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N18 Remark Code (Deactivated): Medicare Allowed Amount

N18 meant payment was based on the Medicare allowed amount. X12 deactivated it and suggested N14, a remark about payment based on a contract or fee schedule that has also been retired; CARC 45 now reports these reductions.

X12 deactivated RARCN18 on January 31, 2004. Payers should no longer use it on new remittances, but it can still appear on older ERAs, corrected claims, and appeals.

Code: N18 (RARC N18)
Status: Deactivated StoppedJanuary 31, 2004 (in use since January 1, 2000).
Code set: Remittance Advice Remark Codes (RARC)
Group codes: CO (Contractual Obligation): Accompanied reductions to the Medicare allowed amount, which were provider write-offs for participating providers.
Official description: Payment based on the Medicare allowed amount.
X12 Remittance Advice Remark Codes, retrieved September 25, 2026
Last verified: September 24, 2026 against the official X12 list.

What N18 meant

Remark N18 explained that the payer used the Medicare allowed amount as the basis for payment. It told the provider where the allowed figure came from, which was useful when a non-Medicare payer priced claims off Medicare’s fee schedule.

What replaced it

X12’s note suggested N14, a remark saying payment was based on a contract, fee schedule, or maximum allowable amount. N14 was retired too, with its own note pointing to reason code 45. So today the reduction is reported with CO-45. Secondary payers that base their payment on Medicare’s processing report the prior payer’s effect with OA-23.

If you still see N18

It only appears in older data. If a payer prices your claims based on Medicare rates, confirm which year’s fee schedule and which locality it uses, and compare the allowed amounts accordingly. Post any matching reduction as a contractual adjustment.

N14Deactivated (Payment based on a contractual amount or agreement, fee schedule, or maximum allowable amount.): The suggested replacement, itself later deactivated.
CO-45 (Charge exceeds fee schedule/maximum allowable or contracted/legislated fee arrangement.): Charge exceeds fee schedule, maximum allowable, or contracted or legislated fee arrangement.
OA-23 (The impact of prior payer(s) adjudication including payments and/or adjustments. (Use only with Group Code OA)): Impact of prior payer adjudication, relevant when a secondary payer bases payment on Medicare.

N18 FAQ

Who used N18?

Medicare contractors, and also other payers that priced claims using Medicare's allowed amounts, for example some supplemental or secondary plans.

How is this reported now?

With CARC 45 for the reduction from billed charges to the allowed amount. The payer's fee schedule basis is a contract matter rather than a remark.

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