What is ICD-10 code F84.2?

F84.2 is the ICD-10-CM diagnosis code for Rett's syndrome. A rare genetic disorder where a child develops normally for the first 6-18 months, then loses previously acquired skills in language, motor function, and social abilities. F84.2 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. F84.2 is part of the FY2026 ICD-10-CM code set published by the CMS National Center for Health Statistics and used on Medicare, Medicaid, and commercial claims in the United States.

Yes. F84.2 is a billable ICD-10-CM code that can be reported on professional and institutional claims as a primary or secondary diagnosis when the provider documentation supports the specific condition described. Billable codes in ICD-10-CM are coded to the highest level of specificity available for the documented diagnosis. F84.2 is current for fiscal year 2026 and remains in the active CMS ICD-10-CM code set published by the National Center for Health Statistics.

What is the V28 vs V24 difference for F84.2?

F84.2 does not map to a payment HCC under either the CMS-HCC V28 risk adjustment model or the older V24 model. V28 reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition. Because neither model rewards F84.2 with a risk adjustment factor, it contributes zero to a Medicare Advantage beneficiary's RAF score in either version. Coders should check whether provider documentation supports a more specific child code that does map to a payment HCC.

What should coders know when reporting F84.2?

Confirm the diagnosis is Rett syndrome specifically, as it has distinct diagnostic criteria including normal development followed by regression Code the associated seizures separately if present (G40 codes)

F84.2: Rett's syndrome — ICD-10

ICD-10-CM Code View

HCC Buddy Code Card

Digital ICD-10 code-book layout with official code detail, always-visible risk models, Code Trumping, and Buddy coding guidance.

Code lookupF84.2Effective periodFY2026 Apr Update (Apr 1-Sep 30)

FY 2026 Apr update / Mental, Behavioral and Neurodevelopmental disorders (F01-F99) / Pervasive and specific developmental disorders (F80-F89)

F84.2

Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidance

Rett's syndrome

A rare genetic disorder where a child develops normally for the first 6-18 months, then loses previously acquired skills in language, motor function, and social abilities.

Buddy Insight

Rett syndrome is a rare genetic neurodevelopmental disorder caused predominantly by mutations in the MECP2 gene, affecting primarily females.

CMS-HCC V28

Not risk-adjusted for this model/period.

RAF 0

CMS-HCC V24

Not risk-adjusted for this model/period.

RAF 0

ACA/HHS

Not risk-adjusted for this model/period.

RAF 0

ESRD/PACE

Not risk-adjusted for this model/period.

RAF 0

RXHCC

Mapped

HCC 133

Personality Disorders, Anxiet...

RAF 0.0

Code Trumping

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Code Book Path

Official

F84Pervasive developmental disorders

F84.2Rett's syndrome

Inclusion Terms

Official

ICD-10-CM does not list inclusion terms for F84.2 in this effective period.

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for F84.2 in this effective period.

Related Child Codes

Official

F84.0Autistic disorder

F84.3Other childhood disintegrative disorder

F84.5Asperger's syndrome

F84.8Other pervasive developmental disorders

F84.9Pervasive developmental disorder, unspecified

Includes

Official

ICD-10-CM does not list Includes notes for F84.2 in this effective period.

Excludes 1

Official

Asperger's syndrome (F84.5)
Autistic disorder (F84.0)
Other childhood disintegrative disorder (F84.3)

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for F84.2 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for F84.2 in this effective period.

Code Also

Official

any associated medical condition and intellectual disabilities

Buddy Documentation Tip

HCC Buddy guidance

Provider documentation of Rett syndrome diagnosis, ideally with genetic confirmation (MECP2 mutation)

Documentation of developmental regression following a period of normal early development

Evidence of characteristic features: loss of purposeful hand skills, stereotypic hand movements, gait abnormalities

Assessment of current functional status including communication, mobility, and feeding

MEAT Support

HCC Buddy guidance

Provider documentation of Rett syndrome diagnosis, ideally with genetic confirmation (MECP2 mutation)

Documentation of developmental regression following a period of normal early development

Evidence of characteristic features: loss of purposeful hand skills, stereotypic hand movements, gait abnormalities

Assessment of current functional status including communication, mobility, and feeding

Audit Caution

HCC Buddy guidance

Coding Rett syndrome as autistic disorder (F84.0) — while they share some features, Rett is a distinct diagnosis

Failing to code all comorbid conditions separately (epilepsy, scoliosis, nutritional difficulties)

Not recognizing atypical Rett syndrome variants that may present differently from classic Rett

Using F84.9 (unspecified PDD) when Rett syndrome has been specifically diagnosed

Common Mistakes

HCC Buddy guidance

F84.3 — Other childhood disintegrative disorder; both involve regression but differ in age of onset and clinical features

F84.0 — Autistic disorder; Rett syndrome has autistic features but is a distinct genetic entity with regression and characteristic hand stereotypies

G40.x — Epilepsy; seizures in Rett syndrome are coded separately

Q87.89 — Other specified congenital malformation syndromes; Rett is a neurodevelopmental disorder, not a malformation syndrome

Last updated: FY2026 ICD-10-CM Apr update, Apr 1, 2026 through Sep 30, 2026. CMS-HCC V28 is 100% phased in for payment year 2026.

