A51.9 ICD-10-CM Code: Early syphilis, unspecified

Coding instructions

Official Tabular Instructional Notes: the inclusion, exclusion and sequencing notes published for A51.9 in the official ICD-10-CM tabular list, quoted as published.

Source: CMS/CDC — ICD-10-CM Tabular ListRelease: FY2026Effective: October 1, 2025

Notes without a marker are published on A51.9 itself; “inherited from” names the category or block whose note applies here.

Coder workflow for A51.9

MedCoder structured workflow — derived from this code’s own official record

Before you code A51.9

1. Unspecified does not mean incorrect. When the record gives no greater specificity, A