2027 ICD-10-CM Diagnosis Code I21.9Acute myocardial infarction, unspecified
ICD-10-CM Codes›I00–I99›I20-I25›I21
- Billable — Valid for Submission
- MCC — Major Complication or Comorbidity
- Risk Adjusts — HCC 228
- Chronic Condition
I21.9 is a billable ICD-10-CM diagnosis code for acute myocardial infarction, unspecified. It is valid on HIPAA claims for fiscal year 2027 (October 1, 2026 through September 30, 2027) and groups to MS-DRG 280 through 285. As a secondary diagnosis, it counts as a major complication or comorbidity (MCC) and places an inpatient stay in the highest severity level of its MS-DRG family. It does not count, however, when the principal diagnosis is one of 25 closely related codes. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Acute myocardial infarction.
For Medicare Advantage risk adjustment, I21.9 maps to CMS-HCC Category 228 (Acute Myocardial Infarction) under the V28 model, adding a risk factor of about 0.252 for a community, non-dual, aged beneficiary in payment year 2027.
Code Identity
Code Classification
Medicare Risk Adjustment (HCC)Billing
I21.9 maps to a payment category in the CMS-HCC model used to risk-adjust Medicare Advantage payments. Weights are the published community factors for payment year 2027.
Source: CMS Payment Year 2027 risk adjustment mappings and model software. Weights are relative factors, not dollar amounts; a beneficiary's total RAF also includes demographics and interactions. Browse all CMS-HCC categories.
Approximate SynonymsGuidance
Alternate terms and clinical phrases that map to this code.
- Acute anteroapical myocardial infarction
- Acute anteroseptal myocardial infarction
- Acute infarction of papillary muscle
- Acute myocardial infarction
- Acute myocardial infarction due to left coronary artery occlusion
- Acute myocardial infarction due to occlusion of circumflex branch of left coronary artery
- Acute myocardial infarction due to right coronary artery occlusion
- Acute myocardial infarction during procedure
- Acute myocardial infarction of anterior wall
- Acute myocardial infarction of anterior wall involving right ventricle
- Acute myocardial infarction of anterolateral wall
- Acute myocardial infarction of apex of heart
- Acute myocardial infarction of apical-lateral wall
- Acute myocardial infarction of atrium
- Acute myocardial infarction of high lateral wall
- Acute myocardial infarction of inferior wall
- Acute myocardial infarction of inferior wall involving right ventricle
- Acute myocardial infarction of inferolateral wall
- Acute myocardial infarction of inferolateral wall with posterior extension
- Acute myocardial infarction of inferoposterior wall
- Acute myocardial infarction of lateral wall
- Acute myocardial infarction of posterobasal wall
- Acute myocardial infarction of posterolateral wall
- Acute myocardial infarction of right ventricle
- Acute myocardial infarction of septum
- Acute posterior myocardial infarction
- Acute Q wave myocardial infarction
- Acute widespread myocardial infarction
- Aneurysm of coronary vessels
- Cardiac rupture due to and following acute myocardial infarction
- Coronary artery embolism
- Coronary artery perforation
- Coronary artery rupture
- Drug-related myocardial necrosis syndrome
- EKG: myocardial infarction
- First myocardial infarction
- Mitral valve regurgitation due to acute myocardial infarction
- Mitral valve regurgitation due to acute myocardial infarction with papillary muscle and chordal rupture
- Mitral valve regurgitation due to acute myocardial infarction without papillary muscle and chordal rupture
- Mixed myocardial ischemia and infarction
- Myocardial infarction
- Myocardial infarction due to atherothrombotic coronary artery disease
- Myocardial infarction following percutaneous coronary intervention
- Myocardial infarction in recovery phase
- Myocardial infarction with non-obstructive coronary artery
- Myocardial necrosis
- New myocardial infarction compared to prior study
- Perforation of coronary artery co-occurrent and due to aneurysm of coronary artery
- Pericarditis secondary to acute myocardial infarction
- Postoperative acute myocardial infarction
- Postoperative myocardial infarction
- Pulmonary embolism due to and following acute myocardial infarction
- Rupture of coronary artery co-occurrent and due to aneurysm of coronary artery
- Rupture of ventricle due to acute myocardial infarction
- True posterior myocardial infarction
Instructional NotesGuidance
Instructions from the official ICD-10-CM Tabular List that apply to I21.9: its own notes plus those printed at I21, block I20-I25, and Chapter 9. A note printed at a category, block or chapter applies to every code under it.
Includes
Further defines, or gives examples of, the content of the category.
- cardiac infarction
- coronary (artery) embolism
- coronary (artery) occlusion
- coronary (artery) rupture
- coronary (artery) thrombosis
- infarction of heart, myocardium, or ventricle
- myocardial infarction specified as acute or with a stated duration of 4 weeks (28 days) or less from onset
Applicable To
Conditions this code is used for: synonyms of the title or, for "other specified" codes, the conditions assigned to it. The list is not exhaustive.
