2026 ICD-10-CM Diagnosis Code A85.1Adenoviral encephalitis

ICD-10-CM Codes›A00–B99›A80-A89›A85

ICD-10-CM A85.1
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

A85.1 is a billable ICD-10-CM diagnosis code for adenoviral encephalitis. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 974 through 976. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Encephalitis and Viral infection.

Code Identity

ICD-10-CM Code
A85.1
Billable Status
Yes — Valid for Submission
Code Describes
Adenoviral encephalitis
Short Description
Adenoviral encephalitis
Same as the full description in the CMS dataset.
Parent Code
A85
Other viral encephalitis, not elsewhere classified

Code Classification

ChapterA00–B99Certain infectious and parasitic diseases
SectionA80-A89Viral and prion infections of the central nervous system
CategoryA85Other viral encephalitis, not elsewhere classified
This CodeA85.1Adenoviral encephalitis

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Acute adenoviral encephalitis
  • Acute adenoviral meningoencephalitis
  • Acute encephalitis
  • Acute meningitis
  • Acute viral encephalitis
  • Adenoviral encephalitis
  • Adenoviral meningitis
  • Adenoviral nervous system infection
  • Encephalitis with AIDS
  • Encephalopathy with AIDS
  • Meningoencephalitis caused by virus
  • Subacute adenoviral encephalitis
  • Subacute adenoviral encephalitis co-occurrent with human immunodeficiency virus infection
  • Subacute adenoviral encephalitis with AIDS

Tabular List NotesGuidance

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

Inclusion Terms

  • Adenoviral meningoencephalitis

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

External Cause of Injuries IndexGuidance

References for this code in the External Cause of Injuries Index.

    • Encephalitis(chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute)
      • in (due to)
        • adenovirus

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

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 A85.1 to ICD-9-CMHistory

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

ICD-9-CM
049.8 Viral encephalitis NEC
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 A85.1Overview

Is A85.1 (Other viral encephalitis, not elsewhere classified) a billable code?

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

What MS-DRG does A85.1 group to?

When adenoviral encephalitis is the principal diagnosis on an inpatient stay, it groups to MS-DRG 974, 975, 976, with relative weights from 0.8945 to 2.8860 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of A85.1?

Under the General Equivalence Mappings, adenoviral encephalitis converts to ICD-9-CM 049.8 (viral encephalitis NEC). The mapping is approximate, so confirm the match fits the documentation.

Footnotes

[1] Not chronic - A diagnosis code that does not fit the criteria for chronic condition (duration, ongoing medical treatment, and limitations) is considered not chronic. Some codes designated as not chronic are acute conditions. Other diagnosis codes that indicate a possible chronic condition, but for which the duration of the illness is not specified in the code description (i.e., we do not know the condition has lasted 12 months or longer) also are considered not chronic.