ICD-10 Code A51.1 – Primary anal syphilis (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code A51.1 – Primary anal syphilis
What it is
A51.1 identifies primary syphilis infection located in the anal area. Use it when the first recognized lesion or ulcer is at or near the anus and documentation supports primary syphilis.
Clinical signs
Typical findings include a painless chancre, local firmness, and nearby lymph node enlargement. Clinical features vary; refer to documentation, especially if the lesion is atypical or the site is described imprecisely.
When to use this code
Use this code when the provider documents primary syphilis affecting the anus, anal canal, or perianal region. It is appropriate when the record clearly identifies the lesion as the initial syphilitic lesion at that site. If the note only says “syphilis” or the stage is unclear, check documentation.
Do not use for
Do not use A51.1 for secondary, latent, or congenital syphilis. Also avoid it when the anal lesion is due to another cause, such as herpes, fissure, hemorrhoids, or nonspecific ulceration.
Coding tip
Confirm both the stage and the anatomic site in the provider note before assigning A51.1.