A81.9
BillableAtypical virus infection of central nervous system, unspecified
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is A81.9 an HCC code?
Yes. A81.9 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 A81.9
For A81.9 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 A81.9 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
A81.9 is the ICD-10-CM diagnosis code for atypical virus infection of central nervous system, unspecified. This code describes an unusual viral infection affecting the brain and spinal cord when the specific virus has not been identified or determined. It is used when a patient has symptoms of a viral brain infection but testing has not pinpointed which virus is responsible. A81.9 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, A81.9 maps to Dementia, Mild or Unspecified (HCC 127) with a community, non-dual, aged base RAF weight of 0.464. A81.9 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 cannot be identified; if the causative organism is known (such as prion disease, measles, or other specified viruses), use the more specific code instead. Because A81.9 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 A81.9 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 cannot be identified; if the causative organism is known (such as prion disease, measles, or other specified viruses), use the more specific code instead
- •This is an unspecified code (ending in .9) and should be avoided when possible; document clinical findings and any test results to support a more specific diagnosis if available
Clinical Significance
This unspecified code represents an atypical virus infection of the central nervous system where the specific causative virus has not been identified. It should be used only when diagnostic workup is incomplete or inconclusive. Providers should be queried for additional specificity whenever possible to avoid this default code.
Documentation Requirements
- ✓Evidence of CNS infection consistent with atypical/slow virus pattern (progressive neurological decline)
- ✓Diagnostic testing performed and results (even if inconclusive) — CSF studies, MRI, EEG
- ✓Documentation of why the specific virus cannot be identified
- ✓Clinical presentation consistent with slow virus infection rather than acute viral encephalitis
- ✓Plans for further diagnostic workup