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

Billing Status: YES. This is a valid, specific, and billable ICD-10-CM reference.

Coding at a Glance

Tabular directives
6 inclusion terms · 2 Excludes1

Inpatient Payment Groups (MS-DRG)

Potential MS-DRG participation — not a DRG assignment.

MS-DRGs this diagnosis helps define, as principal or secondary, per the CMS ICD-10-CM/PCS MS-DRG Definitions Manual v43, Appendix B.

  • MS-DRG 288 — ACUTE AND SUBACUTE ENDOCARDITIS WITH MCC (MDC 05)
  • MS-DRG 289 — ACUTE AND SUBACUTE ENDOCARDITIS WITH CC (MDC 05)
  • MS-DRG 290 — ACUTE AND SUBACUTE ENDOCARDITIS WITHOUT CC/MCC (MDC 05)
  • MS-DRG 791 — PREMATURITY WITH MAJOR PROBLEMS (MDC 15)
  • MS-DRG 793 — FULL TERM NEONATE WITH MAJOR PROBLEMS (MDC 15)
  • MS-DRG 974 — HIV WITH MAJOR RELATED CONDITION WITH MCC (MDC 25)
  • MS-DRG 975 — HIV WITH MAJOR RELATED CONDITION WITH CC (MDC 25)
  • MS-DRG 976 — HIV WITH MAJOR RELATED CONDITION WITHOUT CC/MCC (MDC 25)

A diagnosis appearing in a group's logic does not by itself determine the DRG assigned to a stay; the grouper uses the full claim.

Coding instructions

Official Tabular Instructional Notes: the inclusion, exclusion and sequencing notes published for I33.9 in the official ICD-10-CM tabular list, quoted as published.

Source: CMS/CDC — ICD-10-CM Tabular ListRelease: FY2026Effective: October 1, 2025

Notes without a marker are published on I33.9 itself; “inherited from” names the category or block whose note applies here.

Inclusion Terms

Alternative terms the tabular list files under this code.

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

Excludes1 — Not Coded Here

Conditions generally not reported together with this code (Excludes1) -- an error unless the two conditions are documented as unrelated to each other (ICD-10-CM Official Guidelines, Section I.A.12.a).

Source: inherited from I33

Coder workflow for I33.9

MedCoder structured workflow — derived from this code’s own official record

Before you code I33.9

  1. Unspecified does not mean incorrect. When the record gives no greater specificity, I33.9 may be the appropriate code. Check the record for detail that supports a more specific sibling — in this subcategory the siblings differ by acute versus chronic. An unspecified code is for records that do not provide the detail a more specific code needs; a query, not an assumption, is the route to specificity (Guidelines I.A.9.b, I.B.18).

    ReviewI33.0

    See the relationships section · Guide: Other vs unspecified (NEC vs NOS) →

  2. Check the Excludes1 note: if the documentation supports a condition named there, do not simply proceed with I33.9. Excludes1 conditions are not reported together with this code unless the record shows they are unrelated (Guidelines I.A.12.a).

    See the official tabular notes

Choose the right path

  1. Does the record document the detail a more specific sibling code needs?
    Yes → Review the specific siblings in this subcategory.
    No → Continue — I33.9 is appropriate when the documentation goes no further.

    ReviewI33.0

  2. Does the documentation support a condition named in I33.9’s Excludes1 note?
    Yes → Do not simply proceed: review the excluded code. Both are reported only when the record shows the two conditions are unrelated.
    No → Continue.

    ReviewI01.1, I38

Consider I33.9. Then confirm the code is valid for the date of service in the Verify section.

Documentation check

The provider’s diagnostic statement
Codes are assigned from the provider’s documented diagnosis, not from clinical criteria, test values or a medication list (Guidelines I.A.19).
Acuity
Acute or chronic as documented; when both are documented and separate codes exist, both are reported with the acute code first (Guidelines I.B.8).
Any detail beyond this code’s title
What the record states that a more specific sibling code would capture — or its absence, which itself supports the unspecified code.

