CARC 151: frequency not supported.

What X12 says CARC 151 means

Payment adjusted because the payer deems the information submitted does not support this many/frequency of services.

CARC 151 · Start 10/31/2002, last modified 01/27/2008.

Remark codes that often explain a 151

X12 does not publish fixed CARC-to-RARC pairings, so use the remark on your remittance. Remarks whose text matches this situation: N640, "Exceeds number/frequency approved/allowed within time period", and M86, "Service denied because payment already made for same/similar procedure within set time frame." On Medicare claims you may also see N115 (decision based on a Local Coverage Determination) or N386 (based on a National Coverage Determination), which point to the policy that sets the limit.

Where the limit comes from

For Original Medicare, a covered service must be reasonable and necessary for diagnosis or treatment under 42 CFR 411.15(k)(1). Frequency limits are set in the coverage policy: a National Coverage Determination or the contractor's Local Coverage Determination, both searchable in the CMS Medicare Coverage Database. Other payers publish limits in medical policies or the plan document.

What to check

• Open the policy named by the remark or the payer's medical policy and find the limit: units per date, visits per period, or a minimum interval between services.
• Count what was actually billed and paid in that window, across all claims, including claims from other dates of service in the period.
• Check units for keying errors. A 10 typed for 1 produces a frequency denial that a corrected claim cures.
• If the extra services were needed, check the record. Does the documentation state why the patient needed more than the usual number, in terms the policy accepts?
• Check whether the patient needs to be told in advance. For Medicare, CMS describes the ABN as the notice used where payment is expected to be denied; see the sources.

Appeal or correct?

If the units were wrong, correct the claim. If the units were right and the record supports the need, appeal with the documentation attached. The appeal must argue from the policy's own wording, for example that the record meets a stated exception, not just that the care was needed.

Deadlines for Original Medicare

• Filing the claim: for services on or after January 1, 2010, no later than the close of the period ending 1 calendar year after the date of service (42 CFR 424.44(a)(1)), subject to the exceptions in that section.
• Redetermination: 120 calendar days from receipt of the initial determination, with receipt presumed 5 days after the notice date unless there is evidence to the contrary (42 CFR 405.942). Good cause can extend it.
• Reconsideration: 180 calendar days from receipt of the redetermination notice (405.962).
• ALJ hearing: 60 calendar days from receipt of the reconsideration notice, plus the amount-in-controversy requirement (405.1002).
• Reopening to fix an error: a party may ask the contractor to reopen within 1 year of the determination for any reason, or within 4 years for good cause (405.980(c)).

Medicare Advantage plans and commercial payers set their own deadlines. They are printed on the denial notice or in your contract, and they are often shorter. The calculator on this page covers Original Medicare only.

Worked example (hypothetical)

A policy allows one service per 12 months. The provider billed it twice in 9 months, and the payer paid the first and adjusted the second with CO-151 and remark N640. The record shows a documented change in the patient's condition after the first service. If the policy allows a repeat where the condition changes, an appeal with that record is reasonable. If the policy has no such exception, the adjustment stands.

Sources

• X12: Claim Adjustment Reason Codes — code text, start and last-modified dates (list dated 10/30/2026, retrieved 2026-11-02)
• X12: Remittance Advice Remark Codes — remark text and dates (retrieved 2026-11-02)
• eCFR: 42 CFR 411.15 (particular services excluded from coverage) — regulation text as of the eCFR issue dated 2026-10-29, retrieved 2026-11-02
• CMS: Medicare Coverage Database — where NCDs and LCDs are published (retrieved 2026-11-02)
• eCFR: 42 CFR 405.942 — regulation text as of the eCFR issue dated 2026-10-29, retrieved 2026-11-02
• CMS: Fee-for-Service ABN — what the Advance Beneficiary Notice of Noncoverage is for (page last modified 07/17/2026, retrieved 2026-11-02)

Every code description and rule above was retrieved from these pages on November 2, 2026. X12 code text is copyrighted by X12 and quoted only for the codes explained here; the complete lists are at x12.org. Payers revise code lists, contracts and policies, so confirm anything that affects a claim or a deadline with the payer.