CO · Contractual ObligationCARC 55

Denial Code CO-55

Q: What does CO-55 mean?

CO-55 is a Contractual Obligation (CO) denial: "Procedure/treatment/drug is deemed experimental/investigational by the payer." The payer considers this service, procedure, or drug experimental or investigational under its medical policy — not yet accepted as standard care — so it won't pay. Under the CO group code the provider, not the patient, absorbs it unless valid advance notice shifted liability.

Q: Is CO-55 the patient's responsibility or a provider write-off?

CO-55 carries the CO group code (Contractual Obligation): Provider/contractual responsibility — not billable to the patient while denied. A CO adjustment is the provider's responsibility, not the patient's: balance-billing the patient for a CO amount is a contract (and often compliance) violation. But CO does not automatically mean “write it off.” When the cause is fixable — missing information (CO-16), a modifier problem (CO-4), or NCCI bundling (CO-97) — you correct and resubmit, or appeal with documentation. You only truly write the amount off when it is a final contractual adjustment, such as the fee-schedule difference on CO-45.

Procedure/treatment/drug is deemed experimental/investigational by the payer.

Source: X12 Claim Adjustment Reason Codes (CARC) & Group Codes. Maintained by the D3rx Clinical Billing Team.

What CO-55 means

The payer considers this service, procedure, or drug experimental or investigational under its medical policy — not yet accepted as standard care — so it won't pay. Under the CO group code the provider, not the patient, absorbs it unless valid advance notice shifted liability.

Why CO-55 happens

A newer test, device, or treatment the payer hasn't recognized as standard of care
Off-label use of a drug or procedure
A service the payer's medical policy explicitly lists as experimental/investigational
Emerging genetic or molecular testing not yet in the payer's coverage policy

What to do when you get CO-55

1Pull the payer's medical policy and confirm it lists the service as experimental/investigational for this indication
2Gather the clinical evidence — FDA status, peer-reviewed literature, specialty-society guidelines
3File a formal appeal with that evidence, and request a peer-to-peer/medical-director review where available
4For Medicare, issue an ABN beforehand so liability can shift to the patient if it remains non-covered

Got this denial right now?

Generate a CO-55 appeal letter in 60 seconds — free

Generate CO-55 appeal Ask D3 about CO-55

Free e-sign appeal generator. No signup needed to start — you can review and edit before sending.

CO group code: who absorbs the charge

Contractual Obligation — Provider/contractual responsibility — not billable to the patient while denied.

A CO adjustment is the provider's responsibility, not the patient's: balance-billing the patient for a CO amount is a contract (and often compliance) violation. But CO does not automatically mean “write it off.” When the cause is fixable — missing information (CO-16), a modifier problem (CO-4), or NCCI bundling (CO-97) — you correct and resubmit, or appeal with documentation. You only truly write the amount off when it is a final contractual adjustment, such as the fee-schedule difference on CO-45.

Appeal, correct, or write off CO-55?

CO-55 is won on evidence: appeal with the clinical literature, FDA approval/clearance, and guideline support showing the service is established for this indication, and use a peer-to-peer review when the plan offers one. If the service truly is investigational under the policy, the realistic path is advance notice (ABN/financial waiver) so the patient can elect and accept responsibility — under the CO group code you cannot bill the patient without it.

Timing & deadlines

Appeal within the payer's window (Medicare redetermination 120 days from the remittance; commercial ~180 days). Experimental/investigational appeals often escalate to external/independent review if the first level upholds the denial — preserve each level's deadline.

Example

A practice bills an emerging molecular test the payer's policy lists as investigational, and it returns CO-55. If recent guidelines and FDA status support the test for the patient's indication, an appeal with that evidence and a peer-to-peer review can overturn it.

Prevent CO-55 going forward

Check the payer's medical policy for experimental/investigational status before scheduling
Verify FDA status and coverage for newer tests, devices, and off-label uses
Obtain advance notice (ABN/financial waiver) for likely-non-covered services
Track which emerging services your top payers still classify as investigational

Code families most affected

Emerging genetic/molecular diagnostics
Newer devices and procedures
Off-label drug and biologic uses

Related codes

Denial codes you'll often see alongside CO-55

CO-50 CO-96 CO-167

Payer notes

Experimental/investigational determinations come from each payer's own medical policy, so a service one plan covers another may not. CO-55 is provider liability under the contractual group code; an ABN issued before the service is what lets you bill the patient instead.

Turn this CO-55 denial into a sent appeal

D3rx drafts a ready-to-send, e-signable appeal letter for CO-55 from your claim details — free, backed by CMS, Medicare, and major-payer data.

Generate CO-55 appeal Ask D3 about CO-55 All denial codes

Medical billing disclaimer

CARC/RARC definitions are the standardized X12 set; group-code semantics follow the X12 Claim Adjustment Group Code standard. Filing and appeal windows vary by payer — the standard anchors shown (Medicare redetermination 120 days from the remittance; Medicare timely filing 12 months from date of service; most commercial payers ~90-180 days) are general references, not a guarantee for any specific plan. Always confirm the rule in the payer's provider manual before acting. D3rx is not responsible for claim outcomes.