The service you billed required prior authorization from the payer and none was on file when the claim was processed. The service may have been medically necessary, properly coded, and perfectly documented. None of that matters without the auth. Some payers allow retroactive authorization. Others don't. Your next steps depend entirely on which payer denied the claim and how quickly you act.
CO-55: "Procedure/treatment/drug is deemed experimental, investigational, or not approved by the FDA and is not covered, OR the procedure/revenue code was not authorized."
Both codes mean prior authorization was required. The difference is subtle but matters for your appeal strategy:
| Code | Meaning | Common Scenario |
|---|---|---|
| CO-55 | No authorization exists for this service | Auth was never obtained. Payer has no record of any auth request for this patient/service/date. |
| CO-183 | Authorization exists but doesn't match | Auth was obtained but for a different CPT code, different date range, or different provider. The auth exists but doesn't cover what was billed. |
If you received CO-55, the payer is saying they have zero record of authorization. If you did obtain auth, the fix is finding the auth number and resubmitting. If you didn't obtain auth, the fix is requesting retroactive authorization (if the payer allows it) or appealing on clinical grounds.
The most common cause. The service was scheduled, performed, and billed without anyone checking whether prior auth was required. This happens when auth requirements change (a procedure that was open-access last quarter now requires auth), when a patient switches plans within the same payer (different plan, different auth rules), or when scheduling staff assume auth requirements haven't changed since the last time they checked.
The auth request was submitted but the service was performed before the payer approved it. Some practices submit the auth request and schedule the procedure simultaneously, assuming approval will come through before the service date. If the payer is slow to respond and the service happens before approval, the claim denies.
The auth was obtained, the auth number exists, but nobody entered it on the claim. This is a data entry error, not an auth problem. The fix is simple: resubmit with the auth number in Box 23 (CMS-1500) or the appropriate 837P loop. This should come back as CO-21 (missing information) rather than CO-55, but some payers use CO-55 for both scenarios.
Every authorization has a validity window, typically 30-90 days. If the service was performed after the auth expired, the payer treats it as unauthorized. The auth existed but was no longer valid on the date of service.
Payers can add prior auth requirements for services that previously didn't require them. These changes are published in provider bulletins, but most practices don't monitor bulletins from every payer. The first indication that a service now requires auth is often the CO-55 denial on the first claim.
Whether you can recover a CO-55 denial depends almost entirely on whether the payer accepts retroactive authorization requests. Here's the reality:
| Payer | Retro-Auth Policy | Deadline |
|---|---|---|
| Medicare | Generally not required for most services. When required (certain DME, some outpatient procedures), retro-auth may be available through the MAC. | Varies by MAC |
| UnitedHealthcare | Generally denies retro-auth requests. Very limited exceptions for emergent services only. | N/A (rarely granted) |
| Aetna | Allows retro-auth within a limited window for non-emergent services. | 5 business days from service date |
| BCBS (Anthem) | Varies by state plan. Some allow retro-auth within 14 days, others deny categorically. | Check specific state plan |
| Cigna | Allows retro-auth for urgent/emergent services. Non-emergent retro-auth evaluated case by case. | 14 calendar days |
| Humana | Limited retro-auth for urgent situations with clinical documentation. | 5-10 business days |
UHC is the killer. UnitedHealthcare almost never grants retroactive authorization for non-emergent services. If you perform a service that requires UHC prior auth and don't have it, the revenue is almost certainly gone. This makes UHC auth verification at scheduling the single most important step for any practice with significant UHC volume.
Before assuming no auth was obtained, check your records and the payer's portal. The auth may exist under a different patient ID, a slightly different date range, or a different CPT code. If you find the auth, resubmit the claim with the auth number. If no auth exists, proceed to Step 2.
Check the payer's retro-auth policy using the table above. If retro-auth is available and you're within the deadline, submit the retro-auth request immediately with full clinical documentation supporting medical necessity. Don't wait. The retro-auth deadline is short and non-negotiable.
File a formal appeal arguing that the service was emergent or urgently needed and couldn't be delayed for the auth process. Include documentation of the clinical presentation, the urgency of the condition, and why waiting for authorization would have endangered the patient. Most payer contracts exempt emergent services from prior auth requirements.
For clinical denials, request a peer-to-peer review where the treating physician speaks directly with the payer's medical director. Explain the clinical urgency and why the service couldn't wait for auth. This is your strongest appeal mechanism for CO-55 when retro-auth is denied.
The appeal itself has a filing deadline (typically 60-180 days from the denial date depending on the payer). Track this deadline separately. Missing the appeal deadline makes the denial permanent regardless of whether the appeal would have succeeded.
Verify auth requirements at scheduling, not the day before. When any service is scheduled, the scheduler should query the payer for auth requirements for that specific CPT code, that specific plan, and that specific patient. Auth requirements change. Don't assume last month's rules still apply.
Build an auth tracking system. For every auth obtained, track: patient name, payer, authorized CPT code, auth number, approval date, expiration date, and scheduled service date. Review weekly. Flag any auth expiring within 14 days of the scheduled service for extension. See our complete prior authorization guide for the full tracking workflow.
Subscribe to payer bulletins. Every major payer publishes provider bulletins announcing policy changes including new auth requirements. Subscribe to the bulletin for each payer in your top 5 by volume. This is the only reliable way to learn about new auth requirements before they cause denials.
Never perform a service before the auth is confirmed. "Submitted" is not "approved." Wait for the approval and the auth number before the patient is on the table. If the payer is slow, call to expedite. If the service is urgent, document the urgency in the medical record so you have the clinical justification for an appeal if the retro-auth is denied.
Revenue impact: CO-55 denials average $300-$3,000+ per claim because auth-required services tend to be high-value (imaging, surgery, specialty drugs). A practice averaging 5 CO-55 denials per month at $800 average is losing $48,000/year. With proper auth verification at scheduling, this drops to near zero. The 2 minutes it takes to check auth requirements saves $4,000/month.
Our system verifies auth requirements at scheduling, tracks approval status and expiration dates, and alerts before services are performed without confirmed authorization. CO-55 denials drop to zero.
Learn About Our Denial Management →