I21.A9 Other myocardial infarction type

The ICD-10-CM code for Other myocardial infarction type is I21.A9 (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.A9:

• Infarct, infarction › myocardium, myocardial (acute) (with stated duration of 4 weeks or less) › type 4 (a) (b) (c)
• Infarct, infarction › myocardium, myocardial (acute) (with stated duration of 4 weeks or less) › associated with revascularization procedure
• Infarct, infarction › myocardium, myocardial (acute) (with stated duration of 4 weeks or less) › type 5
• Infarct, infarction › myocardium, myocardial (acute) (with stated duration of 4 weeks or less) › type 3
• Infarct, infarction › myocardium, myocardial (acute) (with stated duration of 4 weeks or less) › subsequent (recurrent) (reinfarction) › type 3
• Infarct, infarction › myocardium, myocardial (acute) (with stated duration of 4 weeks or less) › subsequent (recurrent) (reinfarction) › type 5
• Infarct, infarction › myocardium, myocardial (acute) (with stated duration of 4 weeks or less) › subsequent (recurrent) (reinfarction) › type 4

Inclusion Terms

• Myocardial infarction associated with revascularization procedure
• Myocardial infarction type 3
• Myocardial infarction type 4a
• Myocardial infarction type 4b
• Myocardial infarction type 4c
• Myocardial infarction type 5

U.S. Hospital Utilization

• An estimated 9,405 U.S. inpatient stays in 2023 included I21.A9 among the documented diagnoses.
• 220 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

Do not use I22 for subsequent MI other than type 1/unspecified

Do not assign code I22 for subsequent myocardial infarctions other than type 1 or unspecified. For subsequent type 2 AMI assign only code I21.A1. For subsequent type 4 or type 5 AMI, assign only code I21.A9.

Types 3, 4a-4c, and 5 MI assigned to I21.A9

Acute myocardial infarctions type 3, 4a, 4b, 4c and 5 are assigned to code I21.A9, Other myocardial infarction type.

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)