I21.9 Acute myocardial infarction, unspecified
The ICD-10-CM code for Acute myocardial infarction, unspecified is I21.9 (FY2026). It is a billable, claim-ready diagnosis code.
Classification
- 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 I21.9:
- Infarct, infarction › myocardium, myocardial (acute) (with stated duration of 4 weeks or less)
- Infarct, infarction › myocardium, myocardial (acute) (with stated duration of 4 weeks or less) › type 1
Inclusion Terms
- Myocardial infarction (acute) NOS
U.S. Hospital Utilization
- An estimated 8,255 U.S. inpatient stays in 2023 included I21.9 among the documented diagnoses.
- 1,865 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
Unspecified AMI defaults to I21.9; STEMI w/o site to I21.32) 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.
Presumed relationship due to Index term “with”The classification presumes a causal relationship between hypertension and heart involvement and between hypertension and kidney involvement, as the two conditions are linked by the term “with” in the Alphabetic Index. These conditions should be coded as related even in the absence of provider documentation explicitly linking them, unless the documentation clearly states the conditions are unrelated.
Source: CMS — ICD-10-CM Official Guidelines for Coding and Reporting, FY2026