Insurance Appeal Glossary

CO-4 means the modifier on the claim does not match the procedure code billed. In behavioral health, this most often occurs when the -25 modifier is missing from an E/M code billed on the same day as psychotherapy, or when a telehealth modifier is misapplied. CO-4 is an administrative denial, not a medical necessity denial — fix the modifier and resubmit as a corrected claim unless the modifier was correct, in which case appeal with CPT documentation.

CO-11 means the ICD-10 diagnosis code does not medically justify the CPT code billed. In behavioral health, this arises when the diagnosis is too unspecific (e.g. using F32 instead of F32.1) or when the diagnosis category does not match the service type. The fix is usually a corrected claim with a more specific ICD-10 code plus a brief clinical summary — not a formal appeal.

CO-15 means the prior authorization number on the claim cannot be matched to a valid auth in the payer's system. This differs from CO-197 (auth not obtained at all) — here the auth may exist but the number is wrong, missing, or expired. Verify the exact authorization number from your records, confirm it matches the claim, and submit an appeal with the authorization confirmation document.

CO-16 means a required field is missing or incomplete. Always check the accompanying RARC code — it specifies exactly what is missing (NPI, taxonomy code, rendering provider, place of service, etc.). CO-16 is typically corrected by resubmitting the claim with the missing element, not by a formal appeal. If the data was present and the denial is in error, appeal with proof.

CO-18 means the payer believes this claim was already submitted and adjudicated. Pull the original claim and its RA. If the original was paid, provide the remittance. If the original was denied and you are legitimately resubmitting (not appealing), include the original claim number and the denial RA to distinguish the resubmission from a duplicate.

CO-22 means another insurer should pay first. Verify COB order, submit the primary EOB with the secondary claim, or provide documentation showing your payer is primary. For MHPAEA purposes, COB denials are administrative rather than parity violations — the fix is generally documentation, not a parity argument.

CO-29 means the claim was submitted after the payer's filing deadline. Most commercial payers require claims within 90–365 days of the date of service. Appeal with clearinghouse submission timestamps proving timely filing, or cite a documented exception (payer error, enrollment delay). For government programs, cite the specific regulatory exception.

CO-45 is a contractual reduction to the payer's fee schedule rate — not a denial of coverage. It means your billed charge is higher than the maximum allowed. This does not usually require an appeal unless you believe the wrong fee schedule was applied (wrong contract year, in-network rate applied vs. out-of-network rate). Compare against your contract and file a payment dispute if there is a discrepancy.

CO-50 is the most common and most appealable behavioral health denial. The payer says the service was not medically necessary under its criteria. Under MHPAEA (29 CFR § 2590.712), the medical necessity criteria applied to mental health services must be no more restrictive than those for analogous medical/surgical services. Request the payer's criteria in writing, include clinical notes supporting necessity, and invoke MHPAEA comparative analysis. CO-50 denials have the highest MHPAEA relevance of any code.

CO-96 means the service is not covered under the member's plan. Check whether any ACA essential health benefit or MHPAEA parity requirement overrides the exclusion — non-covered behavioral health services may still be required if the plan covers analogous medical services. Review the Summary of Benefits and Coverage before accepting this denial at face value.

CO-97 means the payer considers this service included in a service already billed. In behavioral health, it most commonly occurs when an E/M visit and psychotherapy are billed on the same day without the -25 modifier. The fix is citing CMS NCCI Edit Policy Chapter 11, which explicitly permits same-day E/M + psychotherapy when -25 is applied, plus documentation that the services were separately identifiable.

CO-119 means the patient has hit the maximum visits, days, or dollars for the benefit. For mental health services, benefit maximums are the most direct form of MHPAEA parity violation — if a plan covers unlimited outpatient medical visits but caps therapy at 30 sessions, that is a per-se MHPAEA violation under 29 CFR § 2590.712(c). Request the payer's comparative analysis document and document the comparable medical/surgical benefit limit.

CO-167 means the diagnosis is not covered under the plan. In behavioral health, this often surfaces as an exclusion for specific mental health diagnoses (e.g. adjustment disorder, V-codes, SUD) that would not be excluded if they were medical diagnoses. Selectively excluding behavioral health diagnoses is an NQTL under MHPAEA — demand the CAA 2021 § 203 comparative analysis.

