Denial Code Decoder
CO-288 — The plan required a referral
This plan makes a referral a condition of paying for specialist care, and none reached the payer. A referral is not an authorization and the two fail differently: an authorization is the plan approving a service, while a referral is the patient's primary care provider directing them to a specialist. A practice can hold a valid authorization and still be denied for a missing referral.
Billable to the patient? Usually not on a participating contract, and the reasoning matters: the patient was routed by their own plan's rules, which is not a decision they made. Check the contract's notice terms before treating it as a patient balance.
Replaced code 165
Referral denials used to arrive as code 165, which combined an absent referral with an exceeded one. The replacement separates them, so a code alone no longer tells you which of the two happened. X12 stopped code 165 on May 1, 2018.
Where this denial is born
- Front desk
- Coding
- Claim build
- Submission
- Adjudication
This denial is usually created at the highlighted stage — that is where prevention lives.
Answer the questions — follow the path
The same questions an experienced biller asks, in order. Your answers draw the route to the right action.
Does this plan require a referral for the service you billed?
Read the eligibility response's plan detail, not the card. Referral rules travel with the plan type, and the same payer sells plans both ways.
The full decision tree
Does this plan require a referral for the service you billed?
Read the eligibility response's plan detail, not the card. Referral rules travel with the plan type, and the same payer sells plans both ways.
- Yes →
Does a valid referral exist for this date of service?
Check both its expiry and its remaining visits — an authorization that is still in date can still be out of visits.
- Yes →
Send the referral and resubmit. Carry the referral on the corrected claim in the element the payer's companion guide specifies, and resubmit as a replacement referencing the original claim number.
- No →
Ask the referring provider, then resubmit. Contact the primary care provider for a referral covering the date of service. Some plans will honour one issued after the fact and some will not — the plan's policy, not the specialist's need, decides.
- Yes →
- No →
Dispute the requirement itself. Attach the eligibility or benefit response showing this plan carries no referral requirement for the service, and ask for reprocessing. Payers do apply another product's rules to the wrong plan.
Prevention
Capture the referral at registration for every plan that requires one, and record its expiry and its visit count — referrals lapse quietly, and the visit that falls off the end denies exactly like the first one would have.
Related reading
The code list, and when it was last checked
X12 maintains the Claim Adjustment Reason Codes list, where this code is listed as 288. The list carries the number alone: the CO in front of it is the adjustment group the payer chose for this line, which decides who owes the balance, and the same code can arrive under a different group from a different payer. It has been on the list since November 1, 2017.
Checked against the list revision dated November 1, 2025 on August 7, 2026, where it reads as an active code.
Sources
The paragraphs behind the guidance above. A page that tells you what to do about money should show you what the instruction rests on.
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