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A86 ICD-10-CM Code: Unspecified viral encephalitis

Billing Status: YES. This is a valid, specific, and billable ICD-10-CM reference.

Coding at a Glance

Tabular directives
2 inclusion terms · 3 Excludes1

Inpatient Payment Groups (MS-DRG)

Potential MS-DRG participation — not a DRG assignment.

MS-DRGs this diagnosis helps define, as principal or secondary, per the CMS ICD-10-CM/PCS MS-DRG Definitions Manual v43, Appendix B.

  • MS-DRG 097 — NON-BACTERIAL INFECTION OF NERVOUS SYSTEM EXCEPT VIRAL MENINGITIS WITH MCC (MDC 01)
  • MS-DRG 098 — NON-BACTERIAL INFECTION OF NERVOUS SYSTEM EXCEPT VIRAL MENINGITIS WITH CC (MDC 01)
  • MS-DRG 099 — NON-BACTERIAL INFECTION OF NERVOUS SYSTEM EXCEPT VIRAL MENINGITIS WITHOUT CC/MCC (MDC 01)
  • MS-DRG 974 — HIV WITH MAJOR RELATED CONDITION WITH MCC (MDC 25)
  • MS-DRG 975 — HIV WITH MAJOR RELATED CONDITION WITH CC (MDC 25)
  • MS-DRG 976 — HIV WITH MAJOR RELATED CONDITION WITHOUT CC/MCC (MDC 25)

A diagnosis appearing in a group's logic does not by itself determine the DRG assigned to a stay; the grouper uses the full claim.

Coding instructions

Official Tabular Instructional Notes: the inclusion, exclusion and sequencing notes published for A86 in the official ICD-10-CM tabular list, quoted as published.

Source: CMS/CDC — ICD-10-CM Tabular ListRelease: FY2026Effective: October 1, 2025

Notes without a marker are published on A86 itself; “inherited from” names the category or block whose note applies here.

Inclusion Terms

Alternative terms the tabular list files under this code.

  • Viral encephalomyelitis NOS
  • Viral meningoencephalitis NOS

Excludes1 — Not Coded Here

Conditions generally not reported together with this code (Excludes1) -- an error unless the two conditions are documented as unrelated to each other (ICD-10-CM Official Guidelines, Section I.A.12.a).

Source: inherited from A80-A89

Coder workflow for A86

MedCoder structured workflow — derived from this code’s own official record

Before you code A86

  1. Unspecified does not mean incorrect. When the record gives no greater specificity, A86 may be the appropriate code. Check the record for detail that supports a more specific sibling. An unspecified code is for records that do not provide the detail a more specific code needs; a query, not an assumption, is the route to specificity (Guidelines I.A.9.b, I.B.18).

    See the relationships section · Guide: Other vs unspecified (NEC vs NOS) →

  2. Check the Excludes1 note: if the documentation supports a condition named there, do not simply proceed with A86. Excludes1 conditions are not reported together with this code unless the record shows they are unrelated (Guidelines I.A.12.a).

    See the official tabular notes

Choose the right path

  1. Does the record document the detail a more specific sibling code needs?
    Yes → Review the specific siblings in this subcategory.
    No → Continue — A86 is appropriate when the documentation goes no further.
  2. Does the documentation support a condition named in A86’s Excludes1 note?
    Yes → Do not simply proceed: review the excluded code. Both are reported only when the record shows the two conditions are unrelated.
    No → Continue.

    ReviewG14, B91, B94.1

Consider A86. Then confirm the code is valid for the date of service in the Verify section.

Documentation check

The provider’s diagnostic statement
Codes are assigned from the provider’s documented diagnosis, not from clinical criteria, test values or a medication list (Guidelines I.A.19).
Any detail beyond this code’s title
What the record states that a more specific sibling code would capture — or its absence, which itself supports the unspecified code.

Official instructions as workflow

  • Excludes1 — check before selecting A86(3 notes)

    Coding workflow: If the documentation supports a condition named in this note, do not simply proceed with A86: the two are not reported together. The one exception is when the record shows the two conditions are unrelated to each other.

