What is ICD-10 code A81.89?

A81.89 is the ICD-10-CM diagnosis code for Other atypical virus infections of central nervous system. It belongs to the ICD-10-CM chapter for certain infectious and parasitic diseases (a00-b99). A81.89 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. A81.89 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. A81.89 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. A81.89 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.

Which HCC does A81.89 map to under V28?

A81.89 maps to CMS-HCC V28 127 (Dementia, Mild or Unspecified). The category number and label come from the CMS-HCC V28 model file for payment year 2026.

What does A81.89 pay in RAF?

A81.89 carries a community, non-dual, aged (CNA) RAF weight of 0.341 under CMS-HCC V28 for payment year 2026. The weight comes from the CMS V28 relative-factor file, not a hand-typed table.

What is the V28 vs V24 difference for A81.89?

CMS phased in the V28 risk adjustment model over payment years 2024 (33%), 2025 (67%), and 2026 (100%), replacing V24. V28 consolidates HCC categories, removes some lower-acuity conditions, and recalibrates RAF weights across the entire model. For A81.89, V28 maps to Dementia, Mild or Unspecified at RAF 0.341 while V24 mapped to Dementia Without Complication at RAF 0.346. Both models can still appear in legacy data and in payer-specific lookback periods.

What documentation supports A81.89?

Use this code only when the specific atypical virus has been identified but doesn't fit into other A81 subcategories (such as Creutzfeldt-Jakob disease, subacute sclerosing panencephalitis, or progressive multifocal leukoencephalopathy) Ensure documentation clearly identifies the causative atypical virus and its effect on the central nervous system; consider querying the provider if the specific virus type is unclear or if a more specific code applies Because A81.89 maps to a payment HCC, the documentation must also satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's risk adjustment score. See the MEAT criteria checklist at /meat-criteria for what Monitor, Evaluate, Assess, or Treat looks like in a note. MEAT is generic documentation evidence, not a payer-specific rule.

2026 ICD-10-CM A81.89: Other atypical virus

A81.89 maps to CMS-HCC V28 127 (RAF 0.341). Documentation must support MEAT. MEAT criteria · RAF calculator · free HCC coding tools

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Code lookupA81.89Effective periodFY2026 Apr Update (Apr 1-Sep 30)

FY 2026 Apr update / Certain infectious and parasitic diseases (A00-B99) / Viral and prion infections of the central nervous system (A80-A89)

A81.89

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

Other atypical virus infections of central nervous system

This code describes rare or unusual viral infections that affect the brain and spinal cord, not classified elsewhere in the coding system. These are atypical viruses that cause inflammation and dysfunction of the central nervous system.

Buddy Insight

This code captures atypical central nervous system viral infections not classified elsewhere in the A81 category, representing rare or emerging viral infections of the brain and spinal cord.

CMS-HCC V28

Mapped

HCC 127

Dementia, Mild or Unspecified

RAF 0.341

CMS-HCC V24

Mapped

HCC 52

Dementia Without Complication

RAF 0.346

ACA/HHS

N/A

—

Not risk-adjusted for this model/period.

Not mapped

ESRD/PACE

Mapped

HCC 52

Dementia Without Complication

RAF 0.042

RXHCC

Mapped

HCC 112

Dementia and Other Specified ...

Not separately weighted

Code Book Path

Official

A81Atypical virus infections of central nervous system

A81.8Other atypical virus infections of central nervous system

A81.89Other atypical virus infections of central nervous system

Inclusion Terms

Official

ICD-10-CM does not list inclusion terms for A81.89 in this effective period.

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for A81.89 in this effective period.

Related Child Codes

Official

A81.81Kuru

A81.82Gerstmann-Straussler-Scheinker syndrome

A81.83Fatal familial insomnia

Includes

Official

ICD-10-CM does not list Includes notes for A81.89 in this effective period.

Excludes 1

Official

ICD-10-CM does not list Excludes 1 notes for A81.89 in this effective period.

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for A81.89 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for A81.89 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for A81.89 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance

Identification of the specific atypical virus or documentation explaining why it cannot be classified under other A81 codes

CNS involvement confirmed by neuroimaging, CSF analysis, or biopsy

Clinical neurological findings documented (cognitive changes, motor deficits, seizures)

Diagnostic testing results: viral PCR, antibody testing, brain biopsy

MEAT Support

HCC Buddy guidance

Identification of the specific atypical virus or documentation explaining why it cannot be classified under other A81 codes

CNS involvement confirmed by neuroimaging, CSF analysis, or biopsy

Clinical neurological findings documented (cognitive changes, motor deficits, seizures)

Diagnostic testing results: viral PCR, antibody testing, brain biopsy

Audit Caution

HCC Buddy guidance

Defaulting to A81.9 (unspecified) when the virus has been identified and A81.89 (other specified) is more accurate

Using generic encephalitis codes when the infection qualifies as an atypical/slow virus CNS infection

Not documenting why the specific virus does not fit into other A81 subcategories

Confusing typical acute viral encephalitis with the progressive course characteristic of atypical virus infections

Common Mistakes

HCC Buddy guidance

A81.9 (Atypical virus infection of CNS, unspecified) — Use A81.89 when the virus is identified but not classified elsewhere; use A81.9 when the virus is truly unknown

A86 (Unspecified viral encephalitis) — General viral encephalitis code; A81.89 is specifically for atypical (slow) virus infections with progressive course

G04.90 (Unspecified acute disseminated encephalomyelitis) — ADEM is typically monophasic and post-infectious; atypical viral CNS infections are progressive

Current with CMS: FY2026 ICD-10-CM Apr 1 update (effective Apr 1 – Sep 30, 2026) · CMS-HCC V28, 100% phased in for payment year 2026. FY2027 code set already staged for October 1, 2026. How HCC Buddy stays current →

Is A81.89 an HCC code?

