What is ICD-10 code I63.321?

I63.321 is the ICD-10-CM diagnosis code for Cerebral infarction due to thrombosis of right anterior cerebral artery. A stroke caused by a blood clot blocking the right anterior cerebral artery, which supplies the front portion of the right side of the brain. I63.321 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. I63.321 is part of the FY2026 ICD-10-CM code set published by the CMS National Center for Health Statistics and used on Medicare, Medicaid, and commercial claims in the United States.

Yes. I63.321 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. I63.321 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.

Which HCC does I63.321 map to under V28?

I63.321 maps to CMS-HCC V28 HCC 249 (Ischemic or Unspecified Stroke). The category numbers, labels, and conditions come from the CMS-HCC V28 model file for payment year 2026.

2026 ICD-10-CM I63.321: Cerebral infarction due

I63.321 maps to CMS-HCC V28 249. A source-labeled RAF reference is available. Documentation must support MEAT. MEAT criteria · RAF Calculator · free HCC coding tools

ICD-10-CM Code View

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Code lookupI63.321Effective periodFY2026 Apr Update (Apr 1-Sep 30)

FY 2026 Apr update / Diseases of the circulatory system (I00-I99) / Cerebrovascular diseases (I60-I69)

I63.321

Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidance

Cerebral infarction due to thrombosis of right anterior cerebral artery

A stroke caused by a blood clot blocking the right anterior cerebral artery, which supplies the front portion of the right side of the brain.

Buddy Insight

This code identifies an acute ischemic stroke caused by thrombosis (in-situ clot formation) of the right anterior cerebral artery, requiring precise documentation of the clot mechanism.

CMS-HCC V28

Mapped

HCC 249

Ischemic or Unspecified Stroke

Code-level coefficient reference

CMS-HCC V24

Historical

Not used for standard CY2026 Medicare Advantage payment. Select a historical period to review V24.

Not used for CY2026 payment

ACA/HHS

Mapped

HCC 146

HHS-HCC 146 adult, RAF varies...

Code-level coefficient reference

ESRD/PACE

Mapped

HCC 100

Ischemic or Unspecified Stroke

Code-level coefficient reference

RXHCC

N/A

—

Not risk-adjusted for this model/period.

Not mapped

Code Book Path

Official

I63.3Cerebral infarction due to thrombosis of cerebral arteries

I63.32Cerebral infarction due to thrombosis of anterior cerebral artery

I63.321Cerebral infarction due to thrombosis of right anterior cerebral artery

Inclusion Terms

Official

ICD-10-CM does not list inclusion terms for I63.321 in this effective period.

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for I63.321 in this effective period.

Related Child Codes

Official

I63.322Cerebral infarction due to thrombosis of left anterior cerebral artery

I63.323Cerebral infarction due to thrombosis of bilateral anterior cerebral arteries

I63.329Cerebral infarction due to thrombosis of unspecified anterior cerebral artery

Includes

Official

ICD-10-CM does not list Includes notes for I63.321 in this effective period.

Excludes 1

Official

ICD-10-CM does not list Excludes 1 notes for I63.321 in this effective period.

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for I63.321 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for I63.321 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for I63.321 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance

Provider documentation of acute cerebral infarction (stroke) as a confirmed diagnosis, not rule-out or suspected

Identification of the affected artery as the anterior cerebral artery

Documentation of laterality (right) supported by clinical findings and/or imaging

Documentation that thrombosis (in-situ clot formation) is the underlying mechanism, typically supported by vascular imaging such as computed tomography angiography or magnetic resonance angiography

MEAT Support

HCC Buddy guidance

Provider documentation of acute cerebral infarction (stroke) as a confirmed diagnosis, not rule-out or suspected

Identification of the affected artery as the anterior cerebral artery

Documentation of laterality (right) supported by clinical findings and/or imaging

Documentation that thrombosis (in-situ clot formation) is the underlying mechanism, typically supported by vascular imaging such as computed tomography angiography or magnetic resonance angiography

Audit Caution

HCC Buddy guidance

Confusing thrombosis with embolism — thrombosis means the clot formed locally in the anterior cerebral artery, while embolism means the clot traveled from elsewhere; query the provider if uncertain

Assigning an acute cerebral infarction code for a history of stroke or residual deficits from a prior stroke — use I69.3xx sequelae codes for follow-up encounters after the acute phase

Failing to capture laterality when it is documented — avoid defaulting to the unspecified laterality code when the provider has documented right

Coding a transient ischemic attack as a cerebral infarction — a transient ischemic attack resolves within 24 hours with no infarction on imaging and uses G45 codes instead

Common Mistakes

HCC Buddy guidance

I63.421 — Cerebral infarction due to embolism of right anterior cerebral artery; use for embolic rather than thrombotic mechanism

I69.3xx — Sequelae of cerebral infarction; use for residual deficits from a prior stroke during follow-up visits, not during the acute event

G45.9 — Transient cerebral ischemic attack, unspecified; use when symptoms resolve completely within 24 hours with no evidence of infarction on imaging

Current with CMS: FY2026 ICD-10-CM Apr 1 update (effective Apr 1 – Sep 30, 2026) · CMS-HCC V28, 100% phased in for payment year 2026. FY2027 code set already staged for October 1, 2026. How HCC Buddy stays current →

Is I63.321 an HCC code?

