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ICD-10 Code I21: Acute myocardial infarction

I21 is an ICD-10 diagnosis entry used to group records related to Acute myocardial infarction. You may see this entry in coding references, medical records, or claims workflows when a broader diagnosis category is being reviewed before a more specific code is chosen. ICD-10 entries help standardize how diagnoses are organized for coding, reporting, analytics, and documentation. This code sits within the broader ICD-10 area for Diseases of the circulatory system (I00-I99).

What is this code?

ICD-10 entries help standardize how diagnoses are organized for coding, reporting, analytics, and documentation. This code sits within the broader ICD-10 area for Diseases of the circulatory system (I00-I99).

đź“‹Clinical overview

This ICD-10 code is commonly used when documentation describes an acute myocardial infarction. The most accurate code depends on STEMI versus NSTEMI pattern, infarct site or culprit coronary artery, and whether the record documents another acute-MI mechanism such as type 2 or microvascular dysfunction.

When is it used?

  • May be used when a clinician documents acute myocardial infarction in a patient's medical record.
  • May appear in hospital records, claims, referrals, and clinical documentation.
  • This code may act more like a grouping or parent code, so a more specific child code may be used in final documentation when available.

What it does not mean

  • A code alone does not explain severity, treatment plan, or outcome.
  • A medical code should not be treated as a substitute for a doctor's diagnosis or advice.
  • This entry may represent a broader category rather than the most specific billable code.

đź©şClinical context

When commonly used
  • Emergency, inpatient, cardiology, and utilization-review workflows where an acute myocardial infarction is part of the documented encounter.
  • Coding workflows where the chart identifies STEMI versus NSTEMI pattern, infarct territory, culprit artery, or another acute-MI mechanism that changes final code specificity.
Common management context
  • Evaluation often includes chest-pain or equivalent symptom review, ECG interpretation, troponin testing, coronary-imaging or catheterization context, and acute cardiology management planning.
  • Documentation usually becomes more specific when the chart identifies anterior versus inferior versus other territory, a named culprit coronary artery, STEMI versus NSTEMI, or another acute-MI mechanism such as type 2 or microvascular dysfunction.
  • Specialist context already linked on this page includes Cardiologist, General Physician.

🔍Key distinctions

Important notes
  • This ICD-10 family captures active acute myocardial infarction rather than chronic ischemic heart disease or a remote prior infarction.
  • STEMI versus NSTEMI pattern, infarct territory, culprit-vessel detail, and alternative acute-MI mechanisms all affect final code specificity inside this family.
  • Documentation that describes acute myocardial infarction is coded differently from unstable angina, chronic ischemic heart disease, or prior healed myocardial infarction.
Important exclusions
  • old myocardial infarction (I25.2)
  • postmyocardial infarction syndrome (I24.1)
  • subsequent type 1 myocardial infarction (I22.-)
Related codes to consider
I21.3ST elevation (STEMI) myocardial infarction of unspecified site
Related acute-myocardial-infarction ICD-10 code; documentation often becomes more specific when the chart identifies STEMI versus NSTEMI, infarct territory, culprit coronary artery, type-2 versus other mechanism, or another acute coronary context.
I21.4Non-ST elevation (NSTEMI) myocardial infarction
Related acute-myocardial-infarction ICD-10 code; documentation often becomes more specific when the chart identifies STEMI versus NSTEMI, infarct territory, culprit coronary artery, type-2 versus other mechanism, or another acute coronary context.
I21.9Acute myocardial infarction, unspecified
Related acute-myocardial-infarction ICD-10 code; documentation often becomes more specific when the chart identifies STEMI versus NSTEMI, infarct territory, culprit coronary artery, type-2 versus other mechanism, or another acute coronary context.
I21.BMyocardial infarction with coronary microvascular dysfunction
Related acute-myocardial-infarction ICD-10 code; documentation often becomes more specific when the chart identifies STEMI versus NSTEMI, infarct territory, culprit coronary artery, type-2 versus other mechanism, or another acute coronary context.

Code hierarchy

chapter
9Diseases of the circulatory system (I00-I99)
block
I20-I25Ischemic heart diseases
currentI20-I25

Official coding notes

Includes
  • cardiac infarction
  • coronary (artery) embolism
  • coronary (artery) occlusion
  • coronary (artery) rupture
  • coronary (artery) thrombosis
  • infarction of heart, myocardium, or ventricle
  • myocardial infarction specified as acute or with a stated duration of 4 weeks (28 days) or less from onset
Applicable to
  • code, if applicable, to identify:
  • exposure to environmental tobacco smoke (Z77.22)
  • history of tobacco dependence (Z87.891)
  • occupational exposure to environmental tobacco smoke (Z57.31)
  • status post administration of tPA (rtPA) in a different facility within the last 24 hours prior to admission to current facility (Z92.82)
  • tobacco dependence (F17.-)
  • tobacco use (Z72.0)
Excludes2 (may code together if documented)
  • old myocardial infarction (I25.2)
  • postmyocardial infarction syndrome (I24.1)
  • subsequent type 1 myocardial infarction (I22.-)
Use additional code
  • code, if applicable, to identify:
  • exposure to environmental tobacco smoke (Z77.22)
  • history of tobacco dependence (Z87.891)
  • occupational exposure to environmental tobacco smoke (Z57.31)
  • status post administration of tPA (rtPA) in a different facility within the last 24 hours prior to admission to current facility (Z92.82)
  • tobacco dependence (F17.-)
  • tobacco use (Z72.0)

Where you may see this code

You may see this entry in coding references, medical records, or claims workflows when a broader diagnosis category is being reviewed before a more specific code is chosen.

Related specialists

CardiologistGeneral Physician

Coding guidelines

Compatibility

Legacy code aligns to an official FY 2026 category. Frontend should resolve to the official category page and surface the billable child codes.
Legacy codes
I21
Replacement codes
I21.3 — ST elevation (STEMI) myocardial infarction of unspecified siteI21.4 — Non-ST elevation (NSTEMI) myocardial infarctionI21.9 — Acute myocardial infarction, unspecifiedI21.B — Myocardial infarction with coronary microvascular dysfunction

Common synonyms

Acute myocardial infarction

Frequently asked questions

Code details

CodeI21
SystemICD10
Display nameAcute myocardial infarction
ChapterDiseases of the circulatory system (I00-I99)
BlockIschemic heart diseases

Flags

BillableNo
Valid clinical useNo

Source

SourceICD-10
Version2026-annual
Releaseannual
Year2026

About this content

This page is prepared by HealthAssure's clinical team using official coding standards from ICD-10. AI tools assist with drafting explanations, which are then reviewed and verified by healthcare professionals for accuracy. This content is for informational purposes and does not replace professional medical advice. Meet our team.