claim carried the wrong one.
• CO-49 — Routine or preventive service. The plan treats it as a screening or routine exam, which many plans exclude. Rare in psychotherapy, common on screening codes.
• CO-203 — Discontinued or reduced service. The payer thinks less was delivered than billed. Check the units and any modifier for a shortened session.

Eligibility Codes: We Don't Have This Person

• PR-31 — Patient cannot be identified as our insured. Nearly always a data problem: a transposed member ID, a maiden name, a dependent billed under the wrong subscriber. Fix it and resubmit rather than appeal.
• CO-32 — Not an eligible dependent and CO-33 — no dependent coverage. The plan covers the subscriber but not this person. Re-verify before the next session.

Clinical-Judgment Codes: They Read It and Said No

• CO-50 — Not deemed a medical necessity by the payer. This is a decision about your clinical reasoning, and a corrected claim will not change it. Request the specific criteria the plan applied, then answer them point by point. This is the code most worth appealing.
• CO-151 — The information submitted doesn't support this many services. Frequency. Common when a client is seen twice a week, or for long sessions. Documentation of necessity is the answer.
• CO-40 — Doesn't qualify as emergent or urgent. Rare in outpatient behavioral health, and usually a place-of-service or coding issue instead.

Paperwork Codes: Fix It, Don't Appeal It

• CO-16 — The claim lacks information or has a billing error. X12 requires at least one remark code with it, and that remark code is the actual answer. M51 means a missing or invalid procedure code.⁴ An appeal here wastes a deadline; correct and resubmit.
• PI-252 — An attachment or documentation is needed to process the claim. Send what they ask for. Note that this arrives as PI or CO, which means it isn't the client's balance.
• PR-227 — Information requested from the client wasn't provided. The payer asked your client something — often a coordination-of-benefits or accident questionnaire — and got nothing. Tell the client, because only they can clear it.
• CO-4, CO-5, CO-11 — The code doesn't match the modifier, the place of service or the diagnosis. Coding problems. Check telehealth modifiers and place-of-service codes first; they cause most of these in a therapy practice.
• OA-18 — Exact duplicate. Often a resubmission sent before the first claim finished processing. Check status before you send again.
• CO-226 / PI-226 — Information requested from you wasn't provided, or came late. The provider-side twin of 227. Somewhere there is a letter asking you for records or a credentialing document.
• CO-222 — Exceeds the contracted maximum units for this provider. Note the wording: this one is about your contract, not the client's benefits, so it isn't patient specific.
• CO-29 — Filing deadline has passed. Usually final. If you have proof of timely filing, that proof is the appeal.
• CO-A1 / PR-A1 — Claim denied, see the remark code. A1 carries no meaning on its own. The remark code beside it is the entire message.

Authorization Codes

• CO-197 — Precertification or authorization absent. No auth on file for a service that needed one. Some payers grant retroactive authorization within a short window, so call before you write it off.
• CO-198 — Authorization exceeded. You had an auth, and used more sessions than it covered. Request additional units with clinical justification.
• CO-39 — Denied at the time authorization was requested. The auth itself was refused. That refusal, not the claim, is what you appeal.
• PR-288 — Referral absent. Different from authorization: the plan wanted a referral from another provider, usually a primary care physician on an HMO.
• CO-B7 — Provider not certified or eligible for this service on this date. Credentialing. Check your effective date with that payer, which is a common surprise for newly credentialed clinicians.

These four are what people mean by a “no auth” denial. Which one you got decides the fix: 197 means get the authorization, 198 means ask for more units, 39 means appeal the refusal itself, and 288 means chase the referral.

Other-Payer Codes

• OA-23 — The impact of a prior payer's decision. Normal on secondary claims: it shows what the first payer already did.
• OA-136 — Failure to follow the prior payer's coverage rules. The secondary plan is refusing because something went wrong at the primary. Fix the primary claim first.
• CO-22 — May be covered by another payer under coordination of benefits. The plan thinks someone else is primary. Usually the client needs to update their COB with the plan.
• CO-24 — Covered under a capitation agreement. Payment flows through a managed care arrangement rather than fee-for-service.
• CO-19 — Work-related injury, so the workers' compensation carrier owns it, and CO-21 — no-fault carrier. The health plan is pointing at a different kind of insurance entirely. Bill that carrier, not the client.

Remark Codes: The Second Line on the Remittance

Reason codes say what happened. Remark codes — the ones starting with M, N or MA — say which detail caused it, and X12 maintains both lists.⁴ Several reason codes are meaningless without the remark beside them: 16, 96, 234, 226 and A1 all require one.

• M51 — Missing, incomplete or invalid procedure code. A CPT problem. Correct and resubmit.
• M62 — Missing, incomplete or invalid treatment authorization code. The auth exists but the number on the claim is wrong or absent.
• M76 — Missing, incomplete or invalid diagnosis. Check the ICD-10 code and its specificity.
• MA130 — Unprocessable, with no appeal rights. Submit a corrected claim; an appeal here achieves nothing.
• N174 — Not a covered service, but the client's liability is limited to the amounts shown under group PR. Read the PR lines before billing anything.
• N30 — Client ineligible for this service. Eligibility, not clinical judgment.
• N56 — The procedure code isn't correct for the service or the date billed. Often a telehealth or place-of-service mismatch.
• N19 — Incidental to the primary procedure. Bundling, in remark form.
• N362 — Units exceed the acceptable maximum. Pairs with 151 or 222.
• N130 — Consult the plan documents for restrictions. A pointer, not an explanation. Ask the payer which restriction applies.
• N115 — Decided under a Local Coverage Determination. Medicare. The LCD is published, so you can read the exact criteria you're being measured against.
• MA04 — Secondary claim can't be considered without the primary payer's information. The primary's remittance never made it. Attach it and resubmit.

The Three-Question Version

Most remittances come down to this, in order:

• Who owns the balance? The group code answers that. PR means the client. CO and PI mean it's yours, unless your contract says otherwise.
• Is this a paperwork problem or a decision? 16, 4, 5, 11, 31, 252 and 18 are paperwork — correct and resubmit. 50, 151, 204 and 197 are decisions — appeal them, with the plan's own criteria.
• What's the deadline? Filing limits and appeal windows both run, and they are different numbers. Write both on the claim the day it arrives.

The workflow for actually answering one — corrected claim versus appeal, what to quote, and the deadlines that apply — is in how to read and answer an insurance denial.

A Note on Reading These With AI

An explanation of benefits carries a client's name, member ID and dates of service. That is protected health information, and pasting it into ChatGPT or Claude sends it to a company you have no business associate agreement with — the point covered in can therapists use ChatGPT under HIPAA.

Hey Eduardo reads the letter on your own Mac instead. Highlight the paragraph, press the shortcut, and ask what the code means and what the deadline is. In local mode nothing is uploaded, so the question of what a vendor keeps never comes up.

Related: How to answer an insurance denial · Panel contracts: what to check first · Hey Eduardo for therapists · Find the deadline, then answer it