ABA Denial CO 24 Duplicate Service: How to Appeal and Win

CO 24 is one of the most frustrating denial codes we see in ABA billing, and it is also one of the most misunderstood. The adjustment reason code CO 24 means the payer has determined the submitted claim is a duplicate of a previously adjudicated service. In theory, that sounds straightforward. In practice, we are seeing CO 24 fire incorrectly on legitimate, separately billable ABA sessions at roughly 60 to 70 percent of the practices we onboard, particularly those billing high-volume CPT 97153 and 97155 combinations across multiple technicians and supervisors in the same date of service.

The pattern we are seeing across approximately 50 behavioral health and ABA practices is that CO 24 denials cluster around three specific billing scenarios, all of which are recoverable with the right documentation and appeal strategy. Left unworked, a mid-sized ABA practice billing 2,500 to 4,000 units of 97153 per month can lose between $8,000 and $22,000 in monthly revenue to CO 24 write-offs that should never have been written off. This post walks through exactly what is triggering these denials and what you can do about it starting tomorrow.

What CO 24 Actually Means in ABA Billing

CO 24 stands for “Charges are covered under a capitation agreement or managed care plan.” Wait, that is not the duplicate language. Let us be precise here because the code has dual usage depending on payer. Under the standard CARCs published by X12, CO 24 is specifically the capitation/managed care overlap code. However, many Medicaid managed care organizations and commercial payers including Optum, Aetna, and BCBS plans repurpose or stack CO 24 alongside RARC codes like M86 (duplicate service, same date, same provider) or MA04 (secondary payer information needed). When you see CO 24 on an ABA claim, your first step is to pull the full EOB and identify the accompanying RARC. That RARC tells you the real story.

The distinction matters because the appeal path for a capitation conflict is entirely different from a true duplicate claim error. Misreading CO 24 as simply “you billed this twice” leads practices to either resubmit unnecessarily or write off valid charges. Neither outcome is acceptable.

The Three Most Common CO 24 Triggers in ABA

Based on our AR analysis across ABA clients ranging from 10-technician operations to multi-site practices with 80-plus BCBAs, here are the three scenarios that generate the overwhelming majority of CO 24 denials:

• Concurrent billing of 97153 and 97155 on the same date of service: CPT 97153 (adaptive behavior treatment by protocol, each 15 minutes) and CPT 97155 (adaptive behavior treatment with protocol modification, each 15 minutes) can and should be billed on the same date when a BCBA conducts direct supervision while the RBT delivers treatment. Payers whose systems lack proper ABA-specific logic flag this as a duplicate. The fix is appending modifier HN (Bachelor’s level clinician) to 97153 and HO (Master’s level clinician) to 97155, and ensuring your place of service code is correct. POS 12 (home) and POS 99 (other) are where we most often see the modifier omission.
• Multiple RBTs billing 97153 for the same client on the same date: When two RBTs work with a single client in the same day across split sessions, payers frequently collapse the second claim as a duplicate if the units, provider NPI, or session times are not clearly differentiated. Each session must carry the rendering provider NPI of the specific RBT, not the group NPI alone, and the claim-level notes must reflect distinct start and stop times.
• Reassociation failures after payer system migrations: We have seen a notable uptick in CO 24 errors in 2025 and into 2026 tied to payer platform migrations at several large Medicaid MCOs. Claims submitted pre-migration that were pending get adjudicated post-migration and then the resubmission gets flagged as a duplicate. This is a payer-side error and is 100 percent appealable.

Step-by-Step CO 24 Appeal Process for ABA Claims

Here is the appeal workflow we build into every ABA client’s denial management protocol. These steps assume a commercial or Medicaid MCO payer, which covers the majority of ABA reimbursement volume in 2026.