Yes. I63.321 (Cerebral infarction due to thrombosis of right anterior cerebral artery) maps to HCC 249, Ischemic or Unspecified Stroke under the CMS-HCC V28 risk adjustment model, with a source-labeled community non-dual aged reference coefficient of 0.239. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes. It is billable for payment year 2026.

Coder answer: I63.321 is billable and maps to V28 HCC 249, Ischemic or Unspecified Stroke. Open it in the Code Book for the tabular path, RAF, and MEAT checklist.

Code: I63.321
Description: Cerebral infarction due to thrombosis of right anterior cerebral artery
HCC (V28): HCC 249 — Ischemic or Unspecified Stroke
RAF reference coefficient: 0.239
Billable: Yes
Payment year: 2026

HCC Category Mapping

V28HCC 249, Ischemic or Unspecified Stroke

0.239

ESRDHCC 100, Ischemic or Unspecified Stroke

Not separately weighted

These are source-labeled model coefficients, not member totals. A category may still be removed by hierarchy or model cleanup rules. Weights are not directly comparable across models: CMS-HCC V28 and V24 use Community, Non-Dual, Aged; ESRD uses the dialysis continuing-enrollee model; RxHCC is the Part D continuing-enrollee, non-low-income, aged weight (a larger scale than CMS-HCC). ACA/HHS has no single weight — it varies by metal level. Actual per-patient RAF contribution depends on member context, hierarchy and cleanup rules, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains available for historical review.

Work I63.321 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →

MEAT Criteria for I63.321

For I63.321 to count as a valid HCC diagnosis in a given encounter, the provider's documentation must show MEAT: Monitor, Evaluate, Assess, or Treat. A diagnosis from a prior year does not carry forward automatically, it has to be re-documented and supported each calendar year.

MMonitor: signs, symptoms, disease progression, or lab trending documented in the note
EEvaluate: test results, medication response, or physical findings reviewed by the provider
AAssess: explicit mention in the assessment or plan with acknowledgment of status
TTreat: medication, referral, procedure, therapy, or counseling tied to the diagnosis

Only one of M/E/A/T is required to support the code, but the documentation must be specific enough to show that the provider actually addressed I63.321 during that encounter, not just copy-forwarded from a problem list.

Coder workflow notes

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What This Code Means

I63.321 is the ICD-10-CM diagnosis code for cerebral infarction due to thrombosis of right anterior cerebral artery. A stroke caused by a blood clot blocking the right anterior cerebral artery, which supplies the front portion of the right side of the brain. I63.321 sits in the ICD-10-CM chapter for diseases of the circulatory system (i00-i99), within the section covering cerebrovascular diseases (i60-i69).

Under the CMS-HCC V28 risk adjustment model, I63.321 maps to Ischemic or Unspecified Stroke (HCC 249) with a source-labeled community, non-dual, aged reference coefficient of 0.239. No V24 mapping is shown for I63.321; use the applicable model and payment year when reviewing the V28 mapping. V28 is the CMS-HCC risk adjustment model that reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes.

Verify documentation specifies 'right' side and 'anterior cerebral artery' to distinguish from middle or posterior cerebral arteries. Because I63.321 maps to an HCC category, the provider's documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) before the diagnosis is used for risk adjustment. When documentation is ambiguous, coders should issue a provider query rather than assume the highest-specificity variant.

HCC Buddy maintains structured V28 and V24 mapping, source-labeled coefficient references, and MEAT documentation criteria for I63.321 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•Verify documentation specifies 'right' side and 'anterior cerebral artery' to distinguish from middle or posterior cerebral arteries
•Confirm thrombosis is the documented mechanism of stroke

Clinical Significance

This code identifies an acute ischemic stroke caused by thrombosis (in-situ clot formation) of the right anterior cerebral artery, requiring precise documentation of the clot mechanism. The anterior cerebral artery supplies the medial surfaces of the frontal and parietal lobes, including the motor and sensory cortex for the lower extremity. Accurate coding of the mechanism, artery, and laterality is critical for risk adjustment and tracking stroke subtypes for quality measures and secondary prevention strategies.

Documentation Requirements

✓Provider documentation of acute cerebral infarction (stroke) as a confirmed diagnosis, not rule-out or suspected
✓Identification of the affected artery as the anterior cerebral artery
✓Documentation of laterality (right) supported by clinical findings and/or imaging
✓Documentation that thrombosis (in-situ clot formation) is the underlying mechanism, typically supported by vascular imaging such as computed tomography angiography or magnetic resonance angiography
✓Neurological examination findings consistent with the identified vascular territory (e.g., contralateral leg weakness greater than arm weakness, personality changes, urinary incontinence, and abulia)
✓Brain imaging (computed tomography or magnetic resonance imaging) confirming acute infarction
✓Timing of symptom onset to confirm acute presentation
✓Documentation of stroke severity (National Institutes of Health Stroke Scale score preferred)
✓Treatment administered (thrombolytics, thrombectomy, antiplatelet therapy, anticoagulation)

Commonly Confused Codes

•I63.421: Cerebral infarction due to embolism of right anterior cerebral artery; use for embolic rather than thrombotic mechanism
•I69.3xx: Sequelae of cerebral infarction; use for residual deficits from a prior stroke during follow-up visits, not during the acute event
•G45.9: Transient cerebral ischemic attack, unspecified; use when symptoms resolve completely within 24 hours with no evidence of infarction on imaging