CO-51 Denial Code (CARC 51) 2026: Pre-Existing Condition Not Covered

CO-51 is a pre-existing condition denial. The payer is saying it won’t cover the service because the condition started before the patient’s coverage did. Under the Affordable Care Act, most health plans can’t do this anymore. So the first question isn’t about the patient’s history. It’s about whether this plan is even allowed to exclude pre-existing conditions. (Primary sources: X12 Claim Adjustment Reason Codes · HealthCare.gov, coverage for pre-existing conditions).

In brief: CO-51 means “these are non-covered services because this is a pre-existing condition.” Check the plan type first. Marketplace plans, Medicaid and CHIP can’t refuse to pay for a condition you had before coverage started. Grandfathered individual plans can. If the plan isn’t allowed to exclude, appeal. If it is, send records to show the condition is new, or look for other coverage.

Part of our denial codes library. Written by Manikandan J, CPC, CRCR, who has more than 10 years of US medical billing and revenue cycle experience. Published 20 October 2026. Last updated: 20 October 2026. Code wording is quoted word for word from X12.

X12 wording for CARC 51

Field	Official X12 text
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.”
Start	01/01/1995
Last modified	07/01/2017

Source: X12 Claim Adjustment Reason Codes, checked 20 October 2026.

Denial code 51, CARC 51 and reason code 51

One X12 code, three common names. When you see denial code 51 or reason code 51 in a portal, the remit will also show a group code. With CO-51 your contract keeps the amount with the provider. With PR-51 the payer assigns it to the patient.

Which plans can still exclude pre-existing conditions?

A pre-existing condition is a health problem a person had before their coverage started.

A grandfathered plan is an individual policy bought on or before 23 March 2010 that keeps some older rules.

Plan type	Can it exclude a pre-existing condition?
Marketplace plan	No
Medicaid or CHIP	No
Grandfathered individual plan	Yes
Coverage outside ACA rules	Check the policy

HealthCare.gov says no plan can refuse to pay for essential health benefits for a condition you had before coverage started. It names grandfathered plans as the exception.

Who pays: group code table

Group + CARC	Who carries it	What to know
CO-51	Provider	Contract bars patient billing. Appeal or write off.
PR-51	Patient	Billable to the patient if the exclusion is valid.
OA-51	Not assigned	Check the primary payer’s handling.

CMS describes CO adjustments as amounts that are “generally… considered a write off for the provider and are not billed to the patient” (Medicare Claims Processing Manual, Ch. 22, §60.1).

Common causes of CO-51

Remark wording is from the X12 RARC list. Pairings vary by payer.

Cause	Typical RARC	Fix
Grandfathered plan with an exclusion clause	N130	Check the clause, then look for other coverage.
Payer reviewing the patient’s history	N204	Send the medical records the payer asks for.
Plan wrongly applied an exclusion	N130	Appeal, citing the plan type.
Condition coded as long-standing when it’s new	N204	Fix the coding and send records showing onset.
Coverage outside ACA rules	N130	Read the policy terms and look for other coverage.

Working a CO-51 step by step

1. Read the policy reference. The 835 REF segment may name the payer policy.
2. Check the plan type. For example, a Marketplace plan can’t apply this exclusion.
3. Check the onset date. Records may show the condition started after coverage began.
4. Send records if the remark asks for them.
5. Appeal when the plan isn’t allowed to exclude or the onset is wrong.
6. Look for other coverage before billing the patient under PR-51.

Appeal approach

Lead with the plan type. If it’s a Marketplace plan, Medicaid or CHIP, cite the HealthCare.gov protection. If the plan is grandfathered, focus on the onset date and send records showing the condition is new. Ask the payer for the exact exclusion clause.

Questions to ask the payer about a CO-51

• Is this plan grandfathered, or is it subject to the ACA?
• Which clause excludes pre-existing conditions?
• What onset date did you use, and where did it come from?
• Which records do you need, and for what period?

Worked example (illustrative)

A patient on a Marketplace plan has knee surgery for an old injury. The remit shows CAS*CO*51*3200.00. The biller confirms the plan was bought on the Marketplace. Because those plans must cover pre-existing conditions, the biller appeals and cites HealthCare.gov. Consider the other case too. If the plan had been grandfathered, the biller would have checked the onset date first.

Prevention checklist

• [ ] Record the plan type at registration, such as Marketplace, job-based or grandfathered.
• [ ] Note any exclusion clauses found during benefit checks.
• [ ] Code onset dates carefully for new injuries.
• [ ] Respond quickly when a payer asks for prior records.
• [ ] Track CO-51 by payer to find wrongly applied exclusions.

Related denial codes

• CO-96: non-covered charges.
• CO-50: not deemed a medical necessity.
• CO-26: expenses incurred before coverage began.
• CO-109: claim not covered by this payer.
• CO-252: attachment or documentation required.
• Denial codes hub for every code we cover.

Key takeaways

CO-51 means the payer excluded a service as a pre-existing condition. Most plans can’t do that under the ACA, so check the plan type first. Appeal when the exclusion isn’t allowed, prove a new onset when it is, and look for other coverage before billing the patient.

FAQ

Sources

How we checked: every code quote on this page was compared word for word with the X12 CARC and RARC lists on 20 October 2026.

• X12, Claim Adjustment Reason Codes (CARC 51, 26). Accessed 20 Oct 2026.
• X12, Remittance Advice Remark Codes (N204, N130). Accessed 20 Oct 2026.
• HealthCare.gov, Coverage for pre-existing conditions. Accessed 20 Oct 2026.
• CMS, Medicare Claims Processing Manual, Ch. 22, §60.1 Group Codes. Accessed 20 Oct 2026.

Last updated: 20 October 2026 · This page is educational and doesn’t replace payer contracts or coding rules.