A81.89
BillableOther atypical virus infections of central nervous system
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is A81.89 an HCC code?
Yes. A81.89 maps to Dementia, Mild or Unspecified under the CMS-HCC V28 risk adjustment model.
HCC Category Mapping
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 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.
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 0.464. A81.89 was not retained as a payment HCC under the older V24 model, so V28 introduced or recategorized it during the 2024–2026 phase-in. 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