    CompareG14, B91, B94.1

    See the official tabular notes · Guidelines I.A.12.a

Coding decision scenarios

Pattern scenarios for this code’s structure — decision rules, not clinical cases

Documentation: The provider documents the condition in the terms of this code’s title and records no further detail.

Coding question: Is a more specific sibling code supportable?

Path: Review the subcategory for a sibling that names the missing detail.

Reason: A more specific code needs documentation of the distinguishing element; without it the unspecified code is appropriate, and a provider query is the route to specificity (Guidelines I.A.9.b, I.B.18).

Documentation: Both the condition A86 describes and a condition named in its Excludes1 note are documented for the same encounter.

Coding question: Can both codes be reported?

Path: Review the Excludes1 note and the excluded code, and look for a provider statement on whether the two conditions are related.

Reason: Excludes1 means the two are not coded together; the exception is when the record shows the conditions are unrelated to each other (Guidelines I.A.12.a).

ReviewG14, B91, B94.1

Every row is derived from this code’s own record — its title, tabular notes, 7th-character family and same-category siblings — with fixed MedCoder wording; nothing is inferred about a patient. The official notes and guideline text are in the sections each row links to, and they control.

Code Overview

Unspecified viral encephalitis is a billable ICD-10-CM diagnosis code (A86).

MedCoder summary Summary composed by MedCoder from this code's official ICD-10-CM record. The tabular instructional notes themselves appear verbatim below.

Indexed Clinical Terms (10)

Official source data — entries quoted as published, in the Index’s own lookup phrasing

Clinical term phrases from the official ICD-10-CM Index to Diseases and Injuries that map to this code. These are alphabetic-index entries shown as the Index writes them — lookup phrasing, not necessarily the wording of a final diagnosis.

Verify Before Coding

  • CC as a secondary diagnosis (FY2026). Can raise the stay's MS-DRG severity tier.

From the code registry, the Medicare Code Editor, and the MS-DRG Definitions Manual. Check it against a full claim in Claim Check.

Relationships & Classification

MedCoder structured relationships — computed from published CMS and AHRQ datasets

Other codes that name A86 or its code family, from the CMS ICD-10-CM tabular instructional notes. Tabular-note edges are stored at the code family level that carries each note.

Referenced by 1 Excludes1 note: A88 — Other viral infections of central nervous system, not elsewhere classified.

These codes’ tabular lists mark this diagnosis as mutually exclusive — not reported together unless the conditions are documented as unrelated.

MS-DRG Grouper Relationships (FY2026)

Potential MS-DRG participation — not a DRG assignment.

FY2026 MS-DRG: CC — Complication or Comorbidity. Reported as a secondary diagnosis, this code raises the stay's MS-DRG severity tier — except when the principal diagnosis is one of 186 clinically related codes on its CMS exclusion list. Does not count toward severity when the stay groups to DRGs 974-976 (HIV with Major Related Condition).

Named in the grouper logic of 6 MS-DRGs: DRG 097 (MDC 01), DRG 098 (MDC 01), DRG 099 (MDC 01), DRG 974 (MDC 25), DRG 975 (MDC 25), DRG 976 (MDC 25).

From the CMS MS-DRG Definitions Manual (Appendices B and C). Actual DRG assignment depends on the complete claim.

Clinical classification (AHRQ CCSR):NVS002 — Encephalitis (default); INF008 — Viral infection.

Clinical Classifications Software Refined (CCSR) for ICD-10-CM Diagnoses. Healthcare Cost and Utilization Project (HCUP), Agency for Healthcare Research and Quality.

Related Codes

Same clinical process (MS-DRG)

Acts as CC — raises the severity of other admissions. CMS groups these diagnoses into one MS-DRG principal-diagnosis exclusion process — a CC/MCC on this list never raises severity when the principal diagnosis is also on it.

