Claim Adjustment Reason Codes on NEMT Remittances: What Each Code Means and the Fix

What a claim adjustment reason code is

Every dollar a payer does not pay on your claim comes with a reason code. X12, the standards body that writes the formats for electronic claims and payments, keeps the list and defines it this way: “These codes describe why a claim or service line was paid differently than it was billed.” The code tells you whether a line was cut, denied, held, or moved to someone else, and why.

The codes are national, so a code means the same thing from any payer that uses it. HIPAA made version 5010 of the X12 835 the required format for electronic remittance advice on January 1, 2012 (45 CFR 162.1602), and the Medicaid program counts as a health plan under HIPAA (45 CFR 160.103). Since January 1, 2014, health plans must also follow a federal operating rule on the uniform use of reason and remark codes, the Phase III CORE 360 rule (45 CFR 162.1603).

X12 adds, changes, and retires codes as requests come in. As of September 2026, the reason code list was last updated November 1, 2025, and the remark code list July 1, 2026.

You find the codes in three places:

• The 835 file. Reason codes sit in the adjustment segments, at the claim level and on each service line.
• The paper or PDF remittance. Indiana prints them in a field marked ARCS, right after its own codes, with up to 20 for the claim and 20 for each line. It defines each one at the end of the remittance.
• Provider-level adjustments. Recoupments and interest use a separate X12 list, the provider adjustment reason codes. Indiana notes that these financial codes are not part of the claim reason code set.

For a full walk through a remittance, see how to read remittance advice.

How to read a code: group, reason, and remark

An adjustment on a remittance looks like “CO 45” or “PR 3”. The letters are the group code and the number is the reason code. X12 gives its own examples, PR 32 and CO 286, and says the group codes “generally assign responsibility for the adjustment amounts.”

Part	Example	What it tells you
Group code	CO	Who carries the amount
Reason code (CARC)	45	Why the amount was not paid
Remark code (RARC)	M22	More detail, often the field or document that failed

Group code	X12 name	What it means for you
CO	Contractual Obligation	You absorb it. A Medicaid provider cannot bill the rider for it.
PR	Patient Responsibility	The rider owes it, such as an allowed copayment
OA	Other Adjustment	Used for exact duplicates, pended lines, and the effect of an earlier payer
PI	Payor Initiated Reduction	A reduction the payer made
CR	Corrections and Reversal	X12 says not to use it with version 5010 and later

Some reason codes only work with certain groups. Code 18 (duplicate), code 23 (the effect of an earlier payer), and code 133 (pended) are for group OA. Code 45 is only for CO or PR.

Some reason codes never stand alone. Codes 16, 96, 252, and A1 must come with at least one remark code that is not an alert, and A1 is only for cases where no more specific reason code fits. When you see one of these, the remark code is the real answer. See remittance advice remark codes.

The normal one: CO 45

CO 45 reads “Charge exceeds fee schedule/maximum allowable or contracted/legislated fee arrangement.” It is the gap between the charge you billed and the rate the payer allows, so it shows up whenever your charge is above the rate. X12 adds that a code 45 amount cannot equal the whole charge, so a fully unpaid line always carries some other reason.

Post CO 45 as a contractual write-off, kept apart from denials. Never bill it to the rider. Federal rule 42 CFR 447.15 limits Medicaid to providers who accept the agency’s payment as payment in full, apart from any cost sharing the state plan allows.

Reason codes behind the usual NEMT problems

The wording in the middle column is X12’s. The last column says what to check on a trip claim.

Eligibility and the wrong payer

Code	X12 wording	What to check
26	Expenses incurred prior to coverage.	The rider’s coverage started after the ride. Rebill if coverage is made retroactive to that date.
27	Expenses incurred after coverage terminated.	Coverage ended before the ride. Check eligibility the day before each ride.
200	Expenses incurred during lapse in coverage	The rider had a gap in coverage on the date of service
31	Patient cannot be identified as our insured.	The Medicaid ID is wrong. Copy it exactly from the eligibility response or the trip assignment.
140	Patient/Insured health identification number and name do not match.	The name or ID has a typo
22	This care may be covered by another payer per coordination of benefits.	Other coverage is on file. Bill that payer first. See third party liability.
24	Charges are covered under a capitation agreement/managed care plan.	The rider’s trips belong to a health plan or its broker
109	Claim/service not covered by this payer/contractor. You must send the claim/service to the correct payer/contractor.	Send the claim to the plan or broker that covers the rider’s rides

