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ICD-10 Code I21.3: ST elevation (STEMI) myocardial infarction of unspecified site

I21.3 is a billable ICD-10 diagnosis code used to classify ST elevation (STEMI) myocardial infarction of unspecified site in medical records and claims. You may see this code in hospital records, discharge summaries, insurance claims, encounter documentation, referrals, or other healthcare billing and coding records. ICD-10 codes are diagnosis classification codes used in healthcare records, reporting, coding workflows, and billing support. This code sits within the broader ICD-10 area for Diseases of the circulatory system (I00-I99).

What is this code?

ICD-10 codes are diagnosis classification codes used in healthcare records, reporting, coding workflows, and billing support. 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 st elevation (stemi) myocardial infarction of unspecified site in a patient's medical record.
  • May appear in hospital records, claims, referrals, and clinical documentation.
  • This code can be used as a clinically usable diagnosis entry in standardized coding workflows.

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.

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.
  • ST-elevation myocardial infarction is coded separately from NSTEMI and other acute myocardial infarction patterns.
  • This title preserves unspecified infarction-site or acute-MI subtype detail; if the chart identifies the site, culprit artery, or mechanism more clearly, a more specific code may be used.
  • Documentation that describes acute myocardial infarction is coded differently from unstable angina, chronic ischemic heart disease, or prior healed myocardial infarction.
Related codes to consider
I21.0ST elevation (STEMI) myocardial infarction of anterior wall
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.
I21.1ST elevation (STEMI) myocardial infarction of inferior wall
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.
I21.2ST elevation (STEMI) myocardial infarction of other sites
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.

Code hierarchy

chapter
9Diseases of the circulatory system (I00-I99)
block
I20-I25Ischemic heart diseases
category
I21Acute myocardial infarction
currentI21

Official coding notes

Inclusion terms
  • Acute transmural myocardial infarction of unspecified site
  • Transmural (Q wave) myocardial infarction NOS
  • Type 1 ST elevation myocardial infarction of unspecified site

Where you may see this code

You may see this code in hospital records, discharge summaries, insurance claims, encounter documentation, referrals, or other healthcare billing and coding records.

Related specialists

CardiologistGeneral Physician

Coverage-related procedures and services

CPT 93350high
ECHOCARDIOGRAPHY, TRANSTHORACIC, REAL-TIME WITH IMAGE DOCUMENTATION (2D), INCLUDES M-MODE RECORDING, WHEN PERFORMED, DURING REST AND CARDIOVASCULAR STRESS TEST USING TREADMILL, BICYCLE EXERCISE AND/OR PHARMACOLOGICALLY INDUCED STRESS, WITH INTERPRETATION AND REPORT;
Source: CMS coverage guidance
CPT 93351high
ECHOCARDIOGRAPHY, TRANSTHORACIC, REAL-TIME WITH IMAGE DOCUMENTATION (2D), INCLUDES M-MODE RECORDING, WHEN PERFORMED, DURING REST AND CARDIOVASCULAR STRESS TEST USING TREADMILL, BICYCLE EXERCISE AND/OR PHARMACOLOGICALLY INDUCED STRESS, WITH INTERPRETATION AND REPORT; INCLUDING PERFORMANCE OF CONTINUOUS ELECTROCARDIOGRAPHIC MONITORING, WITH SUPERVISION BY A PHYSICIAN OR OTHER QUALIFIED HEALTH CARE PROFESSIONAL
Source: CMS coverage guidance
CPT 78454high
MYOCARDIAL PERFUSION IMAGING, PLANAR (INCLUDING QUALITATIVE OR QUANTITATIVE WALL MOTION, EJECTION FRACTION BY FIRST PASS OR GATED TECHNIQUE, ADDITIONAL QUANTIFICATION, WHEN PERFORMED); MULTIPLE STUDIES, AT REST AND/OR STRESS (EXERCISE OR PHARMACOLOGIC) AND/OR REDISTRIBUTION AND/OR REST REINJECTION
Source: CMS coverage guidance
CPT 78453high
MYOCARDIAL PERFUSION IMAGING, PLANAR (INCLUDING QUALITATIVE OR QUANTITATIVE WALL MOTION, EJECTION FRACTION BY FIRST PASS OR GATED TECHNIQUE, ADDITIONAL QUANTIFICATION, WHEN PERFORMED); SINGLE STUDY, AT REST OR STRESS (EXERCISE OR PHARMACOLOGIC)
Source: CMS coverage guidance
CPT 78452high
MYOCARDIAL PERFUSION IMAGING, TOMOGRAPHIC (SPECT) (INCLUDING ATTENUATION CORRECTION, QUALITATIVE OR QUANTITATIVE WALL MOTION, EJECTION FRACTION BY FIRST PASS OR GATED TECHNIQUE, ADDITIONAL QUANTIFICATION, WHEN PERFORMED); MULTIPLE STUDIES, AT REST AND/OR STRESS (EXERCISE OR PHARMACOLOGIC) AND/OR REDISTRIBUTION AND/OR REST REINJECTION
Source: CMS coverage guidance
CPT 78451high
MYOCARDIAL PERFUSION IMAGING, TOMOGRAPHIC (SPECT) (INCLUDING ATTENUATION CORRECTION, QUALITATIVE OR QUANTITATIVE WALL MOTION, EJECTION FRACTION BY FIRST PASS OR GATED TECHNIQUE, ADDITIONAL QUANTIFICATION, WHEN PERFORMED); SINGLE STUDY, AT REST OR STRESS (EXERCISE OR PHARMACOLOGIC)
Source: CMS coverage guidance

Coding guidelines

Compatibility

Legacy and official ICD code match exactly.
Legacy codes
I21.3

Common synonyms

ST elevation (STEMI) myocardial infarction of unspecified siteST elevation (STEMI) myocardial infarction of siteST elevationST elevation STEMI myocardial infarction of unspecified site

Frequently asked questions

Code details

CodeI21.3
SystemICD10
Display nameST elevation (STEMI) myocardial infarction of unspecified site
ChapterDiseases of the circulatory system (I00-I99)
BlockIschemic heart diseases

Flags

BillableYes
Valid clinical useYes

Source

SourceICD-10
Version2026-annual
Releaseannual
Year2026

Index terms

Matched terms
Infarct, infarctionInfarct, infarctionInfarct, infarctionSTEMI
See also
- Infarct, myocardium, ST elevation

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.