A81.9 — Atypical virus infection of central nervous system, unspecified
Is A81.9 billable?
Yes, but A81.9 is an unspecified code. A81.9 is a valid, billable ICD-10-CM code for encounters from October 1, 2026 through September 30, 2027. However, A81.9 is an unspecified or catch-all code. Payers apply additional scrutiny to unspecified codes and many deny them when the medical record supports a more specific option, so check the documentation before selecting A81.9 — the more specific alternatives are listed below.
A81.9 at a glance
| Code | A81.9 |
|---|---|
| Description | Atypical virus infection of central nervous system, unspecified |
| Billable | Yes |
| Code set | ICD-10-CM FY2027 |
| Valid for encounters | October 1, 2026 – September 30, 2027 |
Which MS-DRGs does A81.9 group to?
A81.9 sits in MDC 01 and helps define the logic of 2 MS-DRGs (version 44, FY2027). Which one a stay actually groups to also depends on procedures and secondary diagnoses.
- 056Degenerative Nervous System Disorders with MCCmedical
- 057Degenerative Nervous System Disorders without MCCmedical
As a secondary diagnosis, A81.9 is a complication or comorbidity (CC), which can move a stay into a higher-paying DRG.
How is A81.9 listed in the Alphabetic Index?
These are the routes through the Alphabetic Index that lead to A81.9. They show the wording a clinician may have documented, which often differs from the Tabular description.
- Disease, diseased › prion, central nervous system
- Infection, infected, infective › slow virus (opportunistic)
- Infection, infected, infective › atypical › virus (opportunistic)
- Infection, infected, infective › virus, viral NOS › slow (opportunistic)
- Infection, infected, infective › virus, viral NOS › central nervous system › atypical (opportunistic)
- Infection, infected, infective › virus, viral NOS › central nervous system › slow virus (opportunistic)
Inclusion terms
Alternative wording in documentation that is classified to A81.9.
- Prion diseases of the central nervous system NOS
Excludes1 — never code together — inherited from A80-A89
The conditions below can never be reported together with A81.9 on the same claim. An Excludes1 note means the two conditions cannot occur in the same patient, so reporting both is a coding error.
Use additional code — inherited from Chapter 1
Report an additional code alongside A81.9 to fully describe the condition. A81.9 is sequenced first.
- code to identify resistance to antimicrobial drugs (Z16.-)
Use additional code — inherited from A81
Report an additional code alongside A81.9 to fully describe the condition. A81.9 is sequenced first.
- code, if applicable, to identify:
- dementia with anxiety (F02.84, F02.A4, F02.B4, F02.C4)
- dementia with behavioral disturbance (F02.81-, F02.A1-, F02.B1-, F02.C1-)
- dementia with mood disturbance (F02.83, F02.A3, F02.B3, F02.C3)
- dementia with psychotic disturbance (F02.82, F02.A2, F02.B2, F02.C2)
- dementia without behavioral disturbance (F02.80, F02.A0, F02.B0, F02.C0)
- mild neurocognitive disorder due to known physiological condition (F06.7-)
Excludes2 — not included here — inherited from Chapter 1
The conditions below are not part of A81.9, but a patient may have both at the same time. When documentation supports it, A81.9 and the excluded code may both be reported.
- carrier or suspected carrier of infectious disease (Z22.-)
- certain localized infections - see body system-related chapters
- infectious and parasitic diseases complicating pregnancy, childbirth and the puerperium (O98.-)
- infectious and parasitic diseases specific to the perinatal period (P35-P39)
- influenza and other acute respiratory infections (J00-J22)
How long has A81.9 existed?
A81.9 has been in ICD-10-CM since at least FY2016, the earliest fiscal year on record here.
Derived from the official order files for FY2016–FY2026. FY2017 and FY2020 publish no order file, so those years are not covered.