Authorizations, codes, and miles

Code	X12 wording	What to check
197	Precertification/authorization/notification/pre-treatment absent.	The trip authorization is missing or in the wrong field. See prior authorization.
198	Precertification/notification/authorization/pre-treatment exceeded.	You billed more trips, units, or miles than were approved
284	Precertification/authorization/notification/pre-treatment number may be valid but does not apply to the billed services.	The number belongs to another date, code, or leg
296	Precertification/authorization/notification/pre-treatment number may be valid but does not apply to the provider.	The authorization was issued to another company
4	The procedure code is inconsistent with the modifier used.	An origin and destination modifier or service modifier does not fit the code
181	Procedure code was invalid on the date of service.	The code was not on the payer’s list for that date
182	Procedure modifier was invalid on the date of service.	The modifier was not valid for that date
150	Payer deems the information submitted does not support this level of service.	You billed a higher level than approved, such as wheelchair for an ambulatory rider
151	Payment adjusted because the payer deems the information submitted does not support this many/frequency of services.	The payer cut miles or trips. Compare the miles to your trip record.
96	Non-covered charge(s).	Read the remark. N157 means “Transportation to/from this destination is not covered.”

Your enrollment, the claim itself, and timing

Code	X12 wording	What to check
8	The procedure code is inconsistent with the provider type/specialty (taxonomy).	Your taxonomy code or provider type does not include this service
170	Payment is denied when performed/billed by this type of provider.	Your enrollment type cannot bill this code
B7	This provider was not certified/eligible to be paid for this procedure/service on this date of service.	Your enrollment lapsed, or did not cover this service, on that date
206, 207, 208	National Provider Identifier: missing (206), invalid format (207), not matched (208)	The NPI is missing, mistyped, or not linked to your enrollment. See billing vs rendering provider.
16	Claim/service lacks information or has submission/billing error(s).	Read the remark: M22 is miles, M53 is units, N382 is the rider’s ID, N290 is the rendering provider’s ID
252	An attachment/other documentation is required to adjudicate this claim/service.	The payer needs a document, such as a trip report. N706 means “Missing documentation.”
18	Exact duplicate claim/service	The trip was billed twice. Replace a denied claim instead of resending it.
B13	Previously paid. Payment for this claim/service may have been provided in a previous payment.	Find the earlier payment on an older remittance
29	The time limit for filing has expired.	Send proof of your first, timely filing. See timely filing limits.
133	The disposition of this service line is pending further review.	Nothing to fix yet. X12 requires the payer to reverse and correct the line when the review ends.

State rules decide when these codes appear. Arizona’s fee-for-service manual (Chapter 14, revised July 31, 2026) denies any NEMT claim sent without the standard AHCCCS Daily Trip Report. It also denies a base rate and its mileage sent on separate claims, and treats several claims for one rider on one day as duplicates. Your state manual lists its own triggers. For fixes by denial type, see NEMT claim denials.

State codes printed next to the national ones

Some Medicaid programs add their own codes to the paper remittance. They explain the national codes, but they stay on paper.

Indiana prints four-digit explanation of benefits (EOB) codes on each remittance. Its remittance module (version 7.0, March 13, 2025) says EOB codes are local and are not sent in the 835, which carries only the national codes. Indiana pairs them like this:

Indiana EOB	National codes in the 835	What it means
0201	CARC 206 with RARC N257	The billing NPI or IHCP provider ID is missing
0203	CARC 16 with RARC N382	The member ID is missing
4033	CARC 4 with RARC N519	The modifier does not fit the procedure code

Other payers’ codes matter when you bill Medicaid second. Arizona’s fee-for-service manual (Chapter 9, revised June 3, 2026) requires the other payer’s remittance with every claim, plus the pages that define its reason and remark codes. A claim without those pages is treated as incomplete and denied.

Codes that were retired

X12 stops codes it no longer needs. A stopped code should not appear on a current remittance.

Code	Old meaning	Stopped
15	The authorization number is missing, invalid, or does not apply to the billed services or provider.	May 1, 2018. Authorization problems now use 197, 198, 284, and 296.
28	Coverage not in effect at the time the service was provided.	October 16, 2003. X12 notes it was redundant to 26 and 27.
138	Appeal procedures not followed or time limits not met.	May 1, 2018
B5	Coverage/program guidelines were not met or were exceeded.	May 1, 2016, replaced by 272 and 273

How to work a reason code, step by step

1. Start with the group code. CO means you absorb it or fix it. PR means the rider may owe it, only if your state allows cost sharing. OA and PI tell you to read on.
2. Look up the reason code. Use the code key on the remittance or the X12 list. Match the code to the right line, since one claim can carry different codes on each line.
3. Read every remark code. For 16, 96, and 252 the remark code is required and names the problem.
4. Pick the action. Write off CO 45. Fix your own error with a corrected claim. Appeal when your claim was right, such as a 197 when you hold the authorization. Bill the right payer for 22, 24, or 109. Wait on OA 133.
5. Check the deadline. A fix still has to reach the payer inside its filing limit.
6. Log it. Record each denial by code on a claim denial log. The same code week after week points to a setup problem, not a one-time slip.