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CO-P28 Denial Code: Adjusted Under Liability Coverage Rules

CO-P28 means an auto insurer paid part of the bill but adjusted payment based on the jurisdictional regulations or payment policies for liability coverage benefits. The regulation cited and remark codes explain the reduction.

Quick facts

Code
CO-P28 (CARC P28)
Status
Active In use since November 1, 2017.
Code set
Claim Adjustment Reason Codes (CARC)
Group codes
  • CO (Contractual Obligation): The usual group. The reduction is the provider's to absorb or dispute; state rules govern billing the claimant.
  • PR (Patient Responsibility): Used when the carrier indicates the patient is responsible for part of the amount. Check other coverage and state rules first.
Official description
Payment adjusted based on the Liability Coverage Benefits jurisdictional regulations and/or payment policies. Usage: If adjustment is at the Claim Level, the payer must send and the provider should refer to the 835 Insurance Policy Number Segment (Loop 2100 Other Claim Related Information REF qualifier 'IG') if the jurisdictional regulation applies. If adjustment is at the Line Level, the payer must send and the provider should refer to the 835 Healthcare Policy Identification Segment (loop 2110 Service Payment information REF) if the regulations apply. To be used for Property and Casualty Auto only.
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What CO-P28 means

CARC P28 says payment adjusted based on the liability coverage benefits jurisdictional regulations and/or payment policies. X12 limits it to property and casualty auto claims.

Where CO-P27 is a full denial from a liability carrier, CO-P28 means the carrier paid something but reduced it. The reason isn’t a fee schedule (that’s CO-P29). Instead, it’s another liability rule or carrier payment policy, such as a reasonable-amount determination, apportionment between accident-related and unrelated care, or the policy limit being reached.

Common causes

  • Carrier’s reasonable-amount review reduced the charge.
  • Apportionment of treatment between the accident and other conditions.
  • Policy limits partially exhausted.
  • Treatment frequency or duration questioned under the carrier’s policies.
  • Partial relatedness, where only some services are tied to the accident.

How to fix it

  1. Read the remark codes and cited regulation to understand the reduction.
  2. Request the carrier’s review or explanation if it isn’t clear.
  3. Submit a reconsideration with documentation showing the treatment’s relation to the accident and why the charge is reasonable.
  4. Bill other coverage for the remaining balance where appropriate, such as the patient’s PIP, medical payments, or health plan.
  5. Coordinate with the patient’s attorney if there’s a pending liability claim.
  6. Check state rules before billing the patient.

How to prevent problems

  • Document the accident connection clearly in each note.
  • Separate unrelated conditions in documentation and diagnosis coding.
  • Track liability limits and payments to date on the account.
  • Monitor liability carrier reductions. An ERA Analyzer can surface CO-P28 patterns by carrier.

Specialty notes

Chiropractic, PT, and orthopedic practices treating accident patients often see CO-P28 when pre-existing spine or joint conditions complicate the question of what’s accident-related.

Remark codes that may appear with CO-P28

  • N599 (Our payment for this service is based upon a reasonable amount pursuant to both the terms and conditions of the policy of insurance under which the…): Payment is based on a reasonable amount under the policy and applicable law.
  • N597 (Adjusted based on a medical/dental provider's apportionment of care between related injuries and other unrelated medical/dental conditions/injuries.): Adjusted based on apportionment of care between related injuries and unrelated conditions.
  • N587 (Policy benefits have been exhausted.): Policy benefits have been exhausted, capping the payment.
  • CO-P27 (Payment denied based on the Liability Coverage Benefits jurisdictional regulations and/or payment policies.): Payment denied, rather than adjusted, under liability coverage rules.
  • CO-P29 (Liability Benefits jurisdictional fee schedule adjustment.): A liability benefits fee schedule adjustment, used when pricing is the reason.
  • CO-P22 (Payment adjusted based on the Medical Payments Coverage (MPC) and/or Personal Injury Protection (PIP)): The MPC and PIP equivalent of this adjustment.
  • CO-P32 (Payment adjusted due to Apportionment.): Payment adjusted due to apportionment.

CO-P28 FAQ

What's the difference between CO-P28 and CO-P29?

CO-P29 is a fee schedule pricing reduction. CO-P28 covers other reductions under liability rules or payment policies, such as reasonable-amount determinations, apportionment, or policy limits.

Can I dispute CO-P28?

Yes. Ask for the basis of the reduction, then submit a reconsideration with documentation. If the patient has an attorney, they may also address unpaid amounts in the liability settlement.

Where does the carrier cite the rule?

At claim level, in the 835 insurance policy number segment (loop 2100 REF, qualifier IG). At line level, in the healthcare policy identification segment (loop 2110 REF), when regulations apply.

Is CO-P28 used outside auto claims?

No. X12 limits it to property and casualty auto claims.

Is CO-P28 a denial?

No. Part of the bill was paid. The adjustment shows the portion the liability carrier decided not to pay under its rules or policies.

Should I wait for the liability settlement before billing anyone else?

Not necessarily. Other coverage, such as the patient's own PIP or health plan, may have filing deadlines that keep running. Check them early and bill where appropriate.