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I21.4ICD-10-CM

Non-ST elevation (NSTEMI) myocardial infarction

BillableType 1 onlyThe 4-week rule sits in the Includes noteSix use additional codes, one of which changes the payment group

How the code is built

I

Chapter

Diseases of the circulatory system

21

Category

Acute myocardial infarction

4

Char 4

Non-ST elevation, Type 1

Chapter

I00 to I99

Diseases of the circulatory system

Block

I20 to I25

Ischemic heart diseases

Category

I21

Acute myocardial infarction

Billable

Yes

Complete at 4 characters

Inclusion terms

Acute subendocardial, non-Q wave NOS, nontransmural NOS, and Type 1 non-ST elevation

Four terms, one of which restricts the code to Type 1

Includes at I21

Acute, or a stated duration of 4 weeks or less from onset

The time limit is in the Includes note

Use additional at I21

Six codes, covering tobacco status and prior tPA administration

Excludes2 at I21

Old myocardial infarction, postinfarction syndrome, subsequent Type 1 infarction

DefinitionCode structureClinical pictureDocumentationDecision guideTabular listComparisonClaim contextRisk adjustmentDenialsSources

Key takeaways

  • I21.4 codes a Type 1 non-ST elevation myocardial infarction, and the tabular list restricts it to Type 1
  • A Type 2 infarction is a different code with a code-first instruction; however, the electrocardiogram looks
  • The 4-week limit is not a convention; it is written into the category's Includes note
  • Two conversion rules run in one direction only, and thrombolysis does not change the code
  • It is billable, maps to a risk adjustment category, and carries six additional codes most claims miss
1Definition

What ICD-10 code I21.4 means

I21.4 is the ICD-10-CM code for non-ST elevation myocardial infarction. It applies to an acute infarction without ST-segment elevation on the electrocardiogram.

Four inclusion terms are classified here: acute subendocardial infarction, non-Q wave infarction NOS, nontransmural infarction NOS, and Type 1 non-ST elevation myocardial infarction.

Code

I21.4

Descriptor

Non-ST elevation (NSTEMI) myocardial infarction

Chapter

Diseases of the circulatory system

Block

Ischemic heart diseases

Category

I21, acute myocardial infarction

Billable

Yes

Inclusion terms

Acute subendocardial MI, non-Q wave MI NOS, nontransmural MI NOS, Type 1 NSTEMI

That fourth inclusion term is the most important fact about this code. It restricts I21.4 to Type 1 infarction, meaning an event caused by plaque rupture and thrombosis. The classification says so in the tabular list rather than leaving it to be inferred.

So the absence of ST elevation does not by itself select this code. A Type 2 infarction has no ST elevation either, and it is a different code entirely. Type 2 arises from a supply and demand mismatch rather than from plaque rupture; it classifies to I21.A1, and that code carries a code-first instruction for the underlying cause.

The practical problem follows from that. Providers write NSTEMI for demand ischemia events routinely, because the troponin rises and the electrocardiogram shows no elevation, which is what NSTEMI looks like at the bedside. Sepsis, anemia, tachyarrhythmia, hypotension, and hypoxia all produce that picture. Coding those as I21.4 reports a plaque rupture that did not happen, and it drops the underlying condition that the Type 2 code would have required.

2Code structure

A type axis, a site axis, and a clock

I21.4 has no subdivisions. Three separate things determine whether it is correct.

Axis one, infarction type

Type 1 with ST elevation

I21.0- to I21.3, by wall and artery

Type 1 without ST elevation

I21.4

This code

Type 2, from supply and demand mismatch

I21.A1, with the underlying cause coded first

Type 3, 4, or 5

I21.A9

With coronary microvascular dysfunction

I21.B

Acute, type not stated

I21.9

Axis two, site, which applies only to ST elevation infarction

The ST elevation codes are subdivided by wall and by artery, running to five characters. I21.4 has no site character at all, because a non-ST elevation infarction is not localised on the electrocardiogram the way an ST elevation infarction is. Looking for an anterior or inferior character on this code is looking for something the classification does not provide.

Axis three, the clock, and it is written in the Includes note

The category Includes note covers an infarction specified as acute, or with a stated duration of 4 weeks, meaning 28 days, or less from onset. That is not a coding convention layered on top of the code set. It is part of what I21 means.

Within 28 days of onset

I21.4

This code

A new infarction within 28 days of a previous one

I22.2, subsequent NSTEMI

Beyond 28 days, care continuing

The applicable aftercare code

Beyond 28 days, healed

I25.2, old myocardial infarction

Two conversion rules run in one direction only. A Type 1 NSTEMI that evolves into a STEMI is coded as STEMI. A STEMI that converts to a non-ST elevation pattern after thrombolysis is still coded as STEMI, because the original event was an ST elevation infarction and the treatment changed the tracing rather than the diagnosis.

3Clinical picture

Clinical findings documented under I21.4

A record supporting this code establishes an acute infarction without ST elevation and attributes it to a coronary event.

Symptoms

Chest pain or pressure, dyspnea, diaphoresis, nausea, or an atypical presentation

Electrocardiogram

ST depression, T wave inversion, or non-diagnostic changes, with the absence of ST elevation stated

Biomarkers

Troponin rise and fall, with the pattern rather than a single value

Angiography

Culprit lesion, thrombus, or the absence of obstructive disease

Mechanism

Whether plaque rupture is established, or whether a supply and demand mismatch is documented

Precipitants

Sepsis, anemia, arrhythmia, hypotension, hypoxia, or none

Intervention

Percutaneous intervention, stent placement, thrombolysis, and where each was given

The last three rows decide the code rather than describing the patient. A documented precipitant with no culprit lesion points to Type 2, and a culprit lesion with thrombus points to Type 1. Where the record establishes neither, the type is a query rather than a default.

4Documentation

What the chart must show

  • A provider-stated diagnosis of myocardial infarction
  • Whether ST elevation was present
  • The infarction type, meaning Type 1 or Type 2, stated rather than implied
  • For a Type 2 event, the underlying condition and its causal relationship to the infarction
  • The troponin pattern and the electrocardiographic findings
  • Angiographic findings, including the absence of obstructive disease
  • Date of onset, since the 28-day clock runs from it
  • Whether a previous infarction occurred within the preceding 28 days
  • Whether thrombolysis was given, and whether it was given at another facility within 24 hours
  • Tobacco use, former use, or exposure

The type statement is the single highest-value sentence in the note. Without it, a coder is choosing between two codes with different sequencing rules on the basis of an electrocardiogram that looks the same in both. With it, everything else follows.

5Decision guide

When to assign I21.4, and when not to

NSTEMI, Type 1, or plaque rupture without ST elevation

I21.4

Non-Q wave, nontransmural, or acute subendocardial infarction

I21.4

Type 2 myocardial infarction, or demand ischemia with troponin rise

I21.A1, with the underlying cause sequenced first

NSTEMI documented in a patient with sepsis, anemia, or arrhythmia and no culprit lesion

Query, since this describes a Type 2 event

Type 3, 4 or 5 infarction

I21.A9

Infarction with coronary microvascular dysfunction

I21.B

Acute infarction, type not stated

I21.9

STEMI, by wall and artery

The applicable I21.0 to I21.3 code

NSTEMI that evolved into a STEMI

The STEMI code

STEMI that became non-ST elevation after thrombolysis

The STEMI code, since the original event stands

A new infarction within 28 days of a previous one

I22.2

Beyond 28 days with care continuing

The applicable aftercare code

Healed infarction

I25.2

Postinfarction syndrome

I24.1

Unstable angina, troponin negative

I20.0

Coronary disease with angina and no infarction

The applicable I25.11- combination code

Query rather than default when the note says NSTEMI alongside a documented precipitant and no culprit lesion, when the type is never stated, and when the onset date is unclear against the 28-day boundary.

6Tabular list

Instructional notes at I21

Includes at I21

Cardiac infarction, coronary artery embolism, occlusion, rupture and thrombosis, infarction of heart, myocardium or ventricle, and infarction specified as acute or with a stated duration of 4 weeks or less from onset

Excludes2 at I21

Old myocardial infarction (I25.2)

Excludes2 at I21

Postmyocardial infarction syndrome (I24.1)

Excludes2 at I21

Subsequent Type 1 myocardial infarction (I22.-)

Use additional at I21

Tobacco dependence, tobacco use, history of tobacco dependence, exposure to environmental tobacco smoke, occupational exposure to environmental tobacco smoke

Use additional at I21

Status post administration of tPA in a different facility within the last 24 hours prior to admission

Code first at I21.A1

The underlying cause of the Type 2 infarction

It’s worth noting that the time limit appears in the “Includes” note rather than in the guidelines. Coders treat the 28-day rule as external guidance, and it is part of the code's meaning. An infarction older than 28 days is not an I21 condition at all, which is why the old infarction code sits in the Excludes2 list rather than being an alternative to consider.

All three exclusions are Excludes2, which makes them permissions. A patient with an acute NSTEMI who also has a healed infarction from three years ago carries both codes. So does a patient with a subsequent infarction and a documented earlier one. The notes separate the concepts rather than forbidding the pairs.

Six additional codes accompany this category, and one of them has a direct payment effect. Status post tPA given at another facility within the preceding 24 hours is a transfer scenario, and it appears in the title of a diagnosis-related group in the same way it does for stroke. The five tobacco codes are routinely omitted.

The Type 2 instruction changed. The note at the Type 2 code was formerly a code also and is now a code first, which means the underlying cause is sequenced ahead of the infarction rather than merely reported alongside it. Published advice adds that a causal relationship must be documented rather than assumed. A coder working from a reference written before that change will sequence it wrongly.

7Comparison

Codes frequently confused with I21.4

I21.A1

Descriptor

Myocardial infarction type 2

Use instead when

A supply and demand mismatch is documented, with the cause coded first

I21.A9

Descriptor

Other myocardial infarction type

Use instead when

A Type 3, 4, or 5 infarction is documented

I21.B

Descriptor

Myocardial infarction with coronary microvascular dysfunction

Use instead when

Microvascular dysfunction is documented

I21.9

Descriptor

Acute myocardial infarction, unspecified

Use instead when

An acute infarction is documented with no type stated

I21.01, I21.02, I21.09

Descriptor

ST elevation infarction of the anterior wall, by artery

Use instead when

ST elevation is present in an anterior distribution

I21.11, I21.19

Descriptor

ST elevation infarction of the inferior wall, by artery

Use instead when

ST elevation is present in an inferior distribution

I21.21, I21.29

Descriptor

ST elevation infarction of other sites

Use instead when

ST elevation is present elsewhere

I21.3

Descriptor

ST elevation infarction of unspecified site

Use instead when

ST elevation is present without a site stated

I22.2

Descriptor

Subsequent non-ST elevation myocardial infarction

Use instead when

A new infarction occurs within 28 days of a previous one

I22.0, I22.1, I22.8, I22.9

Descriptor

Subsequent ST elevation infarction, by site

Use instead when

The subsequent infarction has ST elevation

I25.2

Descriptor

Old myocardial infarction

Use instead when

The infarction is healed and beyond the acute window

I24.1

Descriptor

Dressler's syndrome

Use instead when

Postinfarction syndrome is documented, reportable alongside

I24.8, I24.9

Descriptor

Other and unspecified acute ischemic heart disease

Use instead when

An acute ischemic event is documented that is not an infarction

I20.0

Descriptor

Unstable angina

Use instead when

Symptoms occurred without biomarker evidence of infarction

I25.11-

Descriptor

Atherosclerotic heart disease of native coronary artery with angina pectoris

Use instead when

Coronary disease and angina are documented without infarction

I5A

Descriptor

Non-ischemic myocardial injury

Use instead when

Troponin is raised from a non-ischemic cause

R94.39

Descriptor

Abnormal result of other cardiovascular function study

Use instead when

A biomarker abnormality is reported without a diagnosis

Z92.82

Descriptor

Status post tPA administration in a different facility within 24 hours

Use instead when

Reported alongside on a transfer

F17.-, Z72.0, Z87.891, Z77.22, Z57.31

Descriptor

Tobacco dependence, use, history and exposure

Use instead when

Required additionally under the category instruction

The non-ischemic myocardial injury row is worth pausing on. A raised troponin without ischemia is not an infarction of any type, and it has its own code. Between that code, the Type 2 code, and I21.4, there are three different answers to a raised troponin, and only the record distinguishes them.

8Claim context

Services commonly reported with I21.4

I21.4 appears predominantly on inpatient and emergency claims. It supports emergency evaluation, serial biomarker testing, electrocardiography, echocardiography, coronary angiography, percutaneous intervention with or without stenting, and the cardiac rehabilitation that follows.

The diagnosis-related group family for acute infarction is sensitive to what is reported alongside, not to the infarction code alone. Complications and comorbidities move the grouping, and so does the prior tPA status code on a transfer. That code is the clearest example in this glossary of a supplementary code with a direct payment effect, and it appears in the same role on the cerebral infarction page.

Acute myocardial infarction is among the most heavily quality-measured conditions in medicine. Door-to-balloon times, discharge on antiplatelet and statin therapy, and thirty-day mortality and readmission are all measured, and the mortality measures are risk-adjusted using the diagnoses reported. That makes the type distinction a quality reporting matter as well as a coding one, since Type 2 events have a different expected outcome from Type 1.

Cardiac rehabilitation coverage is diagnosis-driven. The qualifying conditions include acute myocardial infarction within a defined preceding period, so the diagnosis and its date both matter to the referral.

Confirm the contractor's determinations for cardiac imaging and rehabilitation, and the applicable national policy for any device therapy.

9Risk adjustment

What I21.4 does to a risk score

The acute code, the old infarction code, and the Type 2 code do not behave alike

These are three different reporting pictures, and they carry different weight.

The acute infarction codes map to a risk adjustment category. Old myocardial infarction is a status rather than an acute condition and carries far less, or nothing, depending on the model. The Type 2 code is separate again, and it brings the underlying condition with it because of the code-first instruction.

The most consequential error is the type, not the specificity.

Coding a Type 2 event as I21.4 reports a plaque rupture, which overstates the event, and it omits the underlying condition that the Type 2 route would have captured. That is a mismatch in both directions at once: the reported condition is wrong, and a capturable one is missing.

The acute window matters here too.

An infarction reported as acute beyond the 28-day boundary describes an event that is no longer occurring, and that is a documentation-to-code mismatch a reviewer will find.

Verify the categories against the payment year.

Categories were renumbered in the transition between model versions, and the ischemic heart disease part of the model was restructured, so an internal crosswalk built for an earlier year will not be reliable.

10Denials

Coding errors that cause denials

Coding a Type 2 infarction as I21.4

Assign the Type 2 code with the underlying cause sequenced first

Sequencing the Type 2 code ahead of its cause

Sequence the cause first, since the instruction is now a code first

Assuming a causal relationship for a Type 2 event

Query, since the relationship must be documented

Reporting I21.4 for an infarction beyond 28 days from onset

Assign the aftercare code or the old infarction code

Reporting I21.4 for a new infarction within 28 days of a previous one

Assign the subsequent NSTEMI code

Coding a STEMI as NSTEMI after thrombolysis changed the tracing

Assign the STEMI code, since the original event stands

Coding an NSTEMI that evolved to STEMI as NSTEMI

Assign the STEMI code

Looking for a site character on I21.4

None exists, since non-ST elevation infarction is not localised in the code set

Coding a raised troponin with no ischemia as an infarction

Assign the non-ischemic myocardial injury code

Omitting the tobacco status code

Add the applicable code, per the category instruction

Omitting the prior tPA status code on a transfer

Add it, since it affects the inpatient payment group

Dropping the old infarction code because an acute one is reported

Report both where documented, since the note is an Excludes2

11Sources

Where to verify I21.4

ICD-10-CM Official Guidelines for Coding and Reporting, Chapter 9

The acute myocardial infarction rules, the subsequent infarction rules, and the guidance on Type 2 infarction sequencing

ICD-10-CM Tabular List at I21

The four inclusion terms, including the Type 1 restriction, the Includes note carrying the 28-day limit, all three Excludes2 conditions, and the six use additional codes

AHA Coding Clinic

Published advice on Type 2 infarction, on the documented causal relationship it requires, and on the conversion rules between ST elevation and non-ST elevation infarction

CMS ICD-10 code page and the FY 2026 files

Current code files and the annual addenda

CMS Medicare Advantage rates and statistics

The current payment year model file and the categories the acute, Type 2, and old infarction codes map to

Medicare Coverage Database

Search your contractor's determinations for cardiac imaging and cardiac rehabilitation.

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Last reviewed August 2026

This page is a free reference for billing and coding teams and is not coding, legal, or clinical advice. Code selection depends on the documentation in the record and on the policy of the payer being billed. Verify every code, instructional note, and sequencing rule against the current ICD-10-CM files and the Official Guidelines for Coding and Reporting for the applicable date of service. ICD-10-CM is maintained by the CDC's National Center for Health Statistics and published by CMS. CPT is a registered trademark of the American Medical Association. TruCare is not affiliated with CMS, NCHS, the AHA, or the AMA.

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