A83.3 ICD-10-CM Code: St Louis encephalitis
HCC Buddy Code Card
Digital ICD-10 code-book layout with official code detail, always-visible risk models, Code Trumping, and Buddy coding guidance.
FY 2026 Apr update / Certain infectious and parasitic diseases (A00-B99) / Viral and prion infections of the central nervous system (A80-A89)
A83.3
Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidanceSt Louis encephalitis
St. Louis encephalitis is a viral infection transmitted by mosquitoes that causes inflammation of the brain, leading to symptoms such as fever, headache, confusion, and potentially serious neurological complications. This disease is most common in North America during summer and fall months.
CMS-HCC V28
00
RAF 0
CMS-HCC V24
00
RAF 0
ACA/HHS
00
RAF 0
ESRD/PACE
00
RAF 0
RXHCC
00
RAF 0
Code Trumping
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Code Book Path
Inclusion Terms
OfficialICD-10-CM does not list inclusion terms for A83.3 in this effective period.
Excludes 2
Official- Venezuelan equine encephalitis (A92.2)
- West Nile fever (A92.3-)
- West Nile virus (A92.3-)
Related Child Codes
Includes
Official- mosquito-borne viral meningoencephalitis
Excludes 1
OfficialICD-10-CM does not list Excludes 1 notes for A83.3 in this effective period.
Code First
OfficialICD-10-CM does not list Code First sequencing instructions for A83.3 in this effective period.
Use Additional
OfficialICD-10-CM does not list Use Additional Code instructions for A83.3 in this effective period.
Code Also
OfficialICD-10-CM does not list Code Also instructions for A83.3 in this effective period.
Buddy Documentation Tip
Last updated: FY2026 ICD-10-CM Apr update, Apr 1, 2026 through Sep 30, 2026. CMS-HCC V28 is 100% phased in for payment year 2026.
Is A83.3 an HCC code?
No. A83.3 is a billable ICD-10-CM code but does not map to any HCC category in V28, V24, ESRD, or RxHCC.
Coder workflow notes
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This code does not map to an HCC category in any model (V28, V24, ESRD, RxHCC).
What This Code Means
A83.3 is the ICD-10-CM diagnosis code for st louis encephalitis. St. Louis encephalitis is a viral infection transmitted by mosquitoes that causes inflammation of the brain, leading to symptoms such as fever, headache, confusion, and potentially serious neurological complications. This disease is most common in North America during summer and fall months. A83.3 sits in the ICD-10-CM chapter for certain infectious and parasitic diseases (a00-b99), within the section covering viral and prion infections of the central nervous system (a80-a89).
A83.3 is a billable ICD-10-CM code but does not map to a payment HCC under the CMS-HCC V28, V24, ESRD, or RxHCC risk adjustment models. It can be reported on Medicare Advantage encounter data submissions but it does not contribute to a beneficiary's RAF score and therefore does not affect risk-adjusted payments to the plan.
Verify the diagnosis is specifically St. Louis encephalitis (not other forms of encephalitis) before assigning this code, as documentation must clearly distinguish it from other arboviral encephalitis conditions.
HCC Buddy maintains structured V28 and V24 mapping, RAF weights, and MEAT documentation criteria for A83.3 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Verify the diagnosis is specifically St. Louis encephalitis (not other forms of encephalitis) before assigning this code, as documentation must clearly distinguish it from other arboviral encephalitis conditions
- •This code should be used in conjunction with any documented complications such as seizures, altered mental status, or other neurological manifestations that may require additional coding