Official instructions as workflow

  • Excludes1 — check before selecting I33.9(2 notes)

    Coding workflow: If the documentation supports a condition named in this note, do not simply proceed with I33.9: the two are not reported together. The one exception is when the record shows the two conditions are unrelated to each other.

    CompareI01.1, I38

    See the official tabular notes · Guidelines I.A.12.a

Coding decision scenarios

Pattern scenarios for this code’s structure — decision rules, not clinical cases

Documentation: The provider documents the condition in the terms of this code’s title and records no further detail.

Coding question: Is a more specific sibling code supportable?

Path: Review the specific siblings in this subcategory and what each requires the record to state.

Reason: A more specific code needs documentation of the distinguishing element; without it the unspecified code is appropriate, and a provider query is the route to specificity (Guidelines I.A.9.b, I.B.18).

ReviewI33.0

Documentation: Both the condition I33.9 describes and a condition named in its Excludes1 note are documented for the same encounter.

Coding question: Can both codes be reported?

Path: Review the Excludes1 note and the excluded code, and look for a provider statement on whether the two conditions are related.

Reason: Excludes1 means the two are not coded together; the exception is when the record shows the conditions are unrelated to each other (Guidelines I.A.12.a).

ReviewI01.1, I38

Every row is derived from this code’s own record — its title, tabular notes, 7th-character family and same-category siblings — with fixed MedCoder wording; nothing is inferred about a patient. The official notes and guideline text are in the sections each row links to, and they control.

Code Overview

Acute and subacute endocarditis, unspecified is a billable ICD-10-CM diagnosis code (I33.9).

MedCoder summary Summary composed by MedCoder from this code's official ICD-10-CM record. The tabular instructional notes themselves appear verbatim below.

Indexed Clinical Terms (6)

Official source data — entries quoted as published, in the Index’s own lookup phrasing

Clinical term phrases from the official ICD-10-CM Index to Diseases and Injuries that map to this code. These are alphabetic-index entries shown as the Index writes them — lookup phrasing, not necessarily the wording of a final diagnosis.

Verify Before Coding

  • MCC as a secondary diagnosis (FY2026). Can raise the stay's MS-DRG severity tier.

From the code registry, the Medicare Code Editor, and the MS-DRG Definitions Manual. Check it against a full claim in Claim Check.

Relationships & Classification

MedCoder structured relationships — computed from published CMS and AHRQ datasets

MS-DRG Grouper Relationships (FY2026)

Potential MS-DRG participation — not a DRG assignment.

FY2026 MS-DRG: MCC — Major Complication or Comorbidity. Reported as a secondary diagnosis, this code raises the stay's MS-DRG severity tier — except when the principal diagnosis is one of 15 clinically related codes on its CMS exclusion list. Does not count toward severity when the stay groups to DRGs 974-976 (HIV with Major Related Condition).

Named in the grouper logic of 8 MS-DRGs: DRG 288 (MDC 05), DRG 289 (MDC 05), DRG 290 (MDC 05), DRG 791 (MDC 15), DRG 793 (MDC 15), DRG 974 (MDC 25), DRG 975 (MDC 25), DRG 976 (MDC 25).

From the CMS MS-DRG Definitions Manual (Appendices B and C). Actual DRG assignment depends on the complete claim.

Clinical classification (AHRQ CCSR):CIR004 — Endocarditis and endocardial disease (default).

Clinical Classifications Software Refined (CCSR) for ICD-10-CM Diagnoses. Healthcare Cost and Utilization Project (HCUP), Agency for Healthcare Research and Quality.

Related Codes

Same clinical process (MS-DRG)

Acts as MCC — raises the severity of other admissions. CMS groups these diagnoses into one MS-DRG principal-diagnosis exclusion process — a CC/MCC on this list never raises severity when the principal diagnosis is also on it.

