I33.9Acute and subacute endocarditis, unspecified

I33.9 is the ICD-10-CM diagnosis code for acute and subacute endocarditis, unspecified. It belongs to category I33 (acute and subacute endocarditis), block I30-I5A (other forms of heart disease) 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 I33, choose I33.9 (unspecified) only when documentation doesn't support a more specific option: infective endocarditis (I33.0).

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 288 (Acute and Subacute Endocarditis with MCC, relative weight 2.6315), DRG 289 (Acute and Subacute Endocarditis with CC, relative weight 1.5362), DRG 290 (Acute and Subacute Endocarditis without CC/MCC, relative weight 0.8367), DRG 791 (Prematurity with Major Problems, relative weight 4.1075), DRG 793 (Full Term Neonate with Major Problems, relative weight 4.2195), DRG 974 (HIV with Major Related Condition with MCC, relative weight 3.0506) and 2 more, in MDC 05 (Diseases and Disorders of the Circulatory System), MDC 15 (Newborns and Other Neonates with Conditions Originating in the Perinatal Period) and MDC 25 (Human Immunodeficiency Virus Infections), 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 has been part of ICD-10-CM since the code set took effect on October 1, 2015 and hasn't changed since.

Before ICD-10, this condition was coded in ICD-9-CM as 421.9 (acute endocarditis, unspecified).