I21.4 ICD-10-CM Code: Non-ST elevation (NSTEMI) myocardial infarction
I21.4 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)
I21.4
Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidanceNon-ST elevation (NSTEMI) myocardial infarction
A heart attack where the blood supply to the heart is blocked but without the characteristic ST segment elevation on an EKG, typically involving partial blockage or smaller vessel occlusion.
Buddy Insight
Non-ST elevation myocardial infarction is the most common type of acute coronary syndrome, characterized by cardiac biomarker elevation without persistent ST elevation on electrocardiogram.
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- Acute subendocardial myocardial infarction
- Non-Q wave myocardial infarction NOS
- Nontransmural myocardial infarction NOS
- Type 1 non-ST elevation myocardial infarction
Excludes 2
Official- old myocardial infarction (I25.2)
- postmyocardial infarction syndrome (I24.1)
- subsequent type 1 myocardial infarction (I22.-)
Related Child Codes
Includes
Official- cardiac infarction
- coronary (artery) embolism
- coronary (artery) occlusion
- coronary (artery) rupture
- coronary (artery) thrombosis
Excludes 1
OfficialICD-10-CM does not list Excludes 1 notes for I21.4 in this effective period.
Code First
OfficialICD-10-CM does not list Code First sequencing instructions for I21.4 in this effective period.
Use Additional
Official- 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)
Code Also
OfficialICD-10-CM does not list Code Also instructions for I21.4 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 I21.4 an HCC code?
Yes. I21.4 (Non-ST elevation (NSTEMI) myocardial infarction) 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: I21.4 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
- I21.4
- Description
- Non-ST elevation (NSTEMI) myocardial infarction
- 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 I21.4 in the Code Book — tabular path, V28 RAF, and MEAT checklist →
MEAT Criteria for I21.4
For I21.4 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 I21.4 during that encounter, not just copy-forwarded from a problem list.
Coder workflow notes
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What This Code Means
I21.4 is the ICD-10-CM diagnosis code for non-st elevation (nstemi) myocardial infarction. A heart attack where the blood supply to the heart is blocked but without the characteristic ST segment elevation on an EKG, typically involving partial blockage or smaller vessel occlusion. I21.4 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, I21.4 maps to Acute Myocardial Infarction (HCC 228) with a community, non-dual, aged base RAF weight of 0.252. Under the older V24 model, I21.4 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.
NSTEMI is confirmed by elevated cardiac biomarkers (troponin) without ST elevation on EKG. Because I21.4 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 I21.4 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •NSTEMI is confirmed by elevated cardiac biomarkers (troponin) without ST elevation on EKG
- •Assign the appropriate 7th character for timing; NSTEMI codes do not specify the culprit vessel location
Clinical Significance
Non-ST elevation myocardial infarction is the most common type of acute coronary syndrome, characterized by cardiac biomarker elevation without persistent ST elevation on electrocardiogram. It represents partial or transient coronary artery occlusion and carries significant short-term and long-term morbidity. Accurate capture is critical as it maps to the highest ischemic heart disease HCC category.
Documentation Requirements
- ✓Explicit documentation of Non-ST elevation myocardial infarction or NSTEMI diagnosis
- ✓Elevated cardiac biomarkers (troponin) with clinical context
- ✓Electrocardiogram findings showing absence of persistent ST elevation (may show ST depression, T-wave inversion, or be normal)
- ✓Clinical symptoms consistent with acute coronary syndrome
- ✓Treatment plan including anticoagulation, antiplatelet therapy, and consideration for cardiac catheterization
- ✓Timing within the 4-week acute window
Commonly Confused Codes
- •I21.01-I21.29: STEMI codes: require persistent ST elevation on electrocardiogram
- •I20.0: Unstable angina: similar presentation but without biomarker elevation; maps to HCC 87, not 86
- •I21.9: Acute myocardial infarction, unspecified: less specific and should not be used when NSTEMI is confirmed
- •I22.2: Subsequent NSTEMI: for recurrent NSTEMI within 4 weeks of initial event
- •I21.A1: Myocardial infarction type 2: caused by supply-demand mismatch, not plaque rupture