I21.B ICD-10-CM Code: Myocardial infarction with coronary microvascular dysfunction
I21.B maps to CMS-HCC V28 228. A source-labeled RAF reference is available. Confirm the documented diagnosis and applicable coding requirements. MEAT criteria · RAF Calculator · free HCC coding tools
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FY 2026 Apr update / Diseases of the circulatory system (I00-I99) / Ischemic heart diseases (I20-I25)
I21.B
Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidanceMyocardial infarction with coronary microvascular dysfunction
A heart attack caused by dysfunction of the smallest blood vessels in the heart rather than blockage of larger coronary arteries.
Buddy Insight
Myocardial infarction with coronary microvascular dysfunction represents a newer diagnostic entity where myocardial damage occurs due to dysfunction of the small coronary vessels rather than obstruction of epicardial arteries.
CMS-HCC V28
MappedHCC 228
Coefficient HCC 228: 0.252 (Community Non-Dual Aged (CNA))
CMS-HCC V24
HistoricalHistorical
Not used for CY2026 payment
ACA/HHS
MappedHCC 131
Code-level coefficient reference
ESRD/PACE
MappedHCC 86
Code-level coefficient reference
RXHCC
MappedHCC 188
Code-level coefficient reference
Code Book Path
Inclusion Terms
Official- Myocardial infarction with coronary microvascular disease
- Myocardial infarction with nonobstructive coronary arteries [MINOCA] with microvascular disease
Excludes 2
Official- certain conditions originating in the perinatal period (P04-P96)Inherited from I00-I99, I21
- certain infectious and parasitic diseases (A00-B99)Inherited from I00-I99, I21
- complications of pregnancy, childbirth and the puerperium (O00-O9A)Inherited from I00-I99, I21
- congenital malformations, deformations, and chromosomal abnormalities (Q00-Q99)Inherited from I00-I99, I21
- endocrine, nutritional and metabolic diseases (E00-E88)Inherited from I00-I99, I21
- injury, poisoning and certain other consequences of external causes (S00-T88)Inherited from I00-I99, I21
- neoplasms (C00-D49)Inherited from I00-I99, I21
- symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified (R00-R94)Inherited from I00-I99, I21
- systemic connective tissue disorders (M30-M36)Inherited from I00-I99, I21
- transient cerebral ischemic attacks and related syndromes (G45.-)Inherited from I00-I99, I21
- old myocardial infarction (I25.2)Inherited from I00-I99, I21
- postmyocardial infarction syndrome (I24.1)Inherited from I00-I99, I21
- subsequent type 1 myocardial infarction (I22.-)Inherited from I00-I99, I21
Related Codes
Includes
Official- cardiac infarctionInherited from I21
- coronary (artery) embolismInherited from I21
- coronary (artery) occlusionInherited from I21
- coronary (artery) ruptureInherited from I21
- coronary (artery) thrombosisInherited from I21
- infarction of heart, myocardium, or ventricleInherited from I21
- myocardial infarction specified as acute or with a stated duration of 4 weeks (28 days) or less from onsetInherited from I21
Excludes 1
OfficialNo Excludes 1 notes are included in this display for I21.B. Check the code and parent instructions in the Code Book.
Code First
OfficialNo Code First sequencing instructions are included in this display for I21.B. Check the code and parent instructions in the Code Book.
Use Additional
Official- code, if applicable, to identify:Inherited from I21
- exposure to environmental tobacco smoke (Z77.22)Inherited from I21
- history of tobacco dependence (Z87.891)Inherited from I21
- occupational exposure to environmental tobacco smoke (Z57.31)Inherited from I21
- status post administration of tPA (rtPA) in a different facility within the last 24 hours prior to admission to current facility (Z92.82)Inherited from I21
- tobacco dependence (F17.-)Inherited from I21
- tobacco use (Z72.0)Inherited from I21
Code Also
Official- the presence of hypertension (I10-I1A)Inherited from I20-I25
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.B an HCC code?
Yes. I21.B (Myocardial infarction with coronary microvascular dysfunction) maps to HCC 228, Acute Myocardial Infarction under the CMS-HCC V28 risk adjustment model, with a source-labeled community non-dual aged reference coefficient of 0.252. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes. It is billable for payment year 2026.
Coder answer: I21.B 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.B
- Description
- Myocardial infarction with coronary microvascular dysfunction
- HCC (V28)
- HCC 228 — Acute Myocardial Infarction
- RAF reference coefficient
- 0.252
- Billable
- Yes
- Payment year
- 2026
HCC Category Mapping
These are source-labeled model coefficients, not member totals. A category may still be removed by hierarchy or model cleanup rules. 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 context, hierarchy and cleanup rules, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains available for historical review.
Work I21.B in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →
MEAT review for I21.B
For I21.B, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule.
- 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
Coder workflow notes
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What This Code Means
I21.B is the ICD-10-CM diagnosis code for myocardial infarction with coronary microvascular dysfunction. A heart attack caused by dysfunction of the smallest blood vessels in the heart rather than blockage of larger coronary arteries. I21.B 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.B maps to Acute Myocardial Infarction (HCC 228) with a source-labeled community, non-dual, aged reference coefficient of 0.252. For CY2026 non-PACE Medicare Advantage, CMS uses 100% of the 2024 CMS-HCC model (V28). PACE uses a separate model blend. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes.
This code requires documentation specifically mentioning coronary microvascular dysfunction or microvascular angina as the cause. For I21.B, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule. 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, source-labeled coefficient references, and MEAT documentation criteria for I21.B sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This code requires documentation specifically mentioning coronary microvascular dysfunction or microvascular angina as the cause
- •Ensure the provider has documented this diagnosis; it may require cardiac catheterization findings to confirm
Clinical Significance
Myocardial infarction with coronary microvascular dysfunction represents a newer diagnostic entity where myocardial damage occurs due to dysfunction of the small coronary vessels rather than obstruction of epicardial arteries. This condition is more common in women and may be underdiagnosed. The creation of this specific code reflects growing recognition of microvascular disease as a significant cause of ischemic heart events.
Documentation Requirements
- ✓Explicit documentation of myocardial infarction with coronary microvascular dysfunction
- ✓Evidence of myocardial infarction (biomarker elevation, ischemic symptoms)
- ✓Cardiac catheterization showing non-obstructive coronary arteries
- ✓Microvascular function testing results (coronary flow reserve, index of microcirculatory resistance) if performed
- ✓Exclusion of epicardial coronary artery obstruction as the cause
Commonly Confused Codes
- •I21.4: Non-ST elevation myocardial infarction: caused by epicardial coronary disease, not microvascular
- •I21.A1: Myocardial infarction type 2: caused by supply-demand mismatch, different mechanism than microvascular dysfunction
- •I24.81: Acute coronary microvascular dysfunction: microvascular dysfunction without actual myocardial infarction
- •I20.89: Other forms of angina pectoris: chest pain without myocardial damage
- •I25.89: Other forms of chronic ischemic heart disease: chronic condition, not acute myocardial infarction