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CO-A1 Denial Code: Claim or Service Denied (See Remark)

CO-A1 means the claim or service was denied, but the payer used a general code because no more specific CARC fits. X12 requires at least one non-alert remark code with it, and that remark code is where the actual denial reason lives.

Quick facts

Code
CO-A1 (CARC A1)
Status
Active In use since January 1, 1995; last modified November 16, 2022.
Code set
Claim Adjustment Reason Codes (CARC)
Group codes
  • CO (Contractual Obligation): The usual group. The provider is responsible for the denied amount while the reason in the remark code is resolved.
  • PR (Patient Responsibility): Used when the payer assigns the denied amount to the patient. Bill only after confirming the remark code supports patient liability.
  • OA (Other Adjustment): Occasionally used when the denial doesn't assign responsibility to either party.
Official description
Claim/Service denied. At least one Remark Code must be provided (may be comprised of either the NCPDP Reject Reason Code, or Remittance Advice Remark Code that is not an ALERT.) Usage: Use this code only when a more specific Claim Adjustment Reason Code is not available.
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What CO-A1 means

CARC A1 says claim or service denied, and X12 attaches two important rules. First, it requires at least one remark code, either a RARC that isn’t an alert or an NCPDP reject reason code. Second, its usage note says to use A1 only when a more specific CARC is not available. In other words, A1 is a catch-all, and the real explanation lives in the remark code.

That makes CO-A1 hard to trend. Two CO-A1 denials can have completely different causes, one a coding error and another a coverage rule. Reporting on A1 alone hides that difference; reporting on the A1 and remark code together shows it.

Example: a payer denies a service under a program-specific rule that has no dedicated CARC. The remittance shows CO-A1 with a remark pointing to the plan’s benefit documents. The billing team needs to read that remark and the policy, not the CARC, to decide next steps.

Common causes

A1 isn’t tied to one cause. Common patterns include:

  • Program-specific rules that don’t map to a standard CARC.
  • Payer systems that default to A1 when their internal denial reason has no direct match.
  • Pharmacy claims where an NCPDP reject code supplies the detail.
  • Mixed issues where the payer reports one general denial with several remarks.

How to fix it

  1. Read every remark code on the claim and service line.
  2. Look up any policy reference in the 835 Healthcare Policy Identification segment if present.
  3. Call the payer if the remark is missing or unclear, and note the reference number.
  4. Choose the right path based on the actual reason: a corrected claim with resubmission code 7 in box 22 for data errors, an appeal for coverage or necessity decisions, or rebilling another payer when the remark points elsewhere.
  5. Don’t bill the patient unless the payer reports PR.

How to prevent it

  • Categorize A1 denials by remark code so you can see the real drivers. See how to read CARC and RARC codes.
  • Ask frequent A1 payers which rules they map to it and build those into claim scrubbing.
  • Track A1 by payer. An ERA Analyzer can group A1 lines with their remark codes to reveal patterns.

Remark codes that may appear with CO-A1

  • N517 (Resubmit a new claim with the requested information.): Resubmit a new claim with the requested information.
  • N56 (Procedure code billed is not correct/valid for the services billed or the date of service billed.): The procedure code billed isn't correct or valid for the services or date.
  • N130 (Consult plan benefit documents/guidelines for information about restrictions for this service.): Points to plan documents for a coverage restriction.
  • CO-16 (Claim/service lacks information or has submission/billing error(s).): Missing or invalid information; payers should use this instead of A1 when the problem is claim data.
  • CO-96 (Non-covered charge(s).): Non-covered charge, used when the service isn't covered and a remark explains why.
  • CO-252 (An attachment/other documentation is required to adjudicate this claim/service.): An attachment or other documentation is required.
  • PR-204 (This service/equipment/drug is not covered under the patient's current benefit plan): Service not covered under the current benefit plan.

CO-A1 FAQ

Why would a payer use A1 instead of a specific code?

X12 allows A1 only when no more specific CARC describes the reason. Some payers use it for unusual situations or for program-specific rules. The remark code gives the detail.

What if there's no remark code with CO-A1?

X12 requires at least one non-alert remark code or NCPDP reject code. If none is present, call the payer and ask for the specific denial reason before deciding whether to correct or appeal.

Should I appeal or correct a CO-A1 denial?

That depends entirely on the remark code. A data problem calls for a corrected claim; a coverage or medical necessity decision calls for an appeal.

Can I bill the patient for CO-A1?

Not under CO. Only if the payer reports PR and the reason supports patient liability.