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

Billable ICD-10-CM code, FY2026. Acts as a Major Complication or Comorbidity (MCC) as a secondary diagnosis.

Applicable to

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

Applies from I21 Acute myocardial infarction

Includes

• cardiac infarction
• coronary (artery) embolism
• coronary (artery) occlusion
• coronary (artery) rupture
• coronary (artery) thrombosis
• infarction of heart, myocardium, or ventricle
• myocardial infarction specified as acute or with a stated duration of 4 weeks (28 days) or less from onset

Excludes2 (not included here)

• old myocardial infarction (I25.2)
• postmyocardial infarction syndrome (I24.1)
• subsequent type 1 myocardial infarction (I22.-)

Use additional code

• code, if applicable, to identify:
• exposure to environmental tobacco smoke (Z77.22)
• history of tobacco dependence (Z87.891)
• occupational exposure to environmental tobacco smoke (Z57.31)
• status post administration of tPA (rtPA) in a different facility within the last 24 hours prior to admission to current facility (Z92.82)
• tobacco dependence (F17.-)
• tobacco use (Z72.0)

Risk adjustment (HCC)

• CMS-HCC V28: HCC 228 — Acute Myocardial Infarction
• CMS-HCC V22: HCC 86 — Acute Myocardial Infarction
• RxHCC V08: HCC 188 — Coronary Artery Disease
• ESRD V24: HCC 86

MS-DRG index

Defines the logic of DRG(s) 280-285 in MDC 05.

Approximate ICD-9-CM (GEMs)

• 410.91 Acute myocardial infarction of unspecified site, initial episode of care (approximate)

Code annotations containing back-references to I21.3

Back-references to I21

• Use Additional: I16.1 Hypertensive emergency ("acute myocardial infarction (I21.-)")
• Type 1 Excludes: I5A Non-ischemic myocardial injury (non-traumatic) ("acute myocardial infarction (I21.-)")
• Type 2 Excludes: I74 Arterial embolism and thrombosis ("coronary embolism and thrombosis (I21-I25)")
• Type 2 Excludes: I82 Other venous embolism and thrombosis ("coronary (I21-I25)")
• Code First: Z92.82 Status post administration of tPA (rtPA) in a different facility within the last 24 hours prior to admission to current facility ("acute myocardial infarction (I21.-, I22.-)")

Diagnosis Index entries for I21.3

• 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

Nearby codes

• I21.11 ST elevation (STEMI) myocardial infarction involving right coronary artery
• I21.19 ST elevation (STEMI) myocardial infarction involving other coronary artery of inferior wall
• I21.2 ST elevation (STEMI) myocardial infarction of other sites
• I21.21 ST elevation (STEMI) myocardial infarction involving left circumflex coronary artery
• I21.29 ST elevation (STEMI) myocardial infarction involving other sites
• I21.3 ST elevation (STEMI) myocardial infarction of unspecified site (this code)
• I21.4 Non-ST elevation (NSTEMI) myocardial infarction
• I21.9 Acute myocardial infarction, unspecified
• I21.A Other type of myocardial infarction
• I21.A1 Myocardial infarction type 2
• I21.A9 Other myocardial infarction type