ICD-10-CM 2027
ICD-10 code for acute encephalitis
From the official ICD-10-CM alphabetic index entry “Encephalitis (chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute) › acute”. Page updated September 29, 2026.
About coding acute encephalitis
The ICD-10-CM code for acute encephalitis is A86 (Unspecified viral encephalitis). The official index lists 10 more specific codes, so check the documentation for details such as type, cause, site or severity before settling on A86.
As a secondary diagnosis it is a CC (complication or comorbidity), which can move an inpatient stay into the "with CC" MS-DRG of its family, unless the principal diagnosis excludes it.
In MS-DRG v44.0, it is part of the grouping logic for DRG 097 (Non-Bacterial Infection of Nervous System Except Viral Meningitis with MCC, relative weight 3.8226), DRG 098 (Non-Bacterial Infection of Nervous System Except Viral Meningitis with CC, relative weight 2.2706), DRG 099 (Non-Bacterial Infection of Nervous System Except Viral Meningitis without CC/MCC, relative weight 1.5256), DRG 974 (HIV with Major Related Condition with MCC, relative weight 3.0506), DRG 975 (HIV with Major Related Condition with CC, relative weight 1.3252) and DRG 976 (HIV with Major Related Condition without CC/MCC, relative weight 0.8124), in MDC 01 (Diseases and Disorders of the Nervous System) and MDC 25 (Human Immunodeficiency Virus Infections), as a principal or secondary diagnosis. The DRG a claim lands in depends on the principal diagnosis, procedures and any CC/MCC secondary diagnoses.
It has been part of ICD-10-CM since the code set took effect on October 1, 2015 and hasn't changed since.
More specific acute encephalitis codes (official index)
- disseminated G04.00 — Acute disseminated encephalitis and encephalomyelitis, unspecified
- infectious G04.01 — Postinfectious acute disseminated encephalitis and encephalomyelitis (postinfectious ADEM)
- noninfectious G04.81 — Other encephalitis and encephalomyelitis
- postimmunization (postvaccination) G04.02 — Postimmunization acute disseminated encephalitis, myelitis and encephalomyelitis
- postinfectious G04.01 — Postinfectious acute disseminated encephalitis and encephalomyelitis (postinfectious ADEM)
- inclusion body A85.8 — Other specified viral encephalitis
- necrotizing hemorrhagic G04.30 — Acute necrotizing hemorrhagic encephalopathy, unspecified
- postimmunization G04.32 — Postimmunization acute necrotizing hemorrhagic encephalopathy
- postinfectious G04.31 — Postinfectious acute necrotizing hemorrhagic encephalopathy
- specified NEC G04.39 — Other acute necrotizing hemorrhagic encephalopathy
Common questions about acute encephalitis ICD-10 codes
What is the ICD-10 code for acute encephalitis?
- A86 — Unspecified viral encephalitis. More specific codes apply when the documentation supports them: G04.00, G04.01, G04.81, G04.02 and others below.
What is the ICD-10 code for disseminated acute encephalitis?
- G04.00 — Acute disseminated encephalitis and encephalomyelitis, unspecified.
What is the ICD-10 code for inclusion body acute encephalitis?
- A85.8 — Other specified viral encephalitis.
What is the ICD-10 code for necrotizing hemorrhagic acute encephalitis?
- G04.30 — Acute necrotizing hemorrhagic encephalopathy, unspecified.
Is A86 billable?
- Yes. A86 is a billable/specific ICD-10-CM code valid for FY2027 (October 1, 2026 – September 30, 2027).
Can A86 be used as a principal diagnosis?
- Yes. The Medicare Code Editor doesn't restrict A86 as a principal diagnosis.
Is A86 a CC or MCC?
- A86 is a CC (complication or comorbidity) under MS-DRG v44.0, unless excluded by the principal diagnosis.