A83.9 — Mosquito-borne viral encephalitis, unspecified, A84.0 — Far Eastern tick-borne encephalitis [Russian spring-summer encephalitis], A84.1 — Central European tick-borne encephalitis, A84.81 — Powassan virus disease, A84.89 — Other tick-borne viral encephalitis, A84.9 — Tick-borne viral encephalitis, unspecified, A85.0 — Enteroviral encephalitis, A85.1 — Adenoviral encephalitis, A85.2 — Arthropod-borne viral encephalitis, unspecified, A85.8 — Other specified viral encephalitis, A87.2 — Lymphocytic choriomeningitis, A88.0 — Enteroviral exanthematous fever [Boston exanthem], A88.8 — Other specified viral infections of central nervous system, A89 — Unspecified viral infection of central nervous system, A92.1 — O'nyong-nyong fever, A92.2 — Venezuelan equine fever, A92.4 — Rift Valley fever, A92.5 — Zika virus disease, A92.8 — Other specified mosquito-borne viral fevers, A93.0 — Oropouche virus disease, +165 more

Same clinical category (CCSR)

AHRQ's Clinical Classifications Software groups these diagnoses under the same clinical categories (Encephalitis, Viral infection).

A84.0 — Far Eastern tick-borne encephalitis [Russian spring-summer encephalitis], A84.1 — Central European tick-borne encephalitis, A84.8 — Other tick-borne viral encephalitis, A84.81 — Powassan virus disease, A84.89 — Other tick-borne viral encephalitis, A84.9 — Tick-borne viral encephalitis, unspecified, A85.0 — Enteroviral encephalitis, A85.1 — Adenoviral encephalitis, A85.2 — Arthropod-borne viral encephalitis, unspecified, A85.8 — Other specified viral encephalitis, A92.2 — Venezuelan equine fever, A92.31 — West Nile virus infection with encephalitis, B00.4 — Herpesviral encephalitis, B01.11 — Varicella encephalitis and encephalomyelitis, B02.0 — Zoster encephalitis, B05.0 — Measles complicated by encephalitis, B06.01 — Rubella encephalitis, B10.01 — Human herpesvirus 6 encephalitis, B10.09 — Other human herpesvirus encephalitis, B26.2 — Mumps encephalitis, +314 more

Same Index main term, other category

The ICD-10-CM Index to Diseases and Injuries files this code under the main terms “Encephalitis”, “Meningoencephalitis”; these codes share that main term but sit in a different category of the Tabular List.

A83.9 — Mosquito-borne viral encephalitis, unspecified (equine), A84.0 — Far Eastern tick-borne encephalitis [Russian spring-summer encephalitis] (Far Eastern), A84.1 — Central European tick-borne encephalitis (diphasic), A84.81 — Powassan virus disease (Powassan), A84.89 — Other tick-borne viral encephalitis (louping ill), A84.9 — Tick-borne viral encephalitis, unspecified (tick-borne), A85.0 — Enteroviral encephalitis (in, enterovirus), A85.1 — Adenoviral encephalitis (in, adenovirus), A85.2 — Arthropod-borne viral encephalitis, unspecified (arthropod-borne NEC), A85.8 — Other specified viral encephalitis (Mengo), A87.2 — Lymphocytic choriomeningitis (lymphatica), A92.2 — Venezuelan equine fever (Venezuelan equine), B00.4 — Herpesviral encephalitis (herpesviral), B01.11 — Varicella encephalitis and encephalomyelitis (postvaricella), B02.0 — Zoster encephalitis (in, zoster), B05.0 — Measles complicated by encephalitis (in, measles), B06.01 — Rubella encephalitis (in, rubella), B09 — Unspecified viral infection characterized by skin and mucous membrane lesions (postexanthematous NEC), B10.01 — Human herpesvirus 6 encephalitis (herpesviral, due to herpesvirus 6), B10.09 — Other human herpesvirus encephalitis (herpesviral, specified NEC), +49 more

Lab tests where this diagnosis supports Medicare coverage (NCD)

Medicare's National Coverage Determination (NCD) program lists this diagnosis as medical justification for these lab tests.

