2026 ICD-10-CM Diagnosis Code A42.9Actinomycosis, unspecified

ICD-10-CM Codes›A00–B99›A30-A49›A42

ICD-10-CM A42.9
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

A42.9 is a billable ICD-10-CM diagnosis code for actinomycosis, unspecified. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 867 through 869, 974 through 976. Coders also document this condition as actinomyces oris or Actinomyces viscosus. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Bacterial infections.

Code Identity

ICD-10-CM Code
A42.9
Billable Status
Yes — Valid for Submission
Code Describes
Actinomycosis, unspecified
Short Description
Actinomycosis, unspecified
Same as the full description in the CMS dataset.
Parent Code
A42
Actinomycosis

Code Classification

ChapterA00–B99Certain infectious and parasitic diseases
SectionA30-A49Other bacterial diseases
CategoryA42Actinomycosis
This CodeA42.9Actinomycosis, unspecified

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Actinomyces oris or Actinomyces viscosus
  • Actinomycotic infection
  • Infection caused by Actinomycetales
  • Infection caused by Tropheryma
  • Propionibacterium infection

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.

    • Actinomycosis, actinomycotic
    • Infection, infected, infective(opportunistic)
      • Actinomyces (israelii)

Clinical ClassificationClinical

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

CCSR INF003
Bacterial infections
Default principal diagnosis: inpatient Yes · outpatient Yes

Clinical InformationClinical

  • Actinomycosis

    infections with bacteria of the genus actinomyces.

  • Actinomycosis, Cervicofacial

    a form of actinomycosis characterized by slow-growing inflammatory lesions of the lymph nodes that drain the mouth (lumpy jaw), reddening of the overlying skin, and intraperitoneal abscesses.

  • Actinomyces

    a genus of gram-positive, rod-shaped bacteria whose organisms are nonmotile. filaments that may be present in certain species are either straight or wavy and may have swollen or clubbed heads.

  • Actinomycosis

    an infectious process caused by bacteria of the actinomyces species. it is characterized by the formation of purulent and painful abscesses in the mouth, lungs and gastrointestinal tract.

Patient EducationClinical

Bacterial Infections

Bacteria are tiny organisms (living things) that have only one cell. Under a microscope, they look like balls, rods, or spirals. They can be found almost everywhere on Earth. There are lots of bacteria in and on your body. In fact, your body has about 10 times more bacteria cells than human cells.

The full article covers:

  • What are bacteria?
  • How do bacterial infections spread?
  • What are the treatments for bacterial infections?
  • Can bacterial infections be prevented?

Read the full article at MedlinePlus

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

Convert A42.9 to ICD-9-CMHistory

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

ICD-9-CM
039.9 Actinomycosis NOS
Approximate The match is approximate rather than exact.

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 A42.9Overview

Is A42.9 (Actinomycosis) a billable code?

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

What MS-DRG does A42.9 group to?

When actinomycosis, unspecified is the principal diagnosis on an inpatient stay, it groups to MS-DRG 867, 868, 869, 974, 975, 976, with relative weights from 0.7297 to 2.8860 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of A42.9?

Under the General Equivalence Mappings, actinomycosis, unspecified converts to ICD-9-CM 039.9 (actinomycosis NOS). The mapping is approximate, so confirm the match fits the documentation.

Footnotes

[1] Not chronic - A diagnosis code that does not fit the criteria for chronic condition (duration, ongoing medical treatment, and limitations) is considered not chronic. Some codes designated as not chronic are acute conditions. Other diagnosis codes that indicate a possible chronic condition, but for which the duration of the illness is not specified in the code description (i.e., we do not know the condition has lasted 12 months or longer) also are considered not chronic.