Denial code · CARC 151
CARC 151 (CO-151): frequency or units not supported on BH claims
What CO-151 means for group therapy units, ABA hours, testing time and TMS sessions, how it differs from benefit limits and authorization limits, and how to answer it.
By the ClaimParrot editorial team · Who we are · Reviewed · 4 min read · 6 cited sources
Quick answer
CARC 151 means the payer accepted the kind of service but decided the information sent does not justify how many units, or how often, it was billed, so it paid fewer or none. In behavioral health it can follow group psychotherapy billed in several units, ABA units above what the plan expects, testing hours and frequent therapy. Fix unit errors; appeal genuine disputes with records.
| Item | Detail |
|---|---|
| Code | CARC 151 · Active on the X12 list; checked October 14, 2026 |
| Plain meaning | The payer accepted the type of service but decided the information sent does not justify the number of units or how often it was billed, so it paid fewer or none. |
| Group codes seen | CO (contractual obligation) |
| Common remark codes | N362, N435, N640, M53 |
| Where it shows up in BH | Outpatient therapy, ABA therapy, Psychiatry and medication management, TMS |
| Usual next step | Fix unit errors; appeal frequency judgments with records |
| Related codes | CARC 119, CARC 198, CARC 50, CARC 97 |
What does CARC 151 mean?
CARC 151 is a quantity judgment: the service type is fine, the amount is not. Sometimes it reflects a unit error, such as an untimed code billed in time-based units; sometimes it reflects the payer's view that the record does not justify the frequency. Tell those apart before responding.
What does CO-151 mean for who pays?
CO-151 is the usual form: the provider carries the units the payer would not accept, and the patient is not billed for them. If a payer reports PR-151, check the plan terms, your contract and any advance agreement with the patient before a statement goes out.
| Group + reason | What it signals | Who usually carries the balance |
|---|---|---|
| CO-151 | Units or frequency judged unsupported | Provider; not the patient |
Which remark codes come with CARC 151?
These remark codes show whether a unit rule or a documentation judgment drove the cut.
| Remark code | What it adds | What to check |
|---|---|---|
| N362 | The days or units billed are more than the payer accepts as a maximum. | The per-day or per-span unit limit applied |
| N435 | The number or frequency billed goes past what is allowed in the period without supporting documentation. | Documentation that would support the higher frequency |
| N640 | The number or frequency billed goes past what was approved or allowed for the period. | The frequency allowed for the period |
| M53 | The days or units of service on the line are missing, incomplete or not valid. | The units entered on the line |
Where does CARC 151 show up in behavioral health?
Behavioral-health 151 denials follow codes with unit or frequency rules. NCCI says a practitioner reports one unit of group psychotherapy per date, however long the session; ABA and testing codes are timed and often capped by policy; TMS courses and high-frequency therapy draw frequency reviews.
| Care setting | Typical trigger | Check first |
|---|---|---|
| Outpatient therapy | Group psychotherapy (90853) billed as several units for one long session | NCCI's one-unit-per-date rule for practitioners |
| ABA therapy | Technician units above what the payer considers supported | Treatment plan hours, session notes and progress data |
| Psychiatry and medication management | Psychological testing hours above the payer's threshold | Testing plan, time records, any approval for extra hours |
| TMS | Sessions beyond the course the policy allows | Session log and response measures |
Corrected claim, reconsideration or appeal: which fits CARC 151?
Separate unit errors from judgments. If units were miscounted or an untimed code was billed in time units, correct the claim. If the payer judged the frequency excessive, send a reconsideration or appeal with the treatment plan, session notes and progress data that justify how often the service was needed.
- 01Recount units against the code's definition and the session times in the notes.
- 02Unit error: correct and resubmit. Do not split units across lines to get around a unit edit.
- 03Frequency judgment: appeal with the treatment plan, the clinical reason for the frequency and measured progress.
What evidence resolves a CARC 151?
For a frequency dispute, show why this patient needed this much, this often. Session-level documentation with times, a treatment plan that sets the frequency, and objective progress or risk measures carry the most weight. For unit errors, the code definition and the time records are enough.
- Session notes with start and stop times
- The treatment plan specifying frequency or hours
- Standardized measures showing need or progress
- The code's unit definition and any payer unit policy
- Approvals covering extra units, if any
How do Medicare, Medicaid and commercial plans differ on CARC 151?
Medicare and Medicaid apply medically unlikely edits that cap the units per day for many codes, and Medicaid states add their own service limits; commercial payers set frequency rules in medical and payment policies. If the limit came from an approval, the question is really CARC 198.
| Program | What differs |
|---|---|
| Medicare (Original) | Medically unlikely edits cap the units per day reported for many codes; using extra lines or modifiers to get around them is discouraged. |
| Medicaid | Medicaid NCCI applies (one unit of group psychotherapy per date for a practitioner), and states set limits, such as Ohio's 12 hours of psychological testing a year before prior authorization. |
| Commercial and ERISA plans | Frequency rules sit in medical and payment policies; ask for the policy the payer applied. |
CO-151 on group therapy units
A fictional IOP billed group psychotherapy as three units for a three-hour group led by one therapist. The payer allowed one unit and returned two as CO-151 with N362.
NCCI treats each group session as one unit for a practitioner, whatever its length, so the program corrected its charge rules and checked how each payer wants IOP group time reported, since some state Medicaid programs allow timed group codes in IOP and PHP.
Names, dates, payers and amounts are invented for illustration.
What a ClaimParrot packet does here
- When CARC 151 needs an appeal rather than a corrected claim, an appeal packet answers the payer's stated reason.
- A person checks every packet: $29, or $99 for level of care, and the first is free (pricing).
- No remittances, records or PHI are sent: just the reason and a de-identified summary (Trust & security).
Every draft is reviewed and approved by your team. ClaimParrot does not make medical-necessity determinations or give legal advice.
Frequently asked questions
How is CARC 151 different from CARC 119?
119 says a benefit limit for the period is used up. 151 says the payer does not think the record supports the number or frequency billed. One is about the plan's count; the other is about justification.
How is CARC 151 different from CARC 198?
198 means you billed past an approval. 151 is the payer's own judgment about frequency or units, with or without an approval in place.
Can we bill group psychotherapy by the hour?
Not with 90853 for a practitioner under NCCI, which counts each group session as one unit regardless of length. Some state Medicaid programs allow timed group codes in IOP and PHP; check yours.
Should we split units across lines to get past a unit edit?
No. NCCI guidance warns against using extra lines or modifiers to get around unit edits. If the units were clinically needed, document why and appeal.
Primary sources and references
- Claim Adjustment Reason Codes (CARC) code list · X12 · checked
- Remittance Advice Remark Codes (RARC) code list · X12 · checked
- 2026 Medicaid NCCI Policy Manual, Chapter XI §C (psychiatric services) and MUE guidance · CMS · checked
- Ohio Medicaid Behavioral Health Provider Requirements and Reimbursement Manual, v1.28.1 (March 2026) · Ohio Department of Medicaid · checked
- Aetna applied behavior analysis medical necessity guide · Aetna · checked
- Aetna provider disputes and appeals overview · Aetna · checked
General information, not legal or medical advice. Payer rules change; confirm deadlines and requirements in the notice and the plan documents. Read our editorial policy.
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