Question 1

What is ICD-10 code A83.1?

Accepted Answer

A83.1 is the ICD-10-CM diagnosis code for western equine encephalitis. Western equine encephalitis is a viral infection that causes inflammation of the brain, transmitted to humans through mosquito bites, primarily occurring in western North America. Symptoms include fever, headache, and neurological complications ranging from mild to severe. A83.1 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).

A83.1 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 specific type of equine encephalitis documented (Western, Eastern, or Venezuelan) as each has a distinct ICD-10-CM code; do not assume based on geographic location alone.

Question 2

Is A83.1 an HCC code?

Accepted Answer

No. A83.1 is a billable ICD-10-CM code but does not map to any payment HCC category under the CMS-HCC V28, V24, ESRD, or RxHCC risk adjustment models. It can still be reported on Medicare Advantage encounter data submissions, but it does not contribute to the beneficiary's Risk Adjustment Factor (RAF) score and therefore does not affect the risk-adjusted capitation payment to the plan. V28 reached 100% phase-in for payment year 2026.

Question 3

Is A83.1 billable?

Accepted Answer

Yes. A83.1 is a billable ICD-10-CM code that can be reported on professional and institutional claims as a primary or secondary diagnosis when the provider documentation supports the specific condition described. Billable codes in ICD-10-CM are coded to the highest level of specificity available for the documented diagnosis. A83.1 is current for fiscal year 2026 and remains in the active CMS ICD-10-CM code set published by the National Center for Health Statistics.

Question 4

What is the V28 vs V24 difference for A83.1?

Accepted Answer

A83.1 does not map to a payment HCC under either the CMS-HCC V28 risk adjustment model or the older V24 model. V28 reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition. Because neither community model rewards A83.1 with a risk adjustment factor, it contributes zero to a Medicare Advantage beneficiary's V28/V24 community RAF score in either version. Coders should check whether provider documentation supports a more specific child code that does map to a community payment HCC.

Question 5

What should coders know when reporting A83.1?

Accepted Answer

Verify the specific type of equine encephalitis documented (Western, Eastern, or Venezuelan) as each has a distinct ICD-10-CM code; do not assume based on geographic location alone Document whether this is the acute phase or if there are residual neurological complications, as sequelae may require additional coding with code category B94.8

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A83.1

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A83.1 ICD-10-CM Code: Western equine encephalitis

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A83.1

Is A83.1 an HCC code?

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