CO-55: Experimental / Investigational Procedure

What Does CO-55 Mean?

CO 55 is a contractual obligation adjustment where the payer has classified the service as experimental or investigational under their medical policy. The provider cannot bill the patient and must either appeal with clinical evidence or write off the denied amount. The appeal process for experimental classifications tends to be more involved than for typical denials because you are challenging the payer's medical policy determination, not just a coding or documentation error.

CARC 55 shows up when the payer has decided that the service you billed is experimental, investigational, or does not have sufficient clinical evidence to meet their coverage standards. This is a coverage denial based on the payer's medical policy, not on whether the service was coded correctly or documented properly. The payer is saying that even with perfect documentation, they do not consider this service proven enough to cover.

This denial code frequently applies to newer medical technologies, off-label drug uses, emerging surgical techniques, and treatments that have Category III CPT codes (temporary codes for emerging technology). Each payer maintains their own medical policy that defines what they consider experimental. A procedure might be covered by one commercial plan and denied as experimental by another based on their respective evidence review processes. Medicare uses NCDs and LCDs to make these determinations, while commercial payers typically rely on internal medical policy committees and technology assessment organizations.

The distinction between CARC 55 (experimental/investigational) and CARC 56 (not effective) is subtle but important. CARC 55 applies when the payer says the treatment lacks enough evidence to determine if it works. CARC 56 applies when the payer has reviewed the evidence and concluded the treatment does not work. The appeal strategy differs: for CARC 55, you need to demonstrate that sufficient evidence exists; for CARC 56, you need to prove the evidence shows the treatment is effective.

Common Causes

How to Resolve

Challenge the payer's experimental classification by presenting peer-reviewed clinical evidence, FDA approval status, and professional society endorsements.

1. Compile clinical evidence Gather peer-reviewed studies, clinical trial data, and FDA documentation that support the treatment as standard of care for the patient's condition.
2. Reference professional guidelines Include endorsements from relevant professional medical societies. For example, NCCN guidelines for oncology treatments carry significant weight with payers.
3. File formal appeal Submit the appeal with a physician narrative explaining the clinical rationale, supported by all compiled evidence. Request a peer-to-peer review with the payer's medical director if the written appeal is denied.

Common RARC Pairings

The RARC code tells you exactly what triggered the CO-55:

How to Prevent CO-55

• Check the payer's experimental/investigational exclusion list before scheduling newer or emerging treatments.
• Obtain prior authorization with supporting clinical evidence before rendering treatments that might be classified as experimental.
• Use Category I CPT codes when available instead of Category III codes, which may automatically trigger experimental classification edits.

General Prevention

• Check the payer's experimental/investigational exclusion list before scheduling the procedure.
• Obtain prior authorization and document the payer's approval before rendering newer or emerging treatments.
• Avoid using Category III CPT codes when a Category I code is available and appropriate for the service.
• Proactively submit published clinical evidence with prior authorization requests for newer treatments.
• Monitor payer medical policy updates for changes in experimental/investigational classifications.

Also Filed As

The same CARC 55 may appear with different Group Codes:

Related Denial Codes