2026 ICD-10-CM Diagnosis Code I21.A9Other myocardial infarction type

ICD-10-CM Codes›I00–I99›I20-I25›I21

ICD-10-CM I21.A9
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

I21.A9 is a billable ICD-10-CM diagnosis code for other myocardial infarction type. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 280 through 285. Coders also document this condition as postoperative myocardial infarction. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Acute myocardial infarction.

Code Identity

ICD-10-CM Code
I21.A9
Billable Status
Yes — Valid for Submission
Code Describes
Other myocardial infarction type
Short Description
Other myocardial infarction type
Same as the full description in the CMS dataset.
Parent Code
I21.A
Other type of myocardial infarction

Code Classification

ChapterI00–I99Diseases of the circulatory system
SectionI20-I25Ischemic heart diseases
CategoryI21Acute myocardial infarction
This CodeI21.A9Other myocardial infarction type

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Postoperative myocardial infarction
  • Right ventricular failure due to right ventricular infarction
  • Type 5 myocardial infarction

Tabular List NotesGuidance

Coding notes and annotation back-references applicable to this code.

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

Code First

  • , if applicable, postprocedural myocardial infarction following cardiac surgery I97.190 I97.790

Code Also

  • complication, if known and applicable, such as:
  • acute stent occlusion T82.897
  • acute stent stenosis T82.855
  • acute stent thrombosis T82.867
  • cardiac arrest due to underlying cardiac condition I46.2
  • complication of percutaneous coronary intervention PCI I97.89
  • occlusion of coronary artery bypass graft T82.218

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

External Cause of Injuries IndexGuidance

References for this code in the External Cause of Injuries Index.

    • 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)
        • subsequent (recurrent) (reinfarction)
          • type 3
    • Infarct, infarction
      • myocardium, myocardial (acute) (with stated duration of 4 weeks or less)
        • subsequent (recurrent) (reinfarction)
          • type 4
    • 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)
        • type 3
    • 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)
        • type 5

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR CIR009
Acute myocardial infarction
Default principal diagnosis: inpatient Yes · outpatient Yes

Patient EducationClinical

Heart Attack

Each year almost 800,000 Americans have a heart attack. A heart attack happens when blood flow to the heart suddenly becomes blocked. Without the blood coming in, the heart can't get oxygen. If not treated quickly, the heart muscle begins to die.

Read the full article at MedlinePlus

Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.

Convert I21.A9 to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
410.90 AMI NOS, unspecified
Approximate The match is approximate rather than exact.
ICD-9-CM
410.91 AMI NOS, initial
Approximate The match is approximate rather than exact.
ICD-9-CM
410.92 AMI NOS, subsequent
Approximate The match is approximate rather than exact.

Code History & ChangesHistory

Replacement I21.A9 replaces the following previously assigned code(s):

  • I21.3 - ST elevation (STEMI) myocardial infarction of unsp site
FY 2018AddedAdded to the ICD-10-CM code setEffective October 1, 2017.
FY 2019–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About I21.A9Overview

Is I21.A9 (Other type of myocardial infarction) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report other myocardial infarction type on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does I21.A9 group to?

When other myocardial infarction type is the principal diagnosis on an inpatient stay, it groups to MS-DRG 280, 281, 282, 283, 284, 285, with relative weights from 0.5989 to 1.9808 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of I21.A9?

Under the General Equivalence Mappings, other myocardial infarction type converts to ICD-9-CM 410.90 (AMI NOS, unspecified), 410.91 (AMI NOS, initial), and 410.92 (AMI NOS, subsequent). The mapping is approximate, so confirm the match fits the documentation.

Footnotes

[1] Chronic - a chronic condition code indicates a condition lasting 12 months or longer and its effect on the patient based on one or both of the following criteria:

  • The condition results in the need for ongoing intervention with medical products,treatment, services, and special equipment
  • The condition places limitations on self-care, independent living, and social interactions.