CO-20: Liability insurer should pay for injury

The payer is telling you the services relate to an injury that should be billed to a liability insurer, such as auto, general liability, or workers compensation. They see the claim as out of scope for the member's regular medical or behavioral health benefit. Mechanically, the claim hits an edit that looks at accident indicators, occurrence codes, E codes, or COB data that suggests a third party might be liable. The payer then denies all or part of the line items as not payable under this coverage and sets that portion as a contractual adjustment under CO, which you cannot bill to the member.

In behavioral health, CO-20 often appears when treatment follows an accident or assault, such as PHP or IOP after a motor vehicle crash with PTSD symptoms. The medical plan sees an accident flag in the file and pushes payment responsibility to the auto or liability carrier, even if your claim is strictly for mental health services. You may also see CO-20 on SUD treatment tied to DUI or job related injuries, for example IOP services after an on the job accident where workers compensation may be involved. Medicaid and MCO plans are especially strict about third party liability and will deny behavioral claims if their system shows an open liability case. If a carve out behavioral vendor administers the benefit, they may not have full visibility to the liability file. That can lead to denials on residential or detox per diem claims on the UB-04, even when the accident is old, the liability claim is closed, or the injury is only indirectly related to the current behavioral episode.

• Confirm with the patient, chart, and intake notes whether the episode is in any way tied to an accident or injury that could involve a liability carrier.
• Check eligibility and COB screens or payer portals for any open auto, liability, or workers compensation coverage or case numbers.
• If a liability policy exists, bill the liability carrier first and obtain their EOB or denial, then submit or resubmit the behavioral claim to the health plan as secondary with that EOB.
• If there is no liability coverage or the liability claim is closed, obtain written proof, such as a denial letter or attestation, and submit a corrected claim or appeal asking the payer to remove CO-20 and process under the behavioral benefit.
• Confirm that occurrence codes, accident indicators, and diagnosis codes on the UB-04 or CMS-1500 accurately reflect the clinical situation and remove accident flags when they are wrong.
• Once corrected, resubmit as a corrected claim if the payer allows reprocessing from a clean claim, or file a formal appeal with supporting COB and accident documentation if required by that payer.

• At intake, ask direct questions about accidents, assaults, workplace injuries, or auto events tied to the behavioral episode and capture any liability carrier details.
• Build registration and EHR fields that clearly separate historical trauma from active accident or liability events so staff do not trigger accident indicators by default.
• Set claim scrubber rules that flag claims with accident dates, occurrence codes, or injury E codes without a documented liability decision or primary carrier EOB before submission.
• Periodically review COB and third party liability rules for your top payers and update front end workflows so behavioral claims are sent to liability carriers first when required.