CO-197 is one of the most contested behavioral health denial codes. The payer denies because no prior authorization was obtained. Under MHPAEA, a prior authorization requirement for outpatient mental health visits — when no authorization is required for comparable outpatient medical visits — is an NQTL violation. Document the payer's authorization requirements for physical therapy, specialist visits, or cardiac rehab and compare. Request the NQTL comparative analysis under CAA 2021 § 203.

OA-23 is used when authorization from another payer is needed, or when the payer uses it to deny ongoing therapy authorization. Clarify whether the required authorization is the payer's own or another insurer's (COB context). If the payer is requiring authorization for mental health without a comparable requirement for medical/surgical outpatient services, invoke MHPAEA. Request the NQTL analysis under CAA 2021 § 203.

The Mental Health Parity and Addiction Equity Act of 2008 (29 USC § 1185a) requires group health plans and insurers to apply the same financial requirements and treatment limitations to mental health and substance use disorder (MH/SUD) benefits as to comparable medical and surgical benefits. It is the primary legal lever in behavioral health appeals. Note: the 2024 MHPAEA Final Rule has been subject to federal non-enforcement since May 2025, but the underlying 2008 statute and 2013 Final Rule (29 CFR § 2590.712) remain fully in force, as does the CAA 2021 § 203 disclosure duty.

Section 203 of the Consolidated Appropriations Act of 2021 added an affirmative duty: group health plans must perform and document a written comparative analysis of their non-quantitative treatment limitations (NQTLs) for MH/SUD benefits versus medical/surgical benefits, and provide that analysis to plan participants or the Departments of Labor, HHS, or Treasury on request. This statutory disclosure obligation applies regardless of the 2025 federal non-enforcement of the 2024 MHPAEA Final Rule — the CAA 2021 text is a separate statute enacted by Congress.

The Employee Retirement Income Security Act of 1974 governs most employer-sponsored (self-funded) health plans. Self-funded plans are generally exempt from state insurance laws but are subject to federal MHPAEA, ERISA's claims and appeals procedures (29 CFR § 2560.503-1), and CAA 2021. When appealing a denial from a self-funded employer plan, you invoke federal law rather than your state's insurance code.

The Affordable Care Act's Essential Health Benefits mandate (ACA § 1302) requires non-grandfathered individual and small-group plans sold on or off the exchanges to cover ten benefit categories, including mental health and substance use disorder services. If a plan excludes a behavioral health service covered under the state EHB benchmark, that exclusion violates federal law independent of MHPAEA.

The Americans with Disabilities Act (42 USC § 12101) prohibits discrimination against individuals with mental health disabilities in places of public accommodation, employment, and under Title II (state and local government). In the insurance context, ADA Title III has been used to challenge health plan benefit designs that discriminate against mental health conditions relative to physical conditions. The ADA supplements MHPAEA — it is a separate, independent legal basis for a parity claim.

The 2013 MHPAEA Final Rule (29 CFR § 2590.712) implemented the 2008 MHPAEA statute and established the framework for evaluating financial requirements, quantitative treatment limitations, and non-quantitative treatment limitations (NQTLs). It remains fully in force and is the primary federal regulation a therapist should cite in an appeal. The subsequent 2024 Final Rule updated NQTL standards, but the 2024 rule's NQTL provisions are under federal non-enforcement as of May 2025 — the 2013 rule fills that gap.

The 2024 MHPAEA Final Rule (89 FR 1), issued in September 2024, significantly expanded NQTL requirements, required outcomes-based comparative analyses, and added network adequacy standards for MH/SUD benefits. On May 15, 2025, the Departments of Labor, HHS, and Treasury issued a joint statement announcing non-enforcement of the 2024 rule's core NQTL provisions pending resolution of ERISA Industry Committee v. DOL. However: (1) the 2013 Final Rule remains in force; (2) the CAA 2021 § 203 disclosure obligation is unaffected; and (3) state parity laws in CA, NY, MA, IL, OR, WA, and CO have adopted independent standards that may exceed the 2013 federal floor.

