A81.9

Atypical virus infection of central nervous system, unspecified

Billable FY2026 2025-10-01 → 2026-09-30

This is the unspecified option. 4 more specific codes sit alongside it. Worth checking whether the documentation supports one of them.

Exclusions 8 never code together

A80-A89block

Excludes1

postpolio syndrome (G14)G14
sequelae of poliomyelitis (B91)B91
sequelae of viral encephalitis (B94.1)B94.1

A00-B99chapter

Excludes1

certain localized infections - see body system-related chapters

Excludes2

carrier or suspected carrier of infectious disease (Z22.-)Z22.-
infectious and parasitic diseases complicating pregnancy, childbirth and the puerperium (O98.-)O98.-
infectious and parasitic diseases specific to the perinatal period (P35-P39)P35-P39
influenza and other acute respiratory infections (J00-J22)J00-J22

Excludes1 — never code together. The excluded condition and this one are mutually exclusive.

Excludes2 — not included here, but the patient may have both. Code both when documented.

Instructional notes 11

A81code

Includes

diseases of the central nervous system caused by prions

Use additional code

code, if applicable, to identify:
dementia with anxiety (F02.84, F02.A4, F02.B4, F02.C4)F02.84 F02.A4 F02.B4 F02.C4
dementia with behavioral disturbance (F02.81-, F02.A1-, F02.B1-, F02.C1-)F02.81-F02.A1-F02.B1-F02.C1-
dementia with mood disturbance (F02.83, F02.A3, F02.B3, F02.C3)F02.83 F02.A3 F02.B3 F02.C3
dementia with psychotic disturbance (F02.82, F02.A2, F02.B2, F02.C2)F02.82 F02.A2 F02.B2 F02.C2
dementia without behavioral disturbance (F02.80, F02.A0, F02.B0, F02.C0)F02.80 F02.A0 F02.B0 F02.C0
mild neurocognitive disorder due to known physiological condition (F06.7-)F06.7-

A81.9code

Inclusion terms

Prion diseases of the central nervous system NOS

A00-B99chapter

Includes

diseases generally recognized as communicable or transmissible

Use additional code

code to identify resistance to antimicrobial drugs (Z16.-)Z16.-

Index entries leading here 2

Infection, infected, infective (opportunistic) B99.9
- atypical
  - virus A81.9
- virus, viral NOS B34.9
  - central nervous system A89
    - atypical A81.9
    - slow virus A81.9
  - slow A81.9
- slow virus A81.9
Disease, diseased — see also Syndrome
- prion, central nervous system A81.9

Alongside this code 4 same parent — A81

A81.0 Creutzfeldt-Jakob disease category A81.1 Subacute sclerosing panencephalitis A81.2 Progressive multifocal leukoencephalopathy A81.8 Other atypical virus infections of central nervous system category

Medications 5

693 Central Nervous System Diseases drugs 80 Central Nervous System Infections drugs 1336 Nervous System Diseases drugs 131 Nervous System Neoplasms drugs 12 Nervous System Malformations drugs

MED-RT files conditions by MeSH, which carves the world differently from ICD-10. These are name matches on this code's description, offered as candidates — not as an assertion that they are this code.

Additional references

AHA Coding Clinic® Licence required

Official coding advice for this code. Published by the American Hospital Association. Not included in this installation.

Risk adjustment

CMS28HCC127 ESRD21HCC52 ESRD24HCC52 RX08HCC112

Presence is a property of the code. Untick a model to hide it everywhere.

This code

Billable: yes
Code set: FY2026
Parent: A81
Block: A80-A89
Siblings: 4

Present FY2024–FY2027.