2026 ICD-10-CM Diagnosis Code A86Unspecified viral encephalitis

ICD-10-CM Codes›A00–B99›A80-A89›A86

ICD-10-CM A86
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

A86 is a billable ICD-10-CM diagnosis code for unspecified viral encephalitis. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 97 through 99, 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 186 closely related codes. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Encephalitis and Viral infection.

Code Identity

ICD-10-CM Code
A86
Billable Status
Yes — Valid for Submission
Code Describes
Unspecified viral encephalitis
Short Description
Unspecified viral encephalitis
Same as the full description in the CMS dataset.
Chapter
A80-A89
Viral and prion infections of the central nervous system

Code Classification

ChapterA00–B99Certain infectious and parasitic diseases
SectionA80-A89Viral and prion infections of the central nervous system
CategoryA86Unspecified viral encephalitis
This CodeA86Unspecified viral encephalitis

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Acute encephalitis
  • Acute necrotizing encephalitis
  • Acute viral encephalitis
  • Arenavirus encephalitis
  • Arenavirus meningitis
  • Chronic echovirus meningoencephalitis
  • Chronic infectious disease of central nervous system
  • Chronic meningitis
  • Chronic viral encephalitis
  • Dementia associated with viral encephalitis
  • Echovirus encephalitis
  • Epidemic encephalitis
  • Infection caused by Lymphocytic choriomeningitis virus
  • Infective ventriculitis
  • Lymphocytic choriomeningitis virus encephalitis
  • Meningitis caused by echovirus
  • Meningitis caused by Enterovirus
  • Meningitis caused by lymphocytic choriomeningitis virus
  • Meningoencephalitis caused by Lymphocytic choriomeningitis virus
  • Meningoencephalitis caused by virus
  • Primary encephalitis
  • Primary viral encephalitis
  • Ventriculitis of the brain
  • Viral encephalitis
  • Viral ventriculitis

Tabular List NotesGuidance

Coding notes and annotation back-references applicable to this code.

Inclusion Terms

  • Viral encephalomyelitis NOS
  • Viral meningoencephalitis NOS

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR NVS002
Encephalitis
Default principal diagnosis: inpatient Yes · outpatient Yes
CCSR INF008
Viral infection
Default principal diagnosis: inpatient No · outpatient No

Clinical InformationClinical

  • Cordonnier Grade 3 Viral Complication, Viral Encephalitis|Grade 3 Any Viral Encephalitis

    any viral encephalitis.

  • Mosquito-Borne Viral Encephalitis

    viral encephalitis that is transmitted by mosquitos.

  • Tick-Borne Viral Encephalitis

    viral encephalitis that is transmitted by ticks.

  • Viral Encephalitis

    encephalitis resulting from viral infection.

  • Epidemic Encephalitis

    a term that may be used to describe encephalitis characterized by sleepiness, which is likely due to a viral cause.

  • Acute Necrotizing Encephalitis

    a rare infectious encephalitis primarily affecting children that is characterized by rapid neurological deterioration and most often follows influenza infection.

Patient EducationClinical

Encephalitis

Encephalitis is inflammation (swelling) of the brain. It can happen when an infection or medical condition that affects the brain activates your immune system. The inflammation can cause a wide range of symptoms. In extreme cases, it can lead to brain damage, stroke, or even death.

The full article covers:

  • What is encephalitis?
  • What causes encephalitis?
  • Who is more likely to get encephalitis?
  • What are the symptoms of encephalitis?
  • How is encephalitis diagnosed?
  • What are the treatments for encephalitis?
  • Can encephalitis be prevented?

Read the full article at MedlinePlus

Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.

Convert A86 to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
049.9 Viral encephalitis NOS
Approximate The match is approximate rather than exact.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About A86Overview

What is the ICD-10 code for unspecified viral encephalitis?

The ICD-10-CM code for unspecified viral encephalitis is A86. It is billable on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

Is A86 (Unspecified viral encephalitis) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report unspecified viral encephalitis on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does A86 group to?

When unspecified viral encephalitis is the principal diagnosis on an inpatient stay, it groups to MS-DRG 97, 98, 99, 974, 975, 976, with relative weights from 0.8945 to 3.6118 depending on complications. Higher weights mean higher Medicare reimbursement.

Is A86 a CC or MCC?

CMS lists A86 as a CC (complication or comorbidity) for FY 2026. 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 186 closely related codes in its exclusion list.

What is the ICD-9 equivalent of A86?

Under the General Equivalence Mappings, unspecified viral encephalitis converts to ICD-9-CM 049.9 (viral encephalitis NOS). The mapping is approximate, so confirm the match fits the documentation.