I21.01 ICD-10-CM Code: ST elevation (STEMI) myocardial infarction involving left main coronary artery
I21.01 maps to CMS-HCC V28 228 (RAF 0.252). Documentation must support MEAT. MEAT criteria · RAF calculator · HCC Buddy coding tools
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FY 2026 Apr update / Diseases of the circulatory system (I00-I99) / Ischemic heart diseases (I20-I25)
I21.01
Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidanceST elevation (STEMI) myocardial infarction involving left main coronary artery
A heart attack caused by a blood clot blocking the left main coronary artery, with characteristic ST segment elevation visible on an EKG.
Buddy Insight
STEMI involving the left main coronary artery represents one of the most life-threatening acute cardiac events, as the left main supplies approximately 75% of the left ventricular myocardium.
CMS-HCC V28
MappedHCC 228
RAF 0.252
CMS-HCC V24
MappedHCC 86
RAF 0.195
ACA/HHS
MappedHCC 131
Varies by metal level
ESRD/PACE
MappedHCC 86
RAF 0.151
RXHCC
MappedHCC 188
RAF 0.052
Code Book Path
Inclusion Terms
Official- Type 1 ST elevation myocardial infarction of anterior wall
Excludes 2
OfficialICD-10-CM does not list Excludes 2 notes for I21.01 in this effective period.
Related Child Codes
Includes
OfficialICD-10-CM does not list Includes notes for I21.01 in this effective period.
Excludes 1
OfficialICD-10-CM does not list Excludes 1 notes for I21.01 in this effective period.
Code First
OfficialICD-10-CM does not list Code First sequencing instructions for I21.01 in this effective period.
Use Additional
OfficialICD-10-CM does not list Use Additional Code instructions for I21.01 in this effective period.
Code Also
OfficialICD-10-CM does not list Code Also instructions for I21.01 in this effective period.
Buddy Documentation Tip
MEAT Support
Audit Caution
Common Mistakes
Current with CMS: FY2026 ICD-10-CM Apr 1 update (effective Apr 1 – Sep 30, 2026) · CMS-HCC V28, 100% phased in for payment year 2026. FY2027 code set already staged for October 1, 2026. How HCC Buddy stays current →
Is I21.01 an HCC code?
Yes. I21.01 (ST elevation (STEMI) myocardial infarction involving left main coronary artery) maps to Acute Myocardial Infarction under the CMS-HCC V28 risk adjustment model (and Acute Myocardial Infarction under V24), with a community non-dual aged RAF of 0.252. It is billable for payment year 2026.
Coder answer: I21.01 is billable and maps to V28 HCC 228, Acute Myocardial Infarction. Open it in the Code Book for the tabular path, RAF, and MEAT checklist.
- Code
- I21.01
- Description
- ST elevation (STEMI) myocardial infarction involving left main coronary artery
- HCC (V28)
- HCC 228 — Acute Myocardial Infarction
- RAF
- 0.252
- Billable
- Yes
- Payment year
- 2026
HCC Category Mapping
Each model's RAF is its CMS base weight for that model's standard population, so weights are not directly comparable across models: CMS-HCC V28 and V24 use Community, Non-Dual, Aged; ESRD uses the dialysis continuing-enrollee model; RxHCC is the Part D continuing-enrollee, non-low-income, aged weight (a larger scale than CMS-HCC). ACA/HHS has no single weight — it varies by metal level. Actual per-patient RAF contribution depends on member segment, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains in use during the transition and for historical data.
Work I21.01 in the Code Book — tabular path, V28 RAF, and MEAT checklist →
MEAT Criteria for I21.01
For I21.01 to count as a valid HCC diagnosis in a given encounter, the provider's documentation must show MEAT: Monitor, Evaluate, Assess, or Treat. A diagnosis from a prior year does not carry forward automatically, it has to be re-documented and supported each calendar year.
- MMonitor: signs, symptoms, disease progression, or lab trending documented in the note
- EEvaluate: test results, medication response, or physical findings reviewed by the provider
- AAssess: explicit mention in the assessment or plan with acknowledgment of status
- TTreat: medication, referral, procedure, therapy, or counseling tied to the diagnosis
Only one of M/E/A/T is required to support the code, but the documentation must be specific enough to show that the provider actually addressed I21.01 during that encounter, not just copy-forwarded from a problem list.
Coder workflow notes
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What This Code Means
I21.01 is the ICD-10-CM diagnosis code for st elevation (stemi) myocardial infarction involving left main coronary artery. A heart attack caused by a blood clot blocking the left main coronary artery, with characteristic ST segment elevation visible on an EKG. I21.01 sits in the ICD-10-CM chapter for diseases of the circulatory system (i00-i99), within the section covering ischemic heart diseases (i20-i25).
Under the CMS-HCC V28 risk adjustment model, I21.01 maps to Acute Myocardial Infarction (HCC 228) with a community, non-dual, aged base RAF weight of 0.252. Under the older V24 model, I21.01 mapped to the same category but with a base RAF weight of 0.195, V28 recalibrated weights across the entire model. V28 is the CMS-HCC risk adjustment model that reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition.
This code requires documentation of STEMI and specific identification of left main coronary artery involvement. Because I21.01 maps to a payment HCC, the provider's documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's Medicare Advantage risk adjustment score. When documentation is ambiguous, coders should issue a provider query rather than assume the highest-specificity variant.
HCC Buddy maintains structured V28 and V24 mapping, RAF weights, and MEAT documentation criteria for I21.01 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This code requires documentation of STEMI and specific identification of left main coronary artery involvement
- •Verify the timing of the MI (initial episode vs. subsequent episode) as this affects the 7th character assignment
Clinical Significance
STEMI involving the left main coronary artery represents one of the most life-threatening acute cardiac events, as the left main supplies approximately 75% of the left ventricular myocardium. This code carries significant risk adjustment weight due to the extreme acuity and resource utilization associated with left main occlusion. Accurate capture is critical for reflecting the true severity of illness in the patient population.
Documentation Requirements
- ✓Explicit documentation of ST elevation myocardial infarction (STEMI)
- ✓Identification of the left main coronary artery as the culprit vessel
- ✓Electrocardiogram findings showing ST elevation pattern consistent with left main involvement
- ✓Cardiac catheterization or angiography results confirming left main occlusion
- ✓Troponin or other cardiac biomarker elevation
- ✓Timing of the event — initial episode of care within first 4 weeks
- ✓Treatment interventions (percutaneous coronary intervention, coronary artery bypass graft, or thrombolytics)
Commonly Confused Codes
- •I21.02: STEMI involving left anterior descending coronary artery: different vessel; left anterior descending is a branch of the left main
- •I21.09: STEMI involving other coronary artery of anterior wall: used when the anterior wall is involved but the vessel is not left main or left anterior descending
- •I21.3: STEMI of unspecified site: used only when the specific vessel and wall location are not documented
- •I21.4: Non-ST elevation myocardial infarction: different electrocardiogram pattern without ST elevation
- •I25.2: Old myocardial infarction: used for healed myocardial infarction, not acute events