2026 ICD-10-CM Diagnosis Code A51.1Primary anal syphilis

ICD-10-CM Codes›A00–B99›A50-A64›A51

ICD-10-CM A51.1
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

A51.1 is a billable ICD-10-CM diagnosis code for primary anal syphilis. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 393 through 395. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Anal and rectal conditions and Sexually transmitted infections (excluding HIV and hepatitis).

Code Identity

ICD-10-CM Code
A51.1
Billable Status
Yes — Valid for Submission
Code Describes
Primary anal syphilis
Short Description
Primary anal syphilis
Same as the full description in the CMS dataset.
Parent Code
A51
Early syphilis

Code Classification

ChapterA00–B99Certain infectious and parasitic diseases
SectionA50-A64Infections with a predominantly sexual mode of transmission
CategoryA51Early syphilis
This CodeA51.1Primary anal syphilis

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Primary anal syphilis
  • Primary extragenital syphilis

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.

    • Induration, indurated
      • chancre
        • anus
    • Syphilis, syphilitic(acquired)
      • anus (late)
        • primary
    • Syphilis, syphilitic(acquired)
      • primary
        • anal

Clinical ClassificationClinical

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

CCSR DIG015
Anal and rectal conditions
Default principal diagnosis: inpatient No · outpatient No
CCSR INF010
Sexually transmitted infections (excluding HIV and hepatitis)
Default principal diagnosis: inpatient Yes · outpatient Yes

Patient EducationClinical

Anal Disorders

Your anus is the opening at the end of your large intestine. It is where stool (poop) leaves your body.

The full article covers:

  • What is the anus?
  • What are anal disorders?
  • What are the symptoms of anal disorders?
  • How are anal disorders diagnosed?
  • How are anal disorders treated?

Read the full article at MedlinePlus

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

Convert A51.1 to ICD-9-CMHistory

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

ICD-9-CM
091.1 Primary anal syphilis
Exact Match The mapping is direct, with no qualifiers.

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 A51.1Overview

Is A51.1 (Early syphilis) a billable code?

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

What MS-DRG does A51.1 group to?

When primary anal syphilis is the principal diagnosis on an inpatient stay, it groups to MS-DRG 393, 394, 395, with relative weights from 0.6490 to 1.5993 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of A51.1?

Under the General Equivalence Mappings, primary anal syphilis converts to ICD-9-CM 091.1 (primary anal syphilis). The mapping is a direct match.

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.