I01.1 — Acute rheumatic endocarditis, I08.1 — Rheumatic disorders of both mitral and tricuspid valves, I08.2 — Rheumatic disorders of both aortic and tricuspid valves, I08.3 — Combined rheumatic disorders of mitral, aortic and tricuspid valves, I09.1 — Rheumatic diseases of endocardium, valve unspecified, I27.841 — Fontan-associated lymphatic dysfunction, I27.848 — Other Fontan-associated condition, I27.849 — Fontan related circulation, unspecified, I33.0 — Acute and subacute infective endocarditis, I38 — Endocarditis, valve unspecified, I87.8 — Other specified disorders of veins, I87.9 — Disorder of vein, unspecified, I99.8 — Other disorder of circulatory system, I99.9 — Unspecified disorder of circulatory system

Same clinical category (CCSR)

AHRQ's Clinical Classifications Software groups these diagnoses under the same clinical category (Endocarditis and endocardial disease).

A18.84 — Tuberculosis of heart, A32.82 — Listerial endocarditis, A39.51 — Meningococcal endocarditis, A52.03 — Syphilitic endocarditis, A52.06 — Other syphilitic heart involvement, A54.83 — Gonococcal heart infection, B33.21 — Viral endocarditis, B37.6 — Candidal endocarditis, I01.1 — Acute rheumatic endocarditis, I01.8 — Other acute rheumatic heart disease, I01.9 — Acute rheumatic heart disease, unspecified, I02.0 — Rheumatic chorea with heart involvement, I09.9 — Rheumatic heart disease, unspecified, I33.0 — Acute and subacute infective endocarditis, I38 — Endocarditis, valve unspecified, I39 — Endocarditis and heart valve disorders in diseases classified elsewhere, M32.11 — Endocarditis in systemic lupus erythematosus

Same Index main term, other category

The ICD-10-CM Index to Diseases and Injuries files this code under the main term “Endocarditis”; these codes share that main term but sit in a different category of the Tabular List.

I07.8 — Other rheumatic tricuspid valve diseases (tricuspid, with, rheumatic fever, inactive or quiescent), I07.9 — Rheumatic tricuspid valve disease, unspecified (tricuspid), I08.0 — Rheumatic disorders of both mitral and aortic valves (mitral, with, aorticdisease), I08.1 — Rheumatic disorders of both mitral and tricuspid valves (mitral, with, tricuspiddisease), I08.2 — Rheumatic disorders of both aortic and tricuspid valves (aortic, with, tricuspiddisease), I08.3 — Combined rheumatic disorders of mitral, aortic and tricuspid valves (tricuspid, with, aorticdisease, mitraldisease), I08.8 — Other rheumatic multiple valve diseases (multiple valves, specified disorders), I08.9 — Rheumatic multiple valve disease, unspecified (multiple valves), I09.1 — Rheumatic diseases of endocardium, valve unspecified (rheumatic), I09.89 — Other specified rheumatic heart diseases (pulmonary, rheumatic), I34.89 — Other nonrheumatic mitral valve disorders (mitral, nonrheumatic), I35.8 — Other nonrheumatic aortic valve disorders (aortic), I36.8 — Other nonrheumatic tricuspid valve disorders (tricuspid, nonrheumatic), I37.8 — Other nonrheumatic pulmonary valve disorders (pulmonary), I38 — Endocarditis, valve unspecified, I42.3 — Endomyocardial (eosinophilic) disease (Löffler's), M32.11 — Endocarditis in systemic lupus erythematosus (verrucous), Q22.2 — Congenital pulmonary valve insufficiency (pulmonary, congenital), Q24.8 — Other specified congenital malformations of heart (congenital), T82.6 — Infection and inflammatory reaction due to cardiac valve prosthesis (due to, prosthetic cardiac valve), +14 more

Contextual Map

Every relationship of I33.9 in one view: hierarchy, official tabular instructions in both directions, clinical classification, risk adjustment, MS-DRG participation, index terms and change history — each edge carrying the CMS source it derives from.

