2026 ICD-10-CM Diagnosis Code S55.999AOther specified injury of unspecified blood vessel at forearm level, unspecified arm, initial encounter

ICD-10-CM Codes›S00–T88›S50-S59›S55

ICD-10-CM S55.999A
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

S55.999A is a billable ICD-10-CM diagnosis code for other specified injury of unspecified blood vessel at forearm level, unspecified arm, initial encounter. The 7th character A marks it as an initial encounter code, used while the patient is receiving active treatment. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 913 through 914, 963 through 965. The code is flagged as an unspecified code, since codes identifying laterality exist in the same family. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Injury to blood vessels, initial encounter.

Code Identity

ICD-10-CM Code
S55.999A
Billable Status
Yes — Valid for Submission
Code Describes
Other specified injury of unspecified blood vessel at forearm level, unspecified arm, initial encounter
Short Description
Inj unsp blood vessel at forearm level, unsp arm, init
Parent Code
S55.999
Other specified injury of unspecified blood vessel at forearm level, unspecified arm

Code Classification

ChapterS00–T88Injury, poisoning and certain other consequences of external causes
SectionS50-S59Injuries to the elbow and forearm
CategoryS55Injury of blood vessels at forearm level
This CodeS55.999AOther specified injury of unspecified blood vessel at forearm level, unspecified arm, initial encounter

Code EditsBilling

Medicare Code Editor checks that affect claim validity for S55.999A.

Unspecified codes
Unspecified codes exist in the ICD-10-CM classification for circumstances when documentation in the medical record does not provide the level of detail needed to support reporting a more specific code. However, in the inpatient setting, there should generally be very limited and rare circumstances for which the laterality (right, left, bilateral) of a condition is unable to be documented and reported. The following pages contain the list of unspecified ICD-10-CM diagnosis codes for which there is a more specific code to identify laterality (right, left, bilateral) within that code family.

Coding GuidelinesGuidance

The appropriate 7th character is to be added to each code from block Injury of blood vessels at forearm level (S55). Use the following options for the applicable episode of care:

  • A - initial encounter
  • D - subsequent encounter
  • S - sequela

Source: ICD-10-CM Official Guidelines for Coding and Reporting, FY 2026, published by CMS and the National Center for Health Statistics.

Clinical ClassificationClinical

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

CCSR INJ016
Injury to blood vessels, initial encounter
Default principal diagnosis: inpatient Yes · outpatient Yes

Patient EducationClinical

Arm Injuries and Disorders

Of the 206 bones in your body, three of them are in your arm: the humerus, radius, and ulna. Your arms are also made up of muscles, joints, tendons, and other connective tissue. Injuries to any of these parts of the arm can occur during sports, a fall, or an accident.

Read the full article at MedlinePlus

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

Convert S55.999A to ICD-9-CMHistory

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

ICD-9-CM
903.9 Injury arm vessel 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 S55.999AOverview

Is S55.999A a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report other specified injury of unspecified blood vessel at forearm level, unspecified arm, initial encounter on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What does the 7th character A in S55.999A mean?

The final character A marks the initial encounter: use it while the patient is receiving active treatment for other specified injury of unspecified blood vessel at forearm level, unspecified arm, such as an emergency visit or first evaluation.

What MS-DRG does S55.999A group to?

When other specified injury of unspecified blood vessel at forearm level, unspecified arm, initial encounter is the principal diagnosis on an inpatient stay, it groups to MS-DRG 913, 914, 963, 964, 965, with relative weights from 0.8855 to 2.7338 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of S55.999A?

Under the General Equivalence Mappings, other specified injury of unspecified blood vessel at forearm level, unspecified arm, initial encounter converts to ICD-9-CM 903.9 (injury arm vessel 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.