I21.3 ICD-10-CM Code: ST elevation (STEMI) myocardial infarction of unspecified site

Code

I21.3

Description

ST elevation (STEMI) myocardial infarction of unspecified site

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 I21.3 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →

MEAT review for I21.3

What This Code Means

HCC Buddy maintains structured V28 and V24 mapping, source-labeled coefficient references, and MEAT documentation criteria for I21.3 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

• •This code should only be used when the documentation does not specify the anatomical site of the STEMI; if the site is documented (anterior, inferior, lateral, posterior, etc.), use the more specific subcategories under I21.0-I21.2
• •Verify the timing of the MI (initial episode vs. subsequent episode within 4 weeks) and whether it's the first occurrence, as this affects code selection and may require additional codes for complications

Clinical Significance

STEMI of unspecified site represents a documentation gap where the location of the myocardial infarction is not recorded. While this code still maps to HCC 86 for risk adjustment, it may trigger audit scrutiny because the anatomical site should typically be identifiable from electrocardiogram and cardiac catheterization data. This code highlights a documentation improvement opportunity.

Documentation Requirements

• ✓Clear documentation of an acute STEMI diagnosis
• ✓Electrocardiogram findings showing ST elevation (even if wall location is not specified in the narrative)
• ✓Cardiac biomarker elevation (troponin)
• ✓Evidence that more specific site documentation is not available
• ✓Timing within the 4-week acute window

Commonly Confused Codes

• •I21.01-I21.29: Site-specific STEMI codes: always preferred when the wall location or vessel is documented
• •I21.4: Non-ST elevation myocardial infarction: different electrocardiogram presentation
• •I21.9: Acute myocardial infarction, unspecified: even less specific, not distinguishing STEMI versus Non-ST elevation
• •I22.9: Subsequent STEMI of unspecified site: for recurrent events within 4 weeks

Child Codes