I21.3 ST elevation (STEMI) myocardial infarction of unspecified site

The ICD-10-CM code for ST elevation (STEMI) myocardial infarction of unspecified site is I21.3 (FY2026). It is a billable, claim-ready diagnosis code.

Classification

Chapter

9: Diseases of the circulatory system (I00-I99)

Section

I20-I25: Ischemic heart diseases (I20-I25)

Category I21

18 codes (13 billable)

FY2026 Status

Stable since FY2024

Also Known As

ICD-10-CM Alphabetic Index entries that lead to I21.3:

• STEMI
• Infarct, infarction › myocardium, myocardial (acute) (with stated duration of 4 weeks or less) › ST elevation (STEMI)
• Infarct, infarction › myocardium, myocardial (acute) (with stated duration of 4 weeks or less) › transmural (see also, Infarct, myocardium, ST elevation, by site)
• Infarct, infarction › myocardium, myocardial (acute) (with stated duration of 4 weeks or less) › Q wave (see also, Infarct, myocardium, ST elevation, by site)

Inclusion Terms

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

U.S. Hospital Utilization

• An estimated 32,130 U.S. inpatient stays in 2023 included I21.3 among the documented diagnoses.
• 18,090 stays listed it as the principal diagnosis.

Source: National Inpatient Sample (NIS), Healthcare Cost and Utilization Project (HCUP), Agency for Healthcare Research and Quality, 2016–2023. National survey-weighted estimates.

Official Coding Guidelines

Type 1 STEMI/NSTEMI code selection and conversion rules

e. Acute myocardial infarction (AMI) 1) Type 1 ST elevation myocardial infarction (STEMI) and non- ST elevation myocardial infarction (NSTEMI) The ICD-10-CM codes for type 1 acute myocardial infarction (AMI) identify the site, such as anterolateral wall or true posterior wall. Subcategories I21.0-I21.2 and code I21.3 are used for type 1 ST elevation myocardial infarction (STEMI). Code I21.4, Non-ST elevation (NSTEMI) myocardial infarction, is used for type 1 non-ST elevation myocardial infarction (NSTEMI) and nontransmural MIs. If a type 1 NSTEMI evolves to STEMI, assign the STEMI code. If a type 1 STEMI converts to NSTEMI due to thrombolytic therapy, it is still coded as STEMI.

Unspecified AMI defaults to I21.9; STEMI w/o site to I21.3

2) Acute myocardial infarction, unspecified Code I21.9, Acute myocardial infarction, unspecified, is the default for unspecified acute myocardial infarction or unspecified type. If only type 1 STEMI or transmural MI without the site is documented, assign code I21.3, ST elevation (STEMI) myocardial infarction of unspecified site.

Source: CMS — ICD-10-CM Official Guidelines for Coding and Reporting, FY2026

References

• CMS — ICD-10-CM Official Code Set and Guidelines, FY2026
• CDC/NCHS — ICD-10-CM Files and Browser Tool
• AHRQ — HCUP National Inpatient Sample, Frequencies by Diagnosis Code (2016–2023)