Experimental or investigational
The payer classifies this procedure, drug or device as not yet proven, so the plan does not cover it.
CO = Contractual Obligation. The provider owes the difference. The patient cannot be billed for this amount. The same reason code can appear with a different group code; the group decides who may be billed.
Usual causes
- New technology or an off-label use the payer's policy calls investigational.
- Unlisted code (see 189) that the payer could not match to a covered service.
- The code is covered for other diagnoses but not this one.
The fix
Find the payer's medical policy for the service. Appeal with peer-reviewed evidence, specialty society guidelines, FDA approval status and the clinical reason for this patient. If a waiver was signed before the service, the patient may be billed.
Appeal or corrected claim?
Clinical appeal, often needing a physician letter. Not a corrected claim.
Deadline to watch
Appeal window per payer, often 180 days for commercial plans.
Exact limits are in your payer contract or provider manual. When in doubt, send the correction or appeal this week.
CO-55 in short
What does CO-55 mean?
The payer classifies this procedure, drug or device as not yet proven, so the plan does not cover it. The official X12 text reads: "Procedure/treatment/drug is deemed experimental/investigational by the payer. Usage: Refer to the 835 Healthcare Policy Identification Segment (loop 2110 Service Payment Information REF), if present."
What usually causes a CO-55 denial?
New technology or an off-label use the payer's policy calls investigational. Unlisted code (see 189) that the payer could not match to a covered service. The code is covered for other diagnoses but not this one.
How do I fix CO-55?
Find the payer's medical policy for the service. Appeal with peer-reviewed evidence, specialty society guidelines, FDA approval status and the clinical reason for this patient. If a waiver was signed before the service, the patient may be billed.
Should I appeal CO-55 or send a corrected claim?
Appeal. Clinical appeal, often needing a physician letter. Not a corrected claim.
What deadline applies to CO-55?
Appeal window per payer, often 180 days for commercial plans.