A Claim Adjustment Reason Code (CARC) is a standardized code published by Washington Publishing Company (WPC) and adopted by CMS that explains why a payer paid a claim at a different amount than billed, or denied it entirely. CARCs appear on the Electronic Remittance Advice (ERA/835) and on paper EOBs. The group prefix (CO = Contractual Obligation, OA = Other Adjustment, PR = Patient Responsibility) determines who is financially responsible. AppealWin covers the 15 most common behavioral health CARC codes.

A Remittance Advice Remark Code (RARC) supplements a CARC with additional detail about why a claim was adjusted or denied. RARCs are also published by WPC and begin with 'N' (informational) or 'M' (supplemental) or other prefixes. CO-16 (missing information) always arrives with a RARC specifying exactly what field is missing — identifying the RARC is the first step to correcting the claim.

An Explanation of Benefits (EOB) is the paper or electronic document a payer sends to the member (and sometimes the provider) after adjudicating a claim. It shows the billed amount, the allowed amount, the plan's payment, the patient's responsibility, and any denial reason codes. The EOB is the primary source document for identifying CARC and RARC codes and triggering an appeal.

The Electronic Remittance Advice (ERA), transmitted as an ANSI X12 835 transaction set, is the electronic version of the EOB sent to providers. It contains the same CARC and RARC codes as the paper EOB. Most billing software and clearinghouses can display and export 835 files. The 835 is the most reliable source for denial code data because paper EOBs sometimes omit remark codes.

The ANSI X12 837 transaction set is the electronic claim format used to bill professional (837P) and institutional (837I) services to health plans. When a claim is submitted via a clearinghouse, it is transmitted as an 837. The clearinghouse submission report with timestamp is the best proof of timely filing for CO-29 (timely filing) appeals.

Current Procedural Terminology (CPT) codes are five-digit codes published by the AMA that identify medical and clinical procedures billed to health plans. For behavioral health, the most common CPT codes are: 90791 (psychiatric diagnostic evaluation), 90832 (psychotherapy 16-37 min), 90834 (psychotherapy 38-52 min), 90837 (psychotherapy 53+ min), 90846 (family therapy without patient), 90847 (family therapy with patient), and 90853 (group therapy). Selecting the wrong CPT code is the most common billing error in behavioral health.

The Healthcare Common Procedure Coding System (HCPCS) consists of Level I codes (identical to CPT codes) and Level II alphanumeric codes (A0000–Z9999) for supplies, equipment, and non-physician services. In behavioral health, Level II HCPCS codes occasionally appear for telehealth platforms (e.g. GT modifier, Q3014 for telehealth facility fee) and certain crisis stabilization services. Medicaid billing often requires HCPCS codes not used in commercial billing.

The International Classification of Diseases, 10th Revision (ICD-10-CM) codes document the patient's diagnosis on a claim. Behavioral health ICD-10 codes fall primarily in the F00–F99 (Mental, Behavioral, and Neurodevelopmental disorders) chapter. The diagnosis must be specific enough to medically justify the CPT code billed — a CO-11 denial (diagnosis inconsistent with procedure) typically means the code is too unspecified or does not map to the service type.

A two-character code appended to a CPT code to indicate that a service was altered in some way without changing its definition. In behavioral health, the most critical modifiers are: -25 (significant, separately identifiable E/M on the same day as another procedure — required when billing same-day E/M + psychotherapy); -59 (distinct procedural service, used to override bundling edits); -GT (synchronous telecommunication, used for telehealth on some payers); and place-of-service modifiers 95/GT. Missing or incorrect modifiers are the #1 cause of CO-4 and CO-97 denials.

A two-digit CMS code on the claim indicating where the service was rendered. Key codes for behavioral health: POS 11 (Office — in-person sessions at a private practice); POS 02 (Telehealth — originating site); POS 10 (Telehealth — patient's home, the most common code since COVID-era waivers). Mismatching POS code and telehealth modifiers is a common billing error generating CO-4 or CO-16 denials.

A Hierarchical Condition Category (HCC) is a risk-adjustment model used by CMS for Medicare Advantage and some Medicaid managed care plans. HCC codes group ICD-10 diagnoses into clinically meaningful risk categories; the higher the HCC score, the higher the plan's capitation payment from CMS. For behavioral health billing, accurately documenting psychiatric diagnoses with their full HCC-mapped ICD-10 specificity is important for risk adjustment accuracy, though HCC is not directly implicated in typical denial appeals.

