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CO-51 Denial Code: Pre-Existing Condition Exclusion

CO-51 means the payer denied the service as treatment for a pre-existing condition that the plan excludes. Most ACA-compliant health plans can no longer apply pre-existing condition exclusions, so check the plan type before accepting it.

Quick facts

Code
CO-51 (CARC 51)
Status
Active In use since January 1, 1995; last modified July 1, 2017.
Code set
Claim Adjustment Reason Codes (CARC)
Group codes
  • CO (Contractual Obligation): Under CO the provider can't bill the patient; the denial must be challenged or written off.
  • PR (Patient Responsibility): Used when the plan's exclusion makes the patient responsible. If the plan can legally apply the exclusion, the patient may be billed.
Official description
These are non-covered services because this is a pre-existing condition. Usage: Refer to the 835 Healthcare Policy Identification Segment (loop 2110 Service Payment Information REF), if present.
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What CO-51 means

CARC 51 says these are non-covered services because this is a pre-existing condition. The payer believes the condition being treated existed before the patient’s coverage began, and its plan excludes coverage for it, at least for a period of time.

This code is much less common today than it once was. The Affordable Care Act prohibits pre-existing condition exclusions for most individual and group health plans. When you see CO-51, the first question is whether the plan is one that can still apply such an exclusion.

The usage note points to the 835 Healthcare Policy Identification segment for the payer’s policy reference.

Where CO-51 still appears

  • Short-term or limited-duration plans that aren’t subject to ACA market rules.
  • Some grandfathered individual policies.
  • Supplemental, indemnity, or specialty coverage, such as certain accident or critical illness policies.
  • International or travel medical plans.
  • Errors where a payer applies an exclusion it isn’t allowed to use.

Common causes

  • Plan type allows exclusions and the condition was diagnosed or treated before coverage started.
  • Payer investigating prior treatment (N204) and records weren’t sent.
  • Misapplied exclusion on an ACA-compliant plan.
  • Diagnosis coding that suggests a long-standing condition when the problem is actually new.

How to fix it

  1. Identify the plan type from the eligibility response or the patient’s plan documents.
  2. If the plan is ACA-compliant, request reprocessing or appeal, noting that the exclusion doesn’t apply.
  3. If the plan can apply exclusions, confirm the condition’s onset date and the plan’s look-back and exclusion periods. Send records that show a new condition or that the exclusion period has passed.
  4. Respond to record requests quickly when the payer is still investigating.
  5. Correct coding if the diagnosis was inaccurate, using resubmission code 7 in box 22.
  6. Bill the patient only if the payer reports PR and the exclusion is valid.

How to prevent it

  • Identify short-term and non-ACA plans at verification and ask about exclusions.
  • Tell patients on such plans that treatment for existing conditions may not be covered.
  • Keep onset dates accurate in documentation.
  • Track payer record requests so they’re answered on time. See eligibility and COB denials.

Remark codes that may appear with CO-51

  • N204 (Services under review for possible pre-existing condition.): Services are under review for a possible pre-existing condition; the payer wants medical records for the prior 12 months.
  • N130 (Consult plan benefit documents/guidelines for information about restrictions for this service.): Directs you to the plan's benefit documents that describe the exclusion.
  • PR-204 (This service/equipment/drug is not covered under the patient's current benefit plan): The service isn't covered under the patient's current benefit plan.
  • CO-179 (Patient has not met the required waiting requirements.): The patient hasn't met the plan's waiting requirements.
  • PR-26 (Expenses incurred prior to coverage.): The service was before coverage started.
  • CO-50 (These are non-covered services because this is not deemed a 'medical necessity' by the payer.): Medical necessity denial, a different reason for non-coverage.

CO-51 FAQ

Can plans still deny for pre-existing conditions?

Under the Affordable Care Act, most individual and group health plans can't exclude pre-existing conditions. Exclusions may still appear in plans not subject to those rules, such as some short-term plans, certain grandfathered individual policies, and some supplemental or non-major-medical coverage.

What should I do when I get CO-51?

Identify the plan type. If it's a standard ACA-compliant plan, ask the payer to reprocess or appeal. If the plan can legally apply the exclusion, check whether the condition really predates coverage and whether any look-back period has ended.

What does the payer mean when it asks for 12 months of records?

Remark N204 signals the payer is investigating whether the condition existed before coverage. Sending records promptly lets it decide; not responding can lead to a denial.