2027 ICD-10-CM Diagnosis Code A81.9Atypical virus infection of central nervous system, unspecified
ICD-10-CM Codes›A00–B99›A80-A89›A81
- Billable — Valid for Submission
- CC — Complication or Comorbidity
- Risk Adjusts — HCC 127
- Chronic Condition
A81.9 is a billable ICD-10-CM diagnosis code for atypical virus infection of central nervous system, unspecified. It is valid on HIPAA claims for fiscal year 2027 (October 1, 2026 through September 30, 2027) and groups to MS-DRG 56 through 57, 974 through 976. As a secondary diagnosis, it counts as a complication or comorbidity (CC) and moves an inpatient stay to a higher severity level within its MS-DRG family. It does not count, however, when the principal diagnosis is one of 11 closely related codes. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Other specified CNS infection and poliomyelitis and Viral infection.
For Medicare Advantage risk adjustment, A81.9 maps to CMS-HCC Category 127 (Dementia, Mild or Unspecified) under the V28 model, adding a risk factor of about 0.341 for a community, non-dual, aged beneficiary in payment year 2027.
Code Identity
Code Classification
Medicare Risk Adjustment (HCC)Billing
A81.9 maps to a payment category in the CMS-HCC model used to risk-adjust Medicare Advantage payments. Weights are the published community factors for payment year 2027.
Source: CMS Payment Year 2027 risk adjustment mappings and model software. Weights are relative factors, not dollar amounts; a beneficiary's total RAF also includes demographics and interactions. Browse all CMS-HCC categories.
Approximate SynonymsGuidance
Alternate terms and clinical phrases that map to this code.
- Acquired prion disease
- Autonomic disorder due to prion disease
- Chorea due to prion disease
- Dementia due to prion disease
- Disorder of autonomic nervous system due to infectious disease
- Familial Alzheimer-like prion disease
- Parkinsonism due to prion disease
- Prion disease
- Secondary tic disorder
- Slow viral central nervous system infection
- Tic disorder due to prion disease
Instructional NotesGuidance
Instructions from the official ICD-10-CM Tabular List that apply to A81.9: its own notes plus those printed at A81, block A80-A89, and Chapter 1. A note printed at a category, block or chapter applies to every code under it.
Includes
Further defines, or gives examples of, the content of the category.
- diseases of the central nervous system caused by prions
- diseases generally recognized as communicable or transmissible
Applicable To
Conditions this code is used for: synonyms of the title or, for "other specified" codes, the conditions assigned to it. The list is not exhaustive.
- Prion diseases of the central nervous system NOS
Excludes1
Not coded here: the excluded code is never reported together with this one, unless the two conditions are unrelated.
Excludes2
Not included here: the excluded condition is not part of this code, but a patient may have both, so both codes may 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)
Use Additional Code
Add a second code, after this one, to fully describe the condition.
- 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-)
- code to identify resistance to antimicrobial drugs (Z16.-)
Source: CMS ICD-10-CM Tabular List. How to read instructional notes.
Referenced in Other NotesGuidance
Instructional notes printed at other codes that name A81.9, its category, or a range that includes it.
Code First 1
These codes say to code first a condition named in the note, and A81.9 is one of the conditions named.
- F02 Dementia in other diseases classified elsewhereprion disease (A81.9)
Index to Diseases and InjuriesGuidance
Alphabetical index entries that point to this code.
Disease, diseased See Also: Syndrome;
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
Patient EducationClinical
Neurologic Diseases
The brain, spinal cord, and nerves make up the nervous system. Together they control all the workings of the body. When something goes wrong with a part of your nervous system, you can have trouble moving, speaking, swallowing, breathing, or learning. You can also have problems with your memory, senses, or mood.
Read the full article at MedlinePlus
Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.
Convert A81.9 to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code HistoryHistory
Questions About A81.9Overview
What is the ICD-10 code for atypical virus infection of central nervous system, unspecified?
The ICD-10-CM code for atypical virus infection of central nervous system, unspecified is A81.9 (sometimes written as A819). It is billable on HIPAA-covered claims from October 1, 2026 through September 30, 2027.
Is A81.9 (Atypical virus infections of central nervous system) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report atypical virus infection of central nervous system, unspecified on HIPAA-covered claims from October 1, 2026 through September 30, 2027.
What MS-DRG does A81.9 group to?
When atypical virus infection of central nervous system, unspecified is the principal diagnosis on an inpatient stay, it groups to MS-DRG 974, 976, 975, 57, 56, with relative weights from 0.7879 to 3.0488 depending on complications. Higher weights mean higher Medicare reimbursement.
Is A81.9 a CC or MCC?
CMS lists A81.9 as a CC (complication or comorbidity) for FY 2027. Reported as a secondary diagnosis, it moves the inpatient stay to a higher-weighted DRG within its severity family. It does not count when the principal diagnosis is one of the 11 closely related codes in its exclusion list.
Can A81.9 be reported with G14?
Not as a rule. The Excludes1 note at block A80-A89, which applies to A81.9, lists "postpolio syndrome (G14)". An Excludes1 note means the excluded code is never reported together with this one; the only exception is when the two conditions are unrelated to each other (Official Guidelines, Section I.A.12.a).
What HCC is A81.9?
A81.9 (atypical virus infection of central nervous system, unspecified) maps to CMS-HCC Category 127 (Dementia, Mild or Unspecified), commonly written as HCC 127, in the CMS-HCC V28 model used for Medicare Advantage risk adjustment in payment year 2027. It mapped to HCC 52 under the retired V24 model. It also maps in the ESRD (V21) and ESRD (V24) models. In the Part D prescription drug model it maps to RxHCC 112.
Does A81.9 risk-adjust for Medicare Advantage payment?
Yes. When documented and reported on a Medicare Advantage encounter, A81.9 adds a risk adjustment factor of about 0.341 to the beneficiary's RAF score for a community, non-dual, aged enrollee (published V28 weights range from 0.000 to 0.438 depending on the payment segment). A more severe related category (HCC 125 and HCC 126) supersedes it when both are reported. See the full factor table on the HCC 127 category page.
