Unspecified viral encephalitis
In Viral and prion infections of the central nervous system (A80-A89), chapter 1: Certain Infectious and Parasitic Diseases
- A86 is an unspecified code: use it only when the record doesn't document anything more specific. If the record has more detail, check the other codes under A80-A89 below.
- As a secondary diagnosis it is a CC (complication or comorbidity), which can raise the MS-DRG payment. It doesn't count when the principal diagnosis is one of 186 closely related codes.
- Doesn't count toward a Medicare Advantage risk score (no HCC in V28).
- AHRQ's Chronic Condition Indicator classes it as not chronic (an acute or short-term condition).
- 125 US deaths in 2024 had an underlying cause in category A86, per the National Vital Statistics System on CDC WONDER. Death records use WHO ICD-10, so the count is for the whole category.
Also indexed as: Encephalitis, acute; Encephalitis, endemic; Encephalitis, epidemic NEC; Encephalitis, infectious NEC; Encephalitis, postviral NEC; Encephalitis, viral, virus; Meningoencephalitis, acute NEC; Meningoencephalitis, infectious; Meningoencephalitis, virus NEC; Meningoencephalomyelitis, acute NEC
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Instructional notes
Includes: conditions that are coded here
- Viral encephalomyelitis NOS
- Viral meningoencephalitis NOS
Also applies, from Viral and prion infections of the central nervous system (A80-A89)
These notes sit on A80-A89 and apply to every code under it, including A86.
Official coding guidelines
From the ICD-10-CM Official Guidelines for Coding and Reporting, FY 2027 (CMS and NCHS). Links open the PDF at the right page.
Rules for coding infections and parasitic diseases, including HIV, organism and drug resistance codes, sepsis and septic shock, MRSA, Zika, and COVID-19.
- Human Immunodeficiency Virus (HIV) Infections (page 19)
- Infectious agents as the cause of diseases classified to other chapters (page 22)
- Infections resistant to antibiotics (page 22)
- Sepsis, Severe Sepsis, and Septic Shock (page 23)
- Methicillin Resistant Staphylococcus aureus (MRSA) Conditions (page 27)
- Zika virus infections (page 28)
- Coronavirus infections (page 28)
Other codes under A80-A89
Viral and prion infections of the central nervous system (A80-A89)
- A80Acute poliomyelitis
- A81Atypical virus infections of central nervous system
- A82Rabies
- A83Mosquito-borne viral encephalitis
- A84Tick-borne viral encephalitis
- A85Other viral encephalitis, not elsewhere classified
- A86Unspecified viral encephalitis(this code)
- A87Viral meningitis
- A88Other viral infections of central nervous system, not elsewhere classified
- A89Unspecified viral infection of central nervous system
A86 MS-DRGs
As the principal diagnosis, A86 groups to MS-DRG 097, 098, 099, 974, 975, or 976 (v44.0). Which one applies depends on procedures performed, secondary diagnoses (CC/MCC), and the patient's age and discharge status.
- DRG 097 Non-Bacterial Infection of Nervous System Except Viral Meningitis with MCC
- DRG 098 Non-Bacterial Infection of Nervous System Except Viral Meningitis with CC
- DRG 099 Non-Bacterial Infection of Nervous System Except Viral Meningitis without CC/MCC
- DRG 974 HIV with Major Related Condition with MCC
- DRG 975 HIV with Major Related Condition with CC
- DRG 976 HIV with Major Related Condition without CC/MCC
Related conditions
Grouped by AHRQ's Clinical Classifications Software Refined (CCSR), which sorts every diagnosis code into clinically meaningful categories.
A86 is in Encephalitis (NVS002, its default category), with 55 codes. Others in it:
A86 is in Viral infection (INF008), with 309 codes. Others in it:
Medicare coverage articles listing A86
These 2 Medicare billing and coding articles list A86 as a diagnosis that supports medical necessity for the services they cover. From the CMS Medicare Coverage Database; check your own contractor's article before billing.
Other codes that refer to A86
Notes on these codes point to A86.
- A88 Excludes1: viral encephalitis NOS (A86)
About the condition
Plain-language background from MedlinePlus (National Library of Medicine), not coding guidance.
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.
Read more about encephalitis on MedlinePlusViral Infections
Viruses are very tiny germs. They are made of genetic material (either DNA or RNA) inside of a protein coating. There are a huge number of viruses on earth. Only a small number of them can infect humans. Those viruses can infect our cells, which may cause disease. Some of the diseases that viruses can cause include the common cold, the flu, COVID-19, and HIV.
Read more about viral infections on MedlinePlusCode history
Valid since FY2016, when ICD-10-CM took effect on October 1, 2015. No changes since.
ICD-9-CM equivalent
Legacy crosswalk from the CMS General Equivalence Mappings (last updated 2018).
- 049.9 Unspecified non-arthropod-borne viral diseases of central nervous system (approximate)