Yes. A81.89 (Other atypical virus infections of central nervous system) maps to Dementia, Mild or Unspecified under the CMS-HCC V28 risk adjustment model (and Dementia Without Complication under V24), with a community non-dual aged RAF of 0.341. It is billable for payment year 2026.

Coder answer: A81.89 is billable and maps to V28 HCC 127, Dementia, Mild or Unspecified. Open it in the Code Book for the tabular path, RAF, and MEAT checklist.

Code: A81.89
Description: Other atypical virus infections of central nervous system
HCC (V28): HCC 127 — Dementia, Mild or Unspecified
RAF: 0.341
Billable: Yes
Payment year: 2026

HCC Category Mapping

V28HCC 127, Dementia, Mild or Unspecified

0.341

V24HCC 52, Dementia Without Complication

0.346

ESRDHCC 52, Dementia Without Complication

0.042

RxHCCHCC 112, Dementia and Other Specified Brain Disorders

Not separately weighted

Each model's RAF is its CMS base weight for that model's standard population, so weights are not directly comparable across models: CMS-HCC V28 and V24 use Community, Non-Dual, Aged; ESRD uses the dialysis continuing-enrollee model; RxHCC is the Part D continuing-enrollee, non-low-income, aged weight (a larger scale than CMS-HCC). ACA/HHS has no single weight — it varies by metal level. 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.

Work A81.89 in the Code Book — tabular path, V28 RAF, and MEAT checklist →

MEAT Criteria for A81.89

For A81.89 to 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 A81.89 during that encounter, not just copy-forwarded from a problem list.

Coder workflow notes

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What This Code Means

A81.89 is the ICD-10-CM diagnosis code for other atypical virus infections of central nervous system. This code describes rare or unusual viral infections that affect the brain and spinal cord, not classified elsewhere in the coding system. These are atypical viruses that cause inflammation and dysfunction of the central nervous system. A81.89 sits in the ICD-10-CM chapter for certain infectious and parasitic diseases (a00-b99), within the section covering viral and prion infections of the central nervous system (a80-a89).

Under the CMS-HCC V28 risk adjustment model, A81.89 maps to Dementia, Mild or Unspecified (HCC 127) with a community, non-dual, aged base RAF weight of 0.341. Under the older CMS-HCC V24 model, A81.89 maps to Dementia Without Complication (HCC 52) with a community, non-dual, aged base RAF weight of 0.346. V28 is the CMS-HCC risk adjustment model that reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition.

Use this code only when the specific atypical virus has been identified but doesn't fit into other A81 subcategories (such as Creutzfeldt-Jakob disease, subacute sclerosing panencephalitis, or progressive multifocal leukoencephalopathy). Because A81.89 maps to a payment HCC, the provider's documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's Medicare Advantage risk adjustment score. When documentation is ambiguous, coders should issue a provider query rather than assume the highest-specificity variant.

HCC Buddy maintains structured V28 and V24 mapping, RAF weights, and MEAT documentation criteria for A81.89 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•Use this code only when the specific atypical virus has been identified but doesn't fit into other A81 subcategories (such as Creutzfeldt-Jakob disease, subacute sclerosing panencephalitis, or progressive multifocal leukoencephalopathy)
•Ensure documentation clearly identifies the causative atypical virus and its effect on the central nervous system; consider querying the provider if the specific virus type is unclear or if a more specific code applies

Clinical Significance

This code captures atypical central nervous system viral infections not classified elsewhere in the A81 category, representing rare or emerging viral infections of the brain and spinal cord. These are typically severe conditions with significant neurological morbidity requiring specialized infectious disease and neurology management.

Documentation Requirements

✓Identification of the specific atypical virus or documentation explaining why it cannot be classified under other A81 codes
✓CNS involvement confirmed by neuroimaging, CSF analysis, or biopsy
✓Clinical neurological findings documented (cognitive changes, motor deficits, seizures)
✓Diagnostic testing results: viral PCR, antibody testing, brain biopsy
✓Exclusion of other classified CNS viral infections

Commonly Confused Codes

•A81.9 (Atypical virus infection of CNS, unspecified): Use A81.89 when the virus is identified but not classified elsewhere; use A81.9 when the virus is truly unknown
•A86 (Unspecified viral encephalitis): General viral encephalitis code; A81.89 is specifically for atypical (slow) virus infections with progressive course
•G04.90 (Unspecified acute disseminated encephalomyelitis): ADEM is typically monophasic and post-infectious; atypical viral CNS infections are progressive