I21.9Acute myocardial infarction, unspecified

I21.9 is the ICD-10-CM diagnosis code for acute myocardial infarction, unspecified. It belongs to category I21 (acute myocardial infarction), block I20-I25 (ischemic heart diseases) and chapter 9 (diseases of the circulatory system). It is billable, so it can be reported on its own for dates of service from October 1, 2026 through September 30, 2027.

Within I21, choose I21.9 (unspecified) only when documentation doesn't support a more specific option: ST elevation (STEMI) myocardial infarction of anterior wall (I21.0), ST elevation (STEMI) myocardial infarction of inferior wall (I21.1), ST elevation (STEMI) myocardial infarction of other sites (I21.2), ST elevation (STEMI) myocardial infarction of unspecified site (I21.3), Non-ST elevation (NSTEMI) myocardial infarction (I21.4), other type of myocardial infarction (I21.A) and myocardial infarction with coronary microvascular dysfunction (I21.B).

Use an additional 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), tobacco dependence (F17.-) and 1 more.

As a secondary diagnosis it is an MCC (major complication or comorbidity), which can move an inpatient stay into the highest-paying "with MCC" MS-DRG of its family, unless the principal diagnosis excludes it.

In MS-DRG v44.0, it is part of the grouping logic for DRG 280 (Acute Myocardial Infarction, Discharged Alive with MCC, relative weight 1.6112), DRG 281 (Acute Myocardial Infarction, Discharged Alive with CC, relative weight 0.9184), DRG 282 (Acute Myocardial Infarction, Discharged Alive without CC/MCC, relative weight 0.7378), DRG 283 (Acute Myocardial Infarction, Expired with MCC, relative weight 1.9619), DRG 284 (Acute Myocardial Infarction, Expired with CC, relative weight 0.7525) and DRG 285 (Acute Myocardial Infarction, Expired without CC/MCC, relative weight 0.539), in MDC 05 (Diseases and Disorders of the Circulatory System), as a principal or secondary diagnosis. The DRG a claim lands in depends on the principal diagnosis, procedures and any CC/MCC secondary diagnoses.

It was added in FY2018, effective October 1, 2017.

Before ICD-10, this condition was coded in ICD-9-CM as 410.90 (acute myocardial infarction of unspecified site, episode of care unspecified), 410.91 (acute myocardial infarction of unspecified site, initial episode of care) and 410.92 (acute myocardial infarction of unspecified site, subsequent episode of care).