Is F84.2 an HCC code?

No. F84.2 is a billable ICD-10-CM code but does not map to any HCC category in V28, V24, ESRD, or RxHCC.

HCC Category Mapping

RxHCCHCC 133, Personality Disorders, Anxiety, and Other Specified Mental Disorders

0.000

RAF weights shown are the community, non-dual, aged base weights from the CMS risk adjustment model file. Actual per-patient RAF contribution depends on member segment, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains in use during the transition and for historical data.

MEAT Criteria for F84.2

For F84.2to count as a valid HCC diagnosis in a given encounter, the provider's documentation must show MEAT: Monitor, Evaluate, Assess, or Treat. A diagnosis from a prior year does not carry forward automatically, it has to be re-documented and supported each calendar year.

MMonitor: signs, symptoms, disease progression, or lab trending documented in the note
EEvaluate: test results, medication response, or physical findings reviewed by the provider
AAssess: explicit mention in the assessment or plan with acknowledgment of status
TTreat: medication, referral, procedure, therapy, or counseling tied to the diagnosis

Only one of M/E/A/T is required to support the code, but the documentation must be specific enough to show that the provider actually addressed F84.2 during that encounter, not just copy-forwarded from a problem list.

What This Code Means

F84.2 is the ICD-10-CM diagnosis code for rett's syndrome. A rare genetic disorder where a child develops normally for the first 6-18 months, then loses previously acquired skills in language, motor function, and social abilities. F84.2 sits in the ICD-10-CM chapter for mental, behavioral and neurodevelopmental disorders (f01-f99), within the section covering pervasive and specific developmental disorders (f80-f89).

F84.2 is a billable ICD-10-CM code but does not map to a payment HCC under the CMS-HCC V28, V24, ESRD, or RxHCC risk adjustment models. It can be reported on Medicare Advantage encounter data submissions but it does not contribute to a beneficiary's RAF score and therefore does not affect risk-adjusted payments to the plan.

F84.2 does not map to any HCC in V28 or V24 models. RxHCC maps to 133 (0.000). No direct RAF score impact. Comorbid conditions such as epilepsy (HCC 135 in V28) may carry independent risk adjustment weight. Coders reviewing F84.2 should check whether additional documentation would support a more specific child code in the same hierarchy that does map to a payment HCC, capturing the correct specificity is the highest-impact RAF improvement available within accurate coding.

HCC Buddy maintains structured V28 and V24 mapping, RAF weights, and MEAT documentation criteria for F84.2 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•Confirm the diagnosis is Rett syndrome specifically, as it has distinct diagnostic criteria including normal development followed by regression
•Code the associated seizures separately if present (G40 codes)

Clinical Significance

Rett syndrome is a rare genetic neurodevelopmental disorder caused predominantly by mutations in the MECP2 gene, affecting primarily females. It is characterized by normal early development followed by regression of language and purposeful hand skills, development of stereotypic hand movements, and gait abnormalities. Patients require lifelong multidisciplinary care for seizures, scoliosis, breathing irregularities, and nutritional challenges.

Documentation Requirements

✓Provider documentation of Rett syndrome diagnosis, ideally with genetic confirmation (MECP2 mutation)
✓Documentation of developmental regression following a period of normal early development
✓Evidence of characteristic features: loss of purposeful hand skills, stereotypic hand movements, gait abnormalities
✓Assessment of current functional status including communication, mobility, and feeding
✓Documentation of comorbid conditions (epilepsy, scoliosis, breathing abnormalities) and current management

Excludes 1, Do NOT code together

Asperger's syndrome (F84.5)
Autistic disorder (F84.0)
Other childhood disintegrative disorder (F84.3)

Commonly Confused Codes

•F84.3: Other childhood disintegrative disorder; both involve regression but differ in age of onset and clinical features
•F84.0: Autistic disorder; Rett syndrome has autistic features but is a distinct genetic entity with regression and characteristic hand stereotypies
•G40.x: Epilepsy; seizures in Rett syndrome are coded separately
•Q87.89: Other specified congenital malformation syndromes; Rett is a neurodevelopmental disorder, not a malformation syndrome