CO-165 denial code
Referral absent or exceeded
How to fix it
Obtain a retroactive referral where the plan permits, or appeal with evidence a valid referral existed.
How to prevent it
Track referral visit counts and expiry dates. Referrals exhaust mid-course exactly like authorisations.
In practice
A patient with an HMO plan is referred to a cardiologist for four visits. On the fifth visit the claim returns CO-165: the referral was absent or its limits exceeded.
The referral was valid and correctly recorded. It simply ran out, in exactly the way an authorisation runs out, and nothing in the specialist's workflow was counting down against it.
Some plans permit a retroactive referral from the primary care physician within a defined window. Where they do, the recovery is a phone call made quickly. Where they do not, the visit is not billable to the plan and, in most HMO contracts, not billable to the patient either.
What sits behind it
The distinction between a referral and an authorisation causes real confusion and they are not the same thing. An authorisation is the plan approving a service. A referral is the primary care physician directing the patient to a specialist, and in gatekeeper HMO designs it is a condition of coverage independent of whether the service itself needs approval.
That means a specialist can hold a perfectly valid authorisation and still be denied for a missing referral, and the reverse. Practices that store one number in one field frequently discover this only when a claim denies for the requirement they were not tracking.
The structural difficulty is that the specialist depends on someone else's administrative action. The referral is issued by the primary care physician's office, expires on their timetable, and is renewed at their discretion. A specialty practice that does not track referral counts and expiry dates as its own responsibility has outsourced its revenue to another office's diary.
Related codes
Terms used here — Prior Authorization · Eligibility Verification · Denial
How we handle it — Prior Authorization · Eligibility Verification · Denial Management
Primary sources
The rules behind CO-165, at the bodies that publish them.
- CMS Interoperability and Prior Authorization Final Rule (CMS-0057-F) (opens in a new tab)
Centers for Medicare & Medicaid Services — The rule imposing prior authorization decision timelines and API requirements on impacted payers. It is the single largest scheduled change to authorization workflow this decade.
- Medicare Claims Processing Manual (opens in a new tab)
Centers for Medicare & Medicaid Services — The operative manual for how Medicare claims must be coded, submitted, adjusted and appealed. When a payer policy and a vendor's advice disagree, this settles it.
- Medicare Coverage Database (LCD/NCD) (opens in a new tab)
Centers for Medicare & Medicaid Services — Searchable national and local coverage determinations. The direct answer to whether a diagnosis supports medical necessity for a given procedure.
Every denial code with a guide
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Data quality
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Patient responsibility
Provider eligibility
Timely filing
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Questions about CO-165
An authorisation is the plan approving a specific service. A referral is the primary care physician directing the patient to a specialist, which gatekeeper HMO plans require as a condition of coverage regardless of whether the service itself needs approval. A claim can hold a valid authorisation and still deny for a missing referral.
Some plans permit it within a defined window, often 30 to 90 days, at the primary care physician's discretion. Where the plan allows it, act quickly — the window is shorter than the filing deadline and it closes without notice.
Usually not. Most HMO contracts treat the referral as a network requirement the provider is responsible for confirming, which means the balance is a contractual write-off rather than patient responsibility.
Track referral visit counts and expiry dates the same way authorised units are tracked, and check both at scheduling rather than at billing. Referrals exhaust mid-course exactly like authorisations, and the specialist bears the loss even though another office issues them.
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