CO-151
Often appealableInformation doesn't support this many services
Group CO — Contractual Obligation — an adjustment tied to the contract between the insurer and the provider. You usually cannot be billed for a CO amount.
CO-151 means the payer decided the submitted information doesn't support the volume or frequency of the services billed — for example, more units or visits than their criteria allow for your condition.
It's a Contractual Obligation (CO). When the frequency was genuinely necessary, this is appealable with clinical documentation that justifies the amount of care.
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Why you're seeing CO-151
- The number of units, visits, or sessions exceeded the payer's frequency limits.
- The documentation didn't explain why the higher frequency was necessary.
- A billing error overstated the number of units.
Can you appeal it?
Appealable when the frequency was medically necessary and can be documented — or correctable if a billing error overstated the units.
What to do next
- 1Confirm whether the billed units/frequency were correct or a billing error.
- 2Ask your provider for documentation justifying why the frequency was necessary.
- 3Request the payer's frequency criteria so you can address them directly.
- 4Appeal in writing with clinical support for the number of services provided.
Evidence that helps
- Chart notes and a treatment plan justifying the frequency of care.
- Clinical guidelines supporting the number of services for your condition.
Frequently asked questions
Why would too many services be denied?
Payers set frequency limits based on clinical criteria. CO-151 means your claim exceeded them. If the extra care was medically necessary, you can appeal with documentation explaining why the frequency was appropriate for your condition.
Related guide: How to appeal a “Not medically necessary” denial →
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Other common denial codes
Last reviewed July 18, 2026. Denial-code lists (CARC/RARC) are updated three times a year; we review these explanations against the current list.
Sources & references
These explanations are written in plain language and based on the authoritative sources below. Always confirm the specifics against your own plan documents and denial letter.
- ASC X12 — Claim Adjustment Reason Codes (CARC)
The standards body that maintains the official CARC list (the CO/PR codes), updated three times a year.
- CMS / X12 — Remittance Advice Remark Codes (RARC)
The remark codes (N-/M- codes) that accompany a CARC and give the specific reason for an adjustment.
- HealthCare.gov — Appealing a health plan decision
Official overview of internal appeals and external review rights for Marketplace and most private plans.
- U.S. Department of Labor — ERISA claims & appeals
Governs claim and appeal procedures and deadlines for most employer-sponsored (ERISA) health plans.
- Your state's insurance regulator
State agencies enforce external review and consumer protections; find yours via the NAIC directory.
Important: this is not legal or medical advice.
AppealBrain is a free, self-help tool that helps you draft an appeal letter using the information you provide. We are not a law firm, medical provider, or insurance company, and using this tool does not create an attorney–client relationship. Appeal rules and deadlines vary by plan and state — always review your own denial letter and plan documents, and consider consulting a licensed professional for your specific situation. We do not guarantee any outcome.