I33.9 Acute and subacute endocarditis, unspecified

The ICD-10-CM code for Acute and subacute endocarditis, unspecified is I33.9 (FY2026). It is a billable, claim-ready diagnosis code.

Classification

Chapter

9: Diseases of the circulatory system (I00-I99)

Section

I30-I5A: Other forms of heart disease (I30-I5A)

Category I33

3 codes (2 billable)

FY2026 Status

Stable since FY2024

Also Known As

ICD-10-CM Alphabetic Index entries that lead to I33.9:

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

Inclusion Terms

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

U.S. Hospital Utilization

• An estimated 1,865 U.S. inpatient stays in 2023 included I33.9 among the documented diagnoses.
• 345 stays listed it as the principal diagnosis.

Source: National Inpatient Sample (NIS), Healthcare Cost and Utilization Project (HCUP), Agency for Healthcare Research and Quality, 2016–2023. National survey-weighted estimates.

Official Coding Guidelines

Presumed relationship due to Index term “with”

The classification presumes a causal relationship between hypertension and heart involvement and between hypertension and kidney involvement, as the two conditions are linked by the term “with” in the Alphabetic Index. These conditions should be coded as related even in the absence of provider documentation explicitly linking them, unless the documentation clearly states the conditions are unrelated.

Provider must link conditions not specifically indexed as related

For hypertension and conditions not specifically linked by relational terms such as “with,” “associated with” or “due to” in the classification, provider documentation must link the conditions in order to code them as related.

Source: CMS — ICD-10-CM Official Guidelines for Coding and Reporting, FY2026

References

• CMS — ICD-10-CM Official Code Set and Guidelines, FY2026
• CDC/NCHS — ICD-10-CM Files and Browser Tool
• AHRQ — HCUP National Inpatient Sample, Frequencies by Diagnosis Code (2016–2023)