1. Pull the original claim and the denied claim side by side. Confirm whether the TCN (Transaction Control Number) on the denial references the same date of service, same CPT, same units, and same rendering NPI. If any element differs, you have grounds for a clean appeal on the basis that these are not duplicate services.
2. Document the clinical distinction. Your appeal letter must include the session notes with timestamped start and stop times, the specific BCBA or RBT rendering the service, and the clinical rationale for why both services were medically necessary on that date. For 97155 specifically, include the protocol modification documentation that justifies BCBA direct involvement.
3. Reference payer policy and CPT editorial guidance. CPT guidelines explicitly allow concurrent billing of 97153 and 97155. Cite the AMA CPT 2026 manual language in your appeal. For Medicaid MCO denials, cite the state’s ABA coverage bulletin. Most states with Medicaid ABA coverage have published guidance permitting these code combinations.
4. Submit with modifier correction if applicable. If the denial was partly triggered by a missing or incorrect modifier, resubmit as a corrected claim (frequency code 7 on the 837P) rather than a straight appeal. Corrected claims typically move faster through the adjudication queue.
5. Track appeal outcomes by denial sub-type. We track CO 24 appeal win rates separately for capitation-related denials versus true duplicate flags. Our ABA clients are averaging a 74 percent first-level appeal overturn rate on CO 24 denials when the appeal includes timestamped session notes and modifier documentation.

What a CO 24 Backlog Actually Costs a Practice

The dollar math here is not abstract. A practice billing an average of 3,000 units of 97153 monthly at a blended commercial rate of $12.50 per unit generates $37,500 in monthly 97153 revenue. If 15 percent of those claims hit CO 24 and go unworked for 90 days, you are looking at $5,625 in monthly exposure per code, before accounting for 97155 and 97157 denials that often travel with the same root cause. Across a full calendar quarter, that is $16,875 per CPT code in potential write-offs. Practices with 97155 billing running at 800 units monthly at $18.00 average add another $2,160 in monthly exposure at the same 15 percent denial rate.

The practices we work with that have implemented structured CO 24 appeal workflows recover between 68 and 80 percent of previously written-off balances when we conduct retroactive denial reviews going back 12 months, within timely filing limits. Not every payer allows 12-month retroactive appeals, so knowing your payer-specific timely filing window is critical before you begin a recovery project.

Modifier Strategy and POS Accuracy as Prevention

Most CO 24 denials in ABA are preventable at the point of claim submission. The three highest-leverage prevention steps are modifier accuracy, rendering NPI discipline, and session note timestamp compliance. We standardize these across our ABA clients through claim scrubbing rules built into the clearinghouse layer before claims ever reach the payer. If you are not scrubbing for modifier completeness on 97153 through 97158 codes before submission, you are generating avoidable denial volume.

Modifier pairs that matter most for CO 24 prevention in ABA billing in 2026: HN on 97153 when billed by a bachelor’s-level technician, HO on 97155 and 97157 when billed by a BCBA, and modifier 59 (distinct procedural service) when two ABA service types are billed on the same date and payer policy requires explicit differentiation. Some payers, particularly certain Blue Cross plans, have moved to modifier XS (separate structure) in place of 59. Know your payer’s current preference before building your scrubbing rules.

Place of service accuracy also drives CO 24 triggers. If your system defaults all claims to POS 11 (office) but your RBTs are delivering services in the home (POS 12) or school (POS 03), payer systems may match incorrectly against prior claims and generate false duplicates. POS code discipline is a foundational billing hygiene issue that compounds across denial types, not just CO 24.

When CO 24 Is Actually a Parity Violation in Disguise

This is a scenario we want ABA billing and compliance teams to take seriously. We have seen payers systematically apply CO 24 to concurrent ABA service combinations that they would not deny for analogous physical health services billed on the same date. When a payer’s internal logic flags concurrent 97153 and 97155 as duplicates but would not flag concurrent physical therapy evaluation and treatment codes on the same date, that is a potential MHPAEA parity issue. If you are seeing a high-volume, systematic pattern of CO 24 on specific code combinations that is not present on the physical health side of the same payer’s book, that pattern is worth documenting and escalating. We cover the MHPAEA appeal framework in detail in our post on mental health parity act appeals and how behavioral health practices are leaving money on the table, and the same analytical approach applies directly to ABA denial patterns.

Start Your CO 24 Recovery Before Timely Filing Closes the Window

If your practice has been writing off CO 24 denials without a structured appeal process, the recovery opportunity is real and it has a hard deadline. Most commercial payers allow 180 days from the original remittance date to file a first-level appeal. Medicaid timely filing rules vary by state but commonly run 90 to 180 days from the denial date. Every week you wait, a portion of your recoverable balance ages out permanently. We offer a free 30-day denial audit where we pull your CO 24 history, quantify the recoverable balance, and map the appeal strategy specific to your payer mix. If you want to see exactly what your practice has left on the table and what can still be recovered, schedule a time directly on our calendar at https://calendar.app.google/zF3c44hYGRjEf5U26. No generalities, no sales pitch — just your numbers.