A formal request to a health plan to reconsider a denied or reduced claim. An appeal is distinct from a grievance (a complaint about service quality) and from a dispute (a billing disagreement outside the formal appeals process). Under ERISA and ACA regulations, plans must provide at least one internal appeal level and, for adverse benefit determinations, access to external review.

A formal appeal filed directly with the insurance company (or its behavioral health subcontractor) asking it to reconsider its own denial. ERISA requires plans to provide at least one level of internal appeal with a decision within 60 days for pre-service claims and 60 days for post-service claims. Internal appeal exhaustion is typically required before external review rights attach.

External review is the right to have a denial reviewed by an independent review organization (IRO) that has no financial relationship with the payer. Under the ACA and ERISA (for grandfathered plans), external review must be available for adverse benefit determinations. The IRO's decision is binding on the plan. Most states have their own external review laws; for self-funded ERISA plans, the federal external review process under the ACA applies.

A telephonic consultation between the treating clinician and the plan's medical director (or a contracted reviewer) to discuss the clinical basis for a prior authorization denial or concurrent review denial. Peer-to-peer is not a formal appeal right — it is a voluntary process offered by most payers. It can be highly effective for medical necessity denials and should be requested before or alongside a formal appeal when the denial is clinical rather than administrative.

A requirement that the treating clinician obtain the payer's approval before providing a service. Under MHPAEA (29 CFR § 2590.712(c)(4)), a prior authorization requirement applied to mental health outpatient services is an NQTL — the payer must demonstrate it applies the same authorization standard to comparable medical/surgical outpatient services. Requiring authorization for therapy while waiving it for physical therapy or specialist visits is a classic parity violation.

Ongoing payer review of a service already in progress (e.g. continued authorization for an inpatient stay or an IOP program). Payers conducting concurrent review must apply the same frequency and burden of proof to behavioral health as to analogous medical/surgical services. A concurrent review denial often generates CARC CO-50 (not medically necessary) or CO-119 (benefit maximum).

A payer's review of a claim after services have already been rendered — typically triggered by a post-service denial. Retrospective review of emergency mental health services is heavily regulated: many states and ACA regulations prohibit retrospective denial of emergency stabilization services even if prior authorization was not obtained.

The standards a payer uses to determine whether a service is clinically appropriate and required. Under MHPAEA, the criteria applied to mental health services must be no more restrictive than the criteria applied to analogous medical/surgical services. Payers must disclose their MH/SUD medical necessity criteria on request (CAA 2021 § 203). The landmark Wit v. United Behavioral Health (9th Cir.) case found UHC's internal criteria to be more restrictive than generally accepted standards.

A Non-Quantitative Treatment Limitation is any restriction on the scope or duration of mental health benefits that is not expressed as a number — including prior authorization requirements, medical necessity criteria, step therapy requirements, and network adequacy standards. Under MHPAEA, NQTLs applied to MH/SUD benefits must be no more restrictive than those applied to comparable medical/surgical benefits, both in terms of the written standards and as applied in practice.

The written document a health plan must produce under CAA 2021 § 203 demonstrating that its NQTLs for MH/SUD benefits are no more restrictive than those for comparable medical/surgical benefits. On request, the plan must provide the analysis to plan participants, treating providers, and the Departments of Labor, HHS, and Treasury. Requesting this document in writing is a powerful step in any MHPAEA-based appeal because noncompliance triggers regulatory action.

Network adequacy refers to whether a plan's provider network has sufficient in-network providers to meet member demand without unreasonable delay. Under the 2024 MHPAEA Final Rule (subject to non-enforcement), network adequacy for MH/SUD must equal that for medical/surgical. Under the 2013 rule and several state laws (CA, OR, WA), inadequate MH networks are still an NQTL violation. Network inadequacy is frequently the root cause of out-of-network balance-billing and OON claim denials.

Services received from a provider who does not have a contract with the member's health plan. OON benefits are often reduced or excluded. However, when a plan's in-network behavioral health provider network is inadequate, members have the right to use OON providers at in-network cost-sharing rates — this is called the 'single case agreement' or 'network gap exception.' Network adequacy failures can be appealed as MHPAEA NQTL violations.

