ICD-10 I21.9
Billable / Specific HCC v28: 86 MCCAcute myocardial infarction, unspecified
About ICD-10-CM I21.9
ICD-10-CM code I21.9 represents Acute myocardial infarction, unspecified. This is a billable/specific code in the Circulatory System chapter (block I21). The 2026 edition of ICD-10-CM I21.9 became effective on October 1, 2025.
Coding Tips for I21.9
Specialist guidance from the PayerReady Medical Coding Team. Specificity warnings, HCC capture rules, sequencing notes.
I21.9 is a CMS-HCC v28 risk-adjustment code (category 86). To count for the patient Risk Adjustment Factor (RAF), document the diagnosis with MEAT language each calendar year: Monitored, Evaluated, Assessed, Treated. A diagnosis on the problem list alone does not satisfy CMS RADV audit standards. Include the diagnosis in the assessment with current status and current treatment plan.
I21.9 is designated MCC for MS-DRG grouping. On inpatient claims, this code can shift the DRG to the with-MCC variant when documented as a present-on-admission secondary diagnosis. Hospital CDI programs flag MCC opportunities during chart review. Failure to capture this code may leave 30 to 80 percent of the inpatient stay revenue unrealized.
Inclusion Terms / Approximate Synonyms
Clinical terms and conditions classified under I21.9. Per CMS ICD-10-CM Tabular 2026.
- Myocardial infarction (acute) NOS
Medicare Advantage HCC Impact
Capture this diagnosis annually for accurate risk adjustment. Missed HCC captures are the #1 revenue leak in Medicare Advantage risk programs.
Inpatient DRG Impact, MCC
codes Acute myocardial infarction, unspecified. As a Major Complication/Comorbidity (MCC), this can shift the DRG assignment to a higher-weighted category, substantial reimbursement impact.
Medicare LCD Coverage for I21.9
Local Coverage Determinations (LCDs) from CMS MACs that list I21.9 as a covered diagnosis.
Showing top 10 of 476 total . Click a CPT for full coverage scope.
Commercial Payer Coverage
Coverage policies from major commercial payers referencing I21.9.
1 Cigna
5 Medicare
CPT Codes Commonly Billed with I21.9
Procedures frequently paired with this diagnosis based on PayerReady's Dx↔Px linkage data.
- 93458 Left heart catheterization — CAD, ACS, chest pain Cardiovascular Disease
- 99285 High-level ER E/M — MI, cardiac arrest, respiratory failure, chest pain, syncope, head injury Emergency Medicine
- 93451 Right heart cath — CAD, native vessel disease, ischemic cardiomyopathy, unstable angina, MI, heart failure, valvular disease, chest pain Cardiology
- 93452 Left heart cath — same as right heart cath indications Cardiology
- 93458 Coronary angiography — CAD, unstable angina, MI, heart failure, chest pain, prior bypass graft evaluation Cardiology
- 99284 ED visit level 4 — MI, PE, COPD exacerbation, asthma exacerbation, syncope, GI bleed, hip fracture, concussion, altered consciousness, DKA Emergency Medicine
- 99285 ED visit level 5 — STEMI, massive PE, respiratory failure, cardiogenic shock, cardiac arrest, ICH, encephalopathy, coma, anaphylaxis, SAH, ARDS, compartment syndrome Emergency Medicine
- 92928 PCI with stent — CAD native vessel, unstable angina, STEMI/NSTEMI all arteries, ischemic cardiomyopathy, bypass graft disease Interventional Cardiology
- 92979 CMS LCD: Billing and Coding: Percutaneous Coronary Interventions CMS LCD
- C9607 CMS LCD: Billing and Coding: Percutaneous Coronary Interventions CMS LCD
Codes Adjacent To I21.9
Other codes in section I20-I25 (Ischemic heart diseases).
ICD-10 I21.9, Billing FAQ
Is ICD-10 code I21.9 billable? +
Yes, I21.9 is a billable ICD-10-CM code that can appear as a primary or secondary diagnosis on claims.
What other names or terms map to I21.9? +
ICD-10 I21.9 includes: Myocardial infarction (acute) NOS.
Does I21.9 affect Medicare Advantage HCC risk adjustment? +
Yes. I21.9 maps to CMS-HCC v28 category 86. Capture this diagnosis annually for accurate Medicare Advantage risk score.
Is I21.9 a CC or MCC for inpatient DRG? +
Yes, this code is designated as MCC. Documenting as a secondary diagnosis on inpatient claims can shift the DRG to a higher-weighted category.
What CPT codes are commonly billed with I21.9? +
Procedures frequently paired with I21.9 include: 93458, 99285, 93451, 93452, 93458.
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Reviewed by the PayerReady Medical Coding Team
Verified against the CMS 2026 code set on May 31, 2026.
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