2026 ICD-10-CM Diagnosis Code I21.21ST elevation (STEMI) myocardial infarction involving left circumflex coronary artery

ICD-10-CM Codes›I00–I99›I20-I25›I21

ICD-10-CM I21.21
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

I21.21 is a billable ICD-10-CM diagnosis code for ST elevation (STEMI) myocardial infarction involving left circumflex coronary artery. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 280 through 285. Coders also document this condition as acute myocardial infarction due to occlusion of circumflex branch of left coronary artery. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Acute myocardial infarction.

This medical diagnosis code is frequently used in Cardiology medical specialties to designate conditions such selected atherosclerosis, ischemia, and infarction.

Code Identity

ICD-10-CM Code
I21.21
Billable Status
Yes — Valid for Submission
Code Describes
ST elevation (STEMI) myocardial infarction involving left circumflex coronary artery
Short Description
STEMI involving left circumflex coronary artery
Parent Code
I21.2
ST elevation (STEMI) myocardial infarction of other sites

Code Classification

ChapterI00–I99Diseases of the circulatory system
SectionI20-I25Ischemic heart diseases
CategoryI21Acute myocardial infarction
This CodeI21.21ST elevation (STEMI) myocardial infarction involving left circumflex coronary artery

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Acute myocardial infarction due to occlusion of circumflex branch of left coronary artery
  • Acute ST segment elevation myocardial infarction due to distal left circumflex coronary artery occlusion
  • Acute ST segment elevation myocardial infarction due to mid left circumflex coronary artery occlusion
  • Acute ST segment elevation myocardial infarction due to obtuse marginal branch of left circumflex coronary artery occlusion
  • Acute ST segment elevation myocardial infarction due to occlusion of circumflex branch of left coronary artery
  • Acute ST segment elevation myocardial infarction due to posterolateral branch of left circumflex coronary artery occlusion
  • Acute ST segment elevation myocardial infarction due to proximal left circumflex coronary artery occlusion

Tabular List NotesGuidance

Coding notes and annotation back-references applicable to this code.

Inclusion Terms

  • ST elevation (STEMI) myocardial infarction involving oblique marginal coronary artery

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

External Cause of Injuries IndexGuidance

References for this code in the External Cause of Injuries Index.

    • Infarct, infarction
      • myocardium, myocardial (acute) (with stated duration of 4 weeks or less)
        • ST elevation (STEMI)
          • involving
            • left circumflex coronary artery
    • Infarct, infarction
      • myocardium, myocardial (acute) (with stated duration of 4 weeks or less)
        • ST elevation (STEMI)
          • involving
            • oblique marginal coronary artery

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR CIR009
Acute myocardial infarction
Default principal diagnosis: inpatient Yes · outpatient Yes

Patient EducationClinical

Heart Attack

Each year almost 800,000 Americans have a heart attack. A heart attack happens when blood flow to the heart suddenly becomes blocked. Without the blood coming in, the heart can't get oxygen. If not treated quickly, the heart muscle begins to die.

Read the full article at MedlinePlus

Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.

Convert I21.21 to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
410.81 AMI NEC, initial
Approximate The match is approximate rather than exact.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About I21.21Overview

Is I21.21 (ST elevation (STEMI) myocardial infarction of other sites) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report ST elevation (STEMI) myocardial infarction involving left circumflex coronary artery on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does I21.21 group to?

When ST elevation (STEMI) myocardial infarction involving left circumflex coronary artery is the principal diagnosis on an inpatient stay, it groups to MS-DRG 280, 281, 282, 283, 284, 285, with relative weights from 0.5989 to 1.9808 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of I21.21?

Under the General Equivalence Mappings, ST elevation (STEMI) myocardial infarction involving left circumflex coronary artery converts to ICD-9-CM 410.81 (AMI NEC, initial). The mapping is approximate, so confirm the match fits the documentation.

Footnotes

[1] Chronic - a chronic condition code indicates a condition lasting 12 months or longer and its effect on the patient based on one or both of the following criteria:

  • The condition results in the need for ongoing intervention with medical products,treatment, services, and special equipment
  • The condition places limitations on self-care, independent living, and social interactions.