HIV Screening Test

Contextual Map

Every relationship of A86 in one view: hierarchy, official tabular instructions in both directions, clinical classification, risk adjustment, MS-DRG participation, index terms and change history — each edge carrying the CMS source it derives from.

Run A86 with these 4 related codes in Claim Check

Hierarchy

Excludes1

Referenced by Excludes1 notes

Clinical classification (CCSR)

MS-DRG Grouper

MDC crossing

  • MDC 01 — Diseases and Disorders of the Nervous System[MDC crossing]: “Diseases and Disorders of the Nervous System — the grouper's crossing between diagnoses and procedures: a principal diagnosis sets the MDC, and same-MDC procedures move the stay to its surgical DRGs. 8,892 same-MDC procedures group here; browse them on the MDC page.”— CMS MS-DRG Definitions Manual · FY2026
  • MDC 25 — Human Immunodeficiency Virus Infections[MDC crossing]: “Human Immunodeficiency Virus Infections — the grouper's crossing between diagnoses and procedures: a principal diagnosis sets the MDC, and same-MDC procedures move the stay to its surgical DRGs.”— CMS MS-DRG Definitions Manual · FY2026

Index entries (10)

  • Encephalitis (chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute), acute[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
  • Encephalitis (chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute), endemic (viral)[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
  • Encephalitis (chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute), epidemic NEC (viral)[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
  • Encephalitis (chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute), infectious NEC (acute) (virus)[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
  • Encephalitis (chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute), postviral NEC[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
  • Encephalitis (chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute), viral, virus[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
  • Meningoencephalitis, acute NEC[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
  • Meningoencephalitis, infectious (acute) (viral)[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
  • and 2 more

Change history

  • FY2016 — In the code set at ICD-10-CM adoption [Change history]— CMS release files (code change ledger) · icd10cm-fy2016

Sources for this page

Codes, titles, notes, index terms and mappings on this page are transcribed from the datasets below. Relationships MedCoder computed and text MedCoder wrote are labelled where they appear.

Code, title, tabular notes and index terms Official source data
CMS/CDC ICD-10-CM FY2026 tabular list, index and tables, effective October 1, 2025 Release, file and checksum · Publisher’s page
Claim edits Official source data
CMS Definitions of Medicare Code Edits — v43.1 (April 2026) Release, file and checksum · Publisher’s page
Inpatient payment groups Official source data
CMS MS-DRG Definitions Manual (incl. Appendix B diagnosis index, Appendix C CC/MCC list, Appendix E procedure index) and IPPS Final Rule tables — v43 Release, file and checksum · Publisher’s page
Change history and date-of-service validity Official source data
CMS ICD-10-CM release addenda, ingested release by release into the change ledger Release, file and checksum · Publisher’s page
Comparisons, relationships and the contextual map MedCoder structured relationship
Computed by MedCoder from the tabular notes and tables above; every derived relationship is marked as derived where it appears
Summary and FAQ answers MedCoder explanation
Written by MedCoder to explain the sources above: drafted with AI assistance, checked by a person against the release files, and labelled as MedCoder text where it appears. Not official text.

Labels on this page: Official source data · MedCoder structured relationship · MedCoder explanation. How to read the labels · All data sources and release dates

Cite this page

Reference this page in a research guide, syllabus or article. The release is included because code content changes each year.

MedCoder.ai. "A86 — Unspecified viral encephalitis." ICD-10-CM FY2026. https://medcoder.ai/icd10/code/a86-unspecified-viral-encephalitis

Change history

  • FY2016 — October 1, 2015
    In the code set at ICD-10-CM adoption
    Unspecified viral encephalitis

No changes since FY2016 — additions, deletions, description changes and billable-status changes are tracked through FY2027 (effective October 1, 2026), and none are recorded for this code. Note changes are tracked from FY2027 only.