- Myocardial infarction (acute) NOS
Excludes2
Not included here: the excluded condition is not part of this code, but a patient may have both, so both codes may be reported.
- certain conditions originating in the perinatal period (P04-P96)
- certain infectious and parasitic diseases (A00-B99)
- complications of pregnancy, childbirth and the puerperium (O00-O9A)
- congenital malformations, deformations, and chromosomal abnormalities (Q00-Q99)
- endocrine, nutritional and metabolic diseases (E00-E88)
- injury, poisoning and certain other consequences of external causes (S00-T88)
- neoplasms (C00-D49)
- symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified (R00-R94)
- systemic connective tissue disorders (M30-M36)
- transient cerebral ischemic attacks and related syndromes (G45.-)
Use Additional Code
Add a second code, after this one, to fully describe the condition.
- 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.-)
- tobacco use (Z72.0)
Code Also
A second code may be needed to fully describe the condition; the note gives no sequencing direction.
Source: CMS ICD-10-CM Tabular List. How to read instructional notes.
Referenced in Other NotesGuidance
Instructional notes printed at other codes that name I21.9, its category, or a range that includes it.
Excludes1 2
These codes carry an Excludes1 note naming I21.9: they are not reported together with it, unless the two conditions are unrelated.
Excludes2 2
These codes carry an Excludes2 note naming I21.9: its condition is not part of those codes, and both may be reported when the patient has both.
Code First 1
These codes say to code first a condition named in the note, and I21.9 is one of the conditions named.
Index to Diseases and InjuriesGuidance
Alphabetical index entries that point to this code.
myocardium, myocardial (acute) (with stated duration of 4 weeks or less) I21.9
type 1 See Also: Infarction, myocardial, by site, or by ST elevation or non-ST elevation; I21.9
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
Clinical InformationClinical
Acute Myocardial Infarction
necrosis of the myocardium, as a result of interruption of the blood supply to the area. it is characterized by a severe and rapid onset of symptoms that may include chest pain, often radiating to the left arm and left side of the neck, dyspnea, sweating, and palpitations.
Patient EducationClinical
Heart Attack
A heart attack, also called a myocardial infarction, is a type of heart disease. It happens when blood flow to the heart suddenly becomes blocked. Without the blood coming in, the heart can't get oxygen. If not treated quickly, the heart muscle begins to die.
The full article covers:
- What is a heart attack?
- What heart attack risk factors can be changed?
- What heart attack risk factors can't be changed?
- What causes a heart attack?
- What are the symptoms of a heart attack?
- How is a heart attack diagnosed?
- What is the treatment for a heart attack?
Read the full article at MedlinePlus
Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.
Convert I21.9 to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code History & ChangesHistory
Replacement I21.9 replaces the following previously assigned code(s):
- I21.3 - ST elevation (STEMI) myocardial infarction of unsp site
Questions About I21.9Overview
What is the ICD-10 code for acute myocardial infarction, unspecified?
The ICD-10-CM code for acute myocardial infarction, unspecified is I21.9 (sometimes written as I219). It is billable on HIPAA-covered claims from October 1, 2026 through September 30, 2027.
Is I21.9 (Acute myocardial infarction) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report acute myocardial infarction, unspecified on HIPAA-covered claims from October 1, 2026 through September 30, 2027.
What MS-DRG does I21.9 group to?
When acute myocardial infarction, unspecified is the principal diagnosis on an inpatient stay, it groups to MS-DRG 282, 285, 281, 280, 283, 284, with relative weights from 0.5197 to 1.9615 depending on complications. Higher weights mean higher Medicare reimbursement.
Is I21.9 a CC or MCC?
CMS lists I21.9 as an MCC (major complication or comorbidity) for FY 2027. Reported as a secondary diagnosis, it places the inpatient stay in the highest-weighted DRG of its severity family. It does not count when the principal diagnosis is one of the 25 closely related codes in its exclusion list.
Does I21.9 need an additional code?
When the condition applies, yes. The Use Additional Code note at I21, which applies to I21.9, reads: 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 listed in the Instructional Notes section. The additional code is reported after I21.9.
What HCC is I21.9?
I21.9 (acute myocardial infarction, unspecified) maps to CMS-HCC Category 228 (Acute Myocardial Infarction), commonly written as HCC 228, in the CMS-HCC V28 model used for Medicare Advantage risk adjustment in payment year 2027. It mapped to HCC 86 under the retired V24 model. It also maps in the PACE (CMS-HCC V22), ESRD (V21), and ESRD (V24) models. In the Part D prescription drug model it maps to RxHCC 188.
Does I21.9 risk-adjust for Medicare Advantage payment?
Yes. When documented and reported on a Medicare Advantage encounter, I21.9 adds a risk adjustment factor of about 0.252 to the beneficiary's RAF score for a community, non-dual, aged enrollee (published V28 weights range from 0.252 to 0.517 depending on the payment segment). HCC 228 sits at the top of its hierarchy, so no other condition category supersedes it. See the full factor table on the HCC 228 category page.