Hierarchy

Clinical classification (CCSR)

MS-DRG Grouper

MDC crossing

  • MDC 05 — Diseases and Disorders of the Circulatory System[MDC crossing]: “Diseases and Disorders of the Circulatory System — the grouper's crossing between diagnoses and procedures: a principal diagnosis sets the MDC, and same-MDC procedures move the stay to its surgical DRGs. 17,209 same-MDC procedures group here; browse them on the MDC page.”— CMS MS-DRG Definitions Manual · FY2026
  • MDC 15 — Newborns and Other Neonates with Conditions Originating in Perinatal Period[MDC crossing]: “Newborns and Other Neonates with Conditions Originating in Perinatal Period — the grouper's crossing between diagnoses and procedures: a principal diagnosis sets the MDC, and same-MDC procedures move the stay to its surgical DRGs.”— CMS MS-DRG Definitions Manual · FY2026

Index entries

  • Endocarditis (chronic) (marantic) (nonbacterial) (thrombotic) (valvular), acute or subacute[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
  • Endocarditis (chronic) (marantic) (nonbacterial) (thrombotic) (valvular), aortic (heart) (nonrheumatic) (valve), acute or subacute[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
  • Endocarditis (chronic) (marantic) (nonbacterial) (thrombotic) (valvular), mitral (chronic) (double) (fibroid) (heart) (inactive) (valve) (with chorea), nonrheumatic, acute or subacute[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
  • Endocarditis (chronic) (marantic) (nonbacterial) (thrombotic) (valvular), pulmonary (chronic) (heart) (valve), acute or subacute[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
  • Endocarditis (chronic) (marantic) (nonbacterial) (thrombotic) (valvular), toxic[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
  • Endocarditis (chronic) (marantic) (nonbacterial) (thrombotic) (valvular), tricuspid (chronic) (heart) (inactive) (rheumatic) (valve) (with chorea), nonrheumatic, acute or subacute[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026

Nearest codes

Change history

  • FY2016 — In the code set at ICD-10-CM adoption [Change history]— CMS release files (code change ledger) · icd10cm-fy2016

Sources for this page

Codes, titles, notes, index terms and mappings on this page are transcribed from the datasets below. Relationships MedCoder computed and text MedCoder wrote are labelled where they appear.

Code, title, tabular notes and index terms Official source data
CMS/CDC ICD-10-CM FY2026 tabular list, index and tables, effective October 1, 2025 Release, file and checksum · Publisher’s page
Claim edits Official source data
CMS Definitions of Medicare Code Edits — v43.1 (April 2026) Release, file and checksum · Publisher’s page
Inpatient payment groups Official source data
CMS MS-DRG Definitions Manual (incl. Appendix B diagnosis index, Appendix C CC/MCC list, Appendix E procedure index) and IPPS Final Rule tables — v43 Release, file and checksum · Publisher’s page
Change history and date-of-service validity Official source data
CMS ICD-10-CM release addenda, ingested release by release into the change ledger Release, file and checksum · Publisher’s page
Comparisons, relationships and the contextual map MedCoder structured relationship
Computed by MedCoder from the tabular notes and tables above; every derived relationship is marked as derived where it appears
Summary and FAQ answers MedCoder explanation
Written by MedCoder to explain the sources above: drafted with AI assistance, checked by a person against the release files, and labelled as MedCoder text where it appears. Not official text.

Labels on this page: Official source data · MedCoder structured relationship · MedCoder explanation. How to read the labels · All data sources and release dates

Cite this page

Reference this page in a research guide, syllabus or article. The release is included because code content changes each year.

MedCoder.ai. "I33.9 — Acute and subacute endocarditis, unspecified." ICD-10-CM FY2026. https://medcoder.ai/icd10/code/i33.9-acute-and-subacute-endocarditis-unspecified

Change history

  • FY2016 — October 1, 2015
    In the code set at ICD-10-CM adoption
    Acute and subacute endocarditis, unspecified

No changes since FY2016 — additions, deletions, description changes and billable-status changes are tracked through FY2027 (effective October 1, 2026), and none are recorded for this code. Note changes are tracked from FY2027 only.

Nearest Codes in This Family

Official ICD-10-CM classifications closest to I33.9 in its code family, with their registry titles.

View all codes in the I33 family