When a patient is covered by more than one health plan, coordination of benefits rules determine which plan pays first (primary) and which pays second (secondary). Most states use the NAIC COB Model Regulation to resolve primary/secondary order. Incorrect COB sequencing generates CARC CO-22 denials. The secondary payer will not adjudicate until it receives the primary EOB.

A claim submitted with all required information, no defects, and no need for additional investigation to adjudicate. State prompt-payment statutes impose time limits for paying clean claims — typically 30–45 days. A claim rejected for missing information (CO-16) may not qualify as a clean claim and restarts the prompt-payment clock once corrected.

State laws requiring payers to pay clean claims within a specified number of calendar days. Penalties for late payment often include interest. Windows vary: CA requires 30 days (EFT) / 45 days (paper) under Cal. Ins. Code § 10123.13; TX requires 30 days EFT under Tex. Ins. Code § 1301.103; WA allows 60 days paper under RCW 48.43.005. Self-funded ERISA plans are generally exempt from state prompt-payment laws.

Individual psychotherapy is one-on-one therapeutic treatment of a mental health condition provided by a licensed clinician (LCSW, LPC, LMFT, psychologist, psychiatrist). Billed with CPT codes 90832 (16-37 min), 90834 (38-52 min), or 90837 (53+ min). These are the most frequently denied behavioral health services and the primary target of MHPAEA parity arguments.

Psychotherapy sessions involving family members to address relational and systemic contributors to a patient's mental health condition. CPT 90847 (family therapy with patient present) and 90846 (family therapy without patient) are used. Family therapy denials often cite the 'not medically necessary' reason (CO-50) — MHPAEA comparisons to analogous family-focused medical services (e.g. family counseling for a child with a chronic physical illness) can be effective.

Psychotherapy conducted with multiple patients simultaneously. Billed with CPT 90853 (group therapy) per patient per session. Group therapy denials frequently cite inadequate medical necessity documentation. Each patient in a group should have documentation of the therapeutic benefit of group format relative to individual treatment.

Short-term intensive services aimed at stabilizing a patient experiencing a psychiatric emergency. Crisis intervention typically uses CPT 90839 (first 60 min) and 90840 (each additional 30 min). ACA regulations and many state laws prohibit retrospective denial of emergency mental health stabilization — authorization obtained after the fact is not required for true emergencies. MHPAEA requires crisis services to be covered comparably to emergency medical/surgical care.

An Intensive Outpatient Program provides structured mental health or SUD treatment, typically 9+ hours per week in three-hour sessions. IOPs are commonly denied via CO-50 (medical necessity) or CO-119 (benefit maximum). These are among the highest-value MHPAEA targets: payers that cover intensive outpatient physical rehabilitation without visit caps cannot impose stricter limits on behavioral health IOP under MHPAEA.

A Partial Hospitalization Program provides hospital-level care without an overnight stay, typically 20+ hours per week. PHPs are the behavioral health equivalent of day surgery or medical day programs. Because PHPs are more intensive and expensive than IOPs, they are frequently subject to stringent prior authorization and concurrent review. PHP denials are strong MHPAEA candidates: compare the payer's medical necessity standard for medical day programs.

Substance Use Disorder (SUD) refers to a cluster of cognitive, behavioral, and physiological symptoms indicating continued use of a substance despite significant related problems. MHPAEA explicitly covers SUD benefits alongside mental health benefits — a plan cannot exclude SUD treatment that it would cover for comparable medical conditions. SUD billing frequently uses H-codes (Medicaid) or revenue codes (facility) alongside CPT procedure codes.

Evaluation and Management (E/M) codes (99202–99215) are used when a prescribing clinician (psychiatrist, psychiatric NP) performs a medical assessment on the same day as psychotherapy. To bill both an E/M and a psychotherapy code on the same date, the E/M must be significant and separately identifiable from the psychotherapy — evidenced by the -25 modifier on the E/M code. Omitting -25 is the leading cause of CO-97 (bundled service) denials. The +25 modifier conflict is the most litigated billing issue in combined psychiatric-therapy billing.