2026 ICD-10-CM Diagnosis Code I33.9Acute and subacute endocarditis, unspecified

ICD-10-CM Codes›I00–I99›I30-I5A›I33

ICD-10-CM I33.9
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

I33.9 is a billable ICD-10-CM diagnosis code for acute and subacute endocarditis, unspecified. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 288 through 290, 974 through 976. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Endocarditis and endocardial disease.

Code Identity

ICD-10-CM Code
I33.9
Billable Status
Yes — Valid for Submission
Code Describes
Acute and subacute endocarditis, unspecified
Short Description
Acute and subacute endocarditis, unspecified
Same as the full description in the CMS dataset.
Parent Code
I33
Acute and subacute endocarditis

Code Classification

ChapterI00–I99Diseases of the circulatory system
SectionI30-I5AOther forms of heart disease
CategoryI33Acute and subacute endocarditis
This CodeI33.9Acute and subacute endocarditis, unspecified

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Abscess at site of interatrial communication
  • Abscess at site of ventricular septal defect
  • Abscess of cardiac septum
  • Acute endocarditis
  • Acute endocarditis co-occurrent with human immunodeficiency virus infection
  • Acute myocarditis
  • Acute myoendocarditis
  • Acute nonbacterial endocarditis
  • Acute periendocarditis
  • Embolism due to non-infective heart valve vegetations
  • Heart disease co-occurrent with human immunodeficiency virus infection
  • Subacute endocarditis
  • Subacute myoendocarditis
  • Subacute pericarditis
  • Subacute periendocarditis

Tabular List NotesGuidance

Coding notes and annotation back-references applicable to this code.

Inclusion Terms

  • Acute endocarditis NOS
  • Acute myoendocarditis NOS
  • Acute periendocarditis NOS
  • Subacute endocarditis NOS
  • Subacute myoendocarditis NOS
  • Subacute periendocarditis NOS

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

External Cause of Injuries IndexGuidance

References for this code in the External Cause of Injuries Index.

    • Endocarditis(chronic) (marantic) (nonbacterial) (thrombotic) (valvular)
      • acute or subacute
    • Endocarditis(chronic) (marantic) (nonbacterial) (thrombotic) (valvular)
      • aortic (heart) (nonrheumatic) (valve)
        • acute or subacute
    • Endocarditis(chronic) (marantic) (nonbacterial) (thrombotic) (valvular)
      • mitral (chronic) (double) (fibroid) (heart) (inactive) (valve) (with chorea)
        • nonrheumatic
          • acute or subacute
    • Endocarditis(chronic) (marantic) (nonbacterial) (thrombotic) (valvular)
      • pulmonary (chronic) (heart) (valve)
        • acute or subacute
    • Endocarditis(chronic) (marantic) (nonbacterial) (thrombotic) (valvular)
      • toxic
    • Endocarditis(chronic) (marantic) (nonbacterial) (thrombotic) (valvular)
      • tricuspid (chronic) (heart) (inactive) (rheumatic) (valve) (with chorea)
        • nonrheumatic
          • acute or subacute

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR CIR004
Endocarditis and endocardial disease
Default principal diagnosis: inpatient Yes · outpatient Yes

Clinical InformationClinical

  • Acute Myocarditis

    the sudden onset of inflammation of heart muscle with myocellular necrosis; this is generally secondary to an infectious cause, and patients often have a recent history of a flu-like illness.

  • Acute and Subacute Endocarditis, Unspecified|Acute and subacute endocarditis, unspecified

    evidence of acute and subacute endocarditis.

  • Acute Endocarditis

    acute inflammation of the endocardium. bacteria is the usual etiologic agent, and the distinction between "acute" and "subacute" has traditionally been made based on the pathogenic organism and clinical presentation.

Patient EducationClinical

Endocarditis

Endocarditis is sometimes called infective endocarditis. It's a rare but life-threatening inflammation of the lining inside your heart's chambers and valves (the endocardium).

The full article covers:

  • What is endocarditis?
  • What causes endocarditis?
  • Who is more likely to develop endocarditis?
  • What are the symptoms of endocarditis?
  • What other problems can endocarditis cause?
  • How is endocarditis diagnosed?
  • What are the treatments for endocarditis?
  • Can endocarditis be prevented?

Read the full article at MedlinePlus

Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.

Convert I33.9 to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
421.9 Ac/subac endocardit NOS
Exact Match The mapping is direct, with no qualifiers.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About I33.9Overview

Is I33.9 (Acute and subacute endocarditis) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report acute and subacute endocarditis, unspecified on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does I33.9 group to?

When acute and subacute endocarditis, unspecified is the principal diagnosis on an inpatient stay, it groups to MS-DRG 288, 289, 290, 974, 975, 976, with relative weights from 0.8945 to 2.8860 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of I33.9?

Under the General Equivalence Mappings, acute and subacute endocarditis, unspecified converts to ICD-9-CM 421.9 (ac/subac endocardit NOS). The mapping is a direct match.

Footnotes

[1] Chronic - a chronic condition code indicates a condition lasting 12 months or longer and its effect on the patient based on one or both of the following criteria:

  • The condition results in the need for ongoing intervention with medical products,treatment, services, and special equipment
  • The condition places limitations on self-care, independent living, and social interactions.