Alphabetic Index FY2027

I33.9

Acute and subacute endocarditis, unspecified

FY2027Chapter 9: Diseases of the circulatory system/I30–I5A Other forms of heart disease

BillableComplete code, valid to report
Care settingsHome Health Primary · MMTA - Cardiac and CirculatoryHospital MCCIPF Comorbidity · Cardiac Conditions

I33.9 (also written I339) is a billable ICD-10-CM diagnosis code for acute and subacute endocarditis, unspecified. It is part of the FY2027 code set, effective October 1, 2026. It is one of the 2 codes in category I33 (Acute and subacute endocarditis). The Alphabetic Index lists it under 6 entries. Its coding notes include 2 Excludes1 entries and 10 Excludes2 entries.

Excludes1

Not coded here. The listed conditions generally should not be reported together.

Inherited from I33 Acute and subacute endocarditis
  • acute rheumatic endocarditis (I01.1)
  • endocarditis NOS (I38)

Excludes2

Not included here. Both codes may be reported if the patient has both conditions.

Also from chapter/block level (10 notes)
Inherited from Chapter 9 Diseases of the circulatory system
  • 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.-)

Care settings

How CMS programs treat this code. These are CMS claim edits and list entries, not coverage or eligibility rules.

Home Health Primary

Accepted as the primary (principal) diagnosis on a home health claim. The Home Health Grouper assigns it to the MMTA - Cardiac and Circulatory clinical group.

Source: CMS Home Health Grouper Software v07.2.26, effective October 1, 2026

Hospital MCC

Counts as a major complication or comorbidity (MCC) when reported as a secondary diagnosis on an inpatient claim. It does not count when the principal diagnosis is one of the 15 codes in its exclusion list. Some MS-DRGs also ignore a CC or MCC that defines the DRG.

Source: CMS MS-DRG Definitions Manual v44, effective October 1, 2026, Appendix C

IPF Comorbidity

On the CMS IPF comorbidity list.

Comorbidity categoryFactor
Cardiac Conditions1.04
It counts as a comorbidity when reported as a secondary diagnosis and actively treated during the stay. An IPF receives only one adjustment within a category and can receive adjustments in more than one category.

Source: CMS IPF PPS FY 2027 final rule (CMS-1847-F), Addendum B-3 (comorbidity codes with adjustment factor); effective for discharges on or after October 1, 2026

Inclusion terms

Conditions classified to this code.

This code
  • Acute endocarditis NOS
  • Acute myoendocarditis NOS
  • Acute periendocarditis NOS
  • Subacute endocarditis NOS
  • Subacute myoendocarditis NOS
  • Subacute periendocarditis NOS

Alphabetic Index terms

Terms the official Alphabetic Index lists for this code. 6 entries