CO-55: experimental or investigational.

What X12 says CARC 55 means

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

CARC 55 · Start 01/01/1995, last modified 07/01/2017.

X12 usage note: Refer to the 835 Healthcare Policy Identification Segment (loop 2110 Service Payment Information REF), if present.

In plain terms: The payer treats the procedure, treatment or drug as experimental or investigational and does not pay for it under its rules.

What it means on a remittance

Payers keep written coverage policies that list services they consider unproven. When a claim matches one, the line is denied with CARC 55. The X12 usage note says to refer to the 835 policy segment, which, if filled in, names the policy. A close relative, CARC 56, is “Procedure/treatment has not been deemed ‘proven to be effective’ by the payer.”

Typical causes

• The service is on the payer's list of experimental or investigational services.
• The procedure code is a new or category III code the payer has not adopted.
• The use falls outside the conditions the policy accepts (indication, age, setting).
• The claim lacks the diagnosis or trial information the policy asks for.

These are common billing patterns, not a diagnosis of your claim. The remittance and the payer's policy decide.

What to check

1. Get the policy named on the denial or search for it on the payer's site. For Medicare, search the Medicare Coverage Database for the NCD or LCD (remark code N115 points to an LCD).
2. Compare the policy's conditions with the diagnosis, setting and date of service.
3. Check that the procedure code and diagnosis on the claim are accurate and specific.
4. Check for a clinical trial or a payer exception process the policy mentions.

Correct, resubmit or appeal

If a coding error put the service under the policy, correct and resubmit. If the service is accurately billed and you disagree, appeal with the policy, the clinical record and any supporting published evidence the policy accepts. For Medicare, the redetermination request is due 120 days from receipt of the notice, with receipt presumed 5 days after the notice date, and a late request can be accepted for good cause (42 CFR 405.942). Other payers set their own windows, found in the denial notice.

Medicare dates that apply

Medicare claims must be filed no later than the close of the period ending 1 calendar year after the date of service (42 CFR 424.44(a)(1)), with exceptions in 424.44(b). A redetermination request must be filed within 120 calendar days from the date a party receives the notice, receipt is presumed 5 days after the notice date (42 CFR 405.942(a)), and a late request can be considered for good cause (405.942(b)). Clerical errors, including inaccurate data entry, are handled by the contractor as reopenings instead of redeterminations (42 CFR 405.980(a)(3)). The tool above runs the date math.

Related codes

X12 does not publish a table that pairs each CARC with remark codes; the payer picks the remark code and prints it beside the CARC on the same remittance line. These X12 remark codes carry wording that speaks to this reason, so look for them:

• N115 — This decision was based on a Local Coverage Determination (LCD). An LCD provides a guide to assist in determining whether a particular item or service is covered. A copy of this policy is available at www.cms.gov/mcd, or if you do not have web access, you may contact the contractor to request a copy of the LCD.
• N569 — Not covered when performed for the reported diagnosis.

Other codes: CARC 50 (medical necessity denials), CARC 97 (bundled services). The full lookup reads any code on this page.

Worked example (hypothetical)

A claim for a procedure denies CO-55 and the remittance names a policy. The biller reads it: the policy accepts the procedure only for a stated indication. The record shows a different indication was documented, so the biller sends a redetermination request with the record and the policy page.

Sources

• X12: Claim Adjustment Reason Codes — code text, usage notes and dates (list dated 10/30/2026, retrieved 2026-11-02)
• X12: Remittance Advice Remark Codes — remark code text (retrieved 2026-11-02)
• 42 CFR 424.44, 42 CFR 405.942 and 42 CFR 405.980 — eCFR, current through 2026-10-29 (retrieved 2026-11-02)

Code descriptions are X12 copyrighted material, quoted here only for the codes on this page and linked to the owner. For the full lists, use X12. Every rule above was read from these pages on November 2, 2026. Payers revise code lists, contracts and policies, so confirm anything that affects a claim or a deadline with the payer.