2026 ICD-10-CM Diagnosis Code I21.3ST elevation (STEMI) myocardial infarction of unspecified site
ICD-10-CM Codes›I00–I99›I20-I25›I21
- Billable — Valid for Submission
- Chronic Condition
I21.3 is a billable ICD-10-CM diagnosis code for ST elevation (STEMI) myocardial infarction of unspecified site. 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. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Acute myocardial infarction.
This medical diagnosis code is frequently used in Cardiology medical specialties to designate conditions such selected atherosclerosis, ischemia, and infarction.
Code Identity
Code Classification
Approximate SynonymsGuidance
Alternate terms and clinical phrases that map to this code.
- Acute anterior ST segment elevation myocardial infarction
- Acute myocardial infarction of apex of heart
- Acute myocardial infarction of atrium
- Acute myocardial infarction of right ventricle
- Acute Q wave infarction - widespread
- Acute Q wave myocardial infarction
- Acute ST segment elevation myocardial infarction
- Acute ST segment elevation myocardial infarction of apex of heart
- Acute ST segment elevation myocardial infarction of atrium
- Acute ST segment elevation myocardial infarction of right ventricle
- Acute widespread myocardial infarction
Tabular List NotesGuidance
Coding notes and annotation back-references applicable to this code.
Inclusion Terms
- Acute transmural myocardial infarction of unspecified site
- Transmural (Q wave) myocardial infarction NOS
- Type 1 ST elevation myocardial infarction of unspecified site
These terms are the conditions for which that code is to be used. The terms may be synonyms of the code title, or, in the case of "other specified" codes, the terms are a list of the various conditions assigned to that code. The inclusion terms are not necessarily exhaustive. Additional terms found only in the Alphabetic Index may also be assigned to a code.
Index to Diseases and InjuriesGuidance
Alphabetical index entries that point to this code.
- STEMI - See Also: - Infarct, myocardium, ST elevation; - I21.3
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)
- Q wave (see also, Infarct, myocardium, ST elevation, by site)
- 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)
- STEMI
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
Clinical InformationClinical
Acute Myocardial Infarction
necrosis of the myocardium, as a result of interruption of the blood supply to the area. it is characterized by a severe and rapid onset of symptoms that may include chest pain, often radiating to the left arm and left side of the neck, dyspnea, sweating, and palpitations.Acute Myocardial Infarction by ECG Finding|ACUTE MYOCARDIAL INFARCTION|Acute Myocardial Infarction|Acute Myocardial Infarction by EKG Finding
an electrocardiographic finding showing a current of injury with st elevation. no specification is provided for localization. (cdisc)Acute Myocardial Infarction From PCI Complication Type|PCI Acute Angiographic Complication|PCI Acute Angiographic Complication|PCI Acute Angiographic Complication|PCIAAC|PCIAAC
a description of the complications from a percutaneous coronary intervention procedure that lead to an acute myocardial ischemic event.Acute Myocardial Infarction Type|ACMITYPE|ACMITYPE
a classification or description of the acute myocardial infarction.Death due to Acute Myocardial Infarction Related to COVID-19|Death due to acute MI
death by any cardiovascular mechanism (e.g., arrhythmia, sudden death, heart failure (hf), stroke, pulmonary embolus, peripheral arterial disease) less than or equal to 30 days after a myocardial infarction (mi), related to the immediate consequences of the mi, such as progressive hf or recalcitrant arrhythmia in a patient with probable or confirmed acute covid-19.Acute Myocardial Infarction by ECG Finding|ACUTE MYOCARDIAL INFARCTION|Acute Myocardial Infarction|Acute Myocardial Infarction by EKG Finding
an electrocardiographic finding showing a current injury of the myocardium.
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
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Convert I21.3 to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code HistoryHistory
Questions About I21.3Overview
Is I21.3 (Acute myocardial infarction) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report ST elevation (STEMI) myocardial infarction of unspecified site on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
What MS-DRG does I21.3 group to?
When ST elevation (STEMI) myocardial infarction of unspecified site 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 I21.3?
Under the General Equivalence Mappings, ST elevation (STEMI) myocardial infarction of unspecified site converts to ICD-9-CM 410.91 (AMI NOS, initial). 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.
