2026 ICD-10-CM Diagnosis Code A42.0Pulmonary actinomycosis
ICD-10-CM Codes›A00–B99›A30-A49›A42
- Billable — Valid for Submission
- CC — Complication or Comorbidity
- Not Chronic
A42.0 is a billable ICD-10-CM diagnosis code for pulmonary actinomycosis. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 177 through 179, 974 through 976. As a secondary diagnosis, it counts as a complication or comorbidity (CC) and moves an inpatient stay to a higher severity level within its MS-DRG family. It does not count, however, when the principal diagnosis is one of 10 closely related codes. Coders also document this condition as actinomycotic pneumonia. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Bacterial infections.
A42.0 no longer risk-adjusts for Medicare Advantage: it mapped to HCC 115 under the retired CMS-HCC V24 model through payment year 2025 but maps to no category in the live V28 model. It still risk-adjusts in the PACE (CMS-HCC V22) category 115, ESRD (V21) category 115, and ESRD (V24) category 115 for payment year 2026.
Code Identity
Code Classification
Medicare Risk Adjustment (HCC)Billing
A42.0 no longer risk-adjusts for Medicare Advantage: it maps to no payment category in the live CMS-HCC V28 model, although it still risk-adjusts in the other CMS models shown below.
Source: CMS Payment Year 2026 risk adjustment mappings and model software. Weights are relative factors, not dollar amounts; a beneficiary's total RAF also includes demographics and interactions. Browse all CMS-HCC categories.
Approximate SynonymsGuidance
Alternate terms and clinical phrases that map to this code.
- Actinomycotic pneumonia
- Pneumonia caused by anaerobic bacteria
- Pulmonary actinomycosis
Index to Diseases and InjuriesGuidance
Alphabetical index entries that point to this code.
Actinomycosis, actinomycotic A42.9
with pneumonia A42.0
pulmonary A42.0
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
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 A42.0 to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code HistoryHistory
Questions About A42.0Overview
What is the ICD-10 code for pulmonary actinomycosis?
The ICD-10-CM code for pulmonary actinomycosis is A42.0 (sometimes written as A420). It is billable on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
Is A42.0 (Actinomycosis) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report pulmonary actinomycosis on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
What MS-DRG does A42.0 group to?
When pulmonary actinomycosis is the principal diagnosis on an inpatient stay, it groups to MS-DRG 177, 178, 179, 974, 975, 976, with relative weights from 0.7550 to 2.8860 depending on complications. Higher weights mean higher Medicare reimbursement.
Is A42.0 a CC or MCC?
CMS lists A42.0 as a CC (complication or comorbidity) for FY 2026. Reported as a secondary diagnosis, it moves the inpatient stay to a higher-weighted DRG within its severity family. It does not count when the principal diagnosis is one of the 10 closely related codes in its exclusion list.
What is the ICD-9 equivalent of A42.0?
Under the General Equivalence Mappings, pulmonary actinomycosis converts to ICD-9-CM 039.1 (pulmonary actinomycosis). The mapping is approximate, so confirm the match fits the documentation.
Does A42.0 risk-adjust for Medicare Advantage payment?
Not for Medicare Advantage. A42.0 mapped to HCC 115 in the retired CMS-HCC V24 model, which last determined payment in 2025, but it maps to no category in the live V28 model; see all codes that no longer risk-adjust. It still risk-adjusts in the PACE (CMS-HCC V22) category 115 (Pneumococcal Pneumonia/Empyema/Lung Abscess), ESRD (V21) category 115 (Pneumococcal Pneumonia/Empyema/Lung Abscess), and ESRD (V24) category 115 (Pneumococcal Pneumonia/Empyema/Lung Abscess).
