Claim adjustment reason code
Denial code CO-55 meaning
CARC 55, usually seen as CO-55: The payer considers the service, treatment or drug experimental.
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- Reason code
- CARC 55
- Short name
- Experimental or investigational
- Plain meaning
- The payer considers the service, treatment or drug experimental.
- Group codes
- CO-55: Contractual obligation. The provider absorbs the amount under its contract or the payer's rules. Do not bill the patient for it.
Plain-English summary written by Rivvi. X12 owns the CARC list and its exact wording, which is not reproduced here. Read it on the X12 CARC list (list dated November 1, 2025, checked October 23, 2026). This page does not decide how a claim is paid.
Common causes
- The payer's policy lists the service as investigational.
What to check before resubmitting
- Read the payer's policy.
- Appeal with published evidence if the service is standard care.
Related reason codes
Related pages
Questions
What does denial code CO-55 mean?
The payer considers the service, treatment or drug experimental. This is Rivvi's plain-English summary of CARC 55. X12's exact wording is on x12.org.
How do I fix CO-55?
Read the payer's policy. Appeal with published evidence if the service is standard care.
Can I bill the patient for CO-55?
With group code CO, no. CO means the provider absorbs the amount under its contract or the payer's rules.
Which group codes come with CARC 55?
Most often CO (contractual obligation). The group code, not the reason code, says who owes the amount.
What is the difference between a CARC and a RARC?
A claim adjustment reason code (CARC) says why a claim line was paid differently than billed. A remittance advice remark code (RARC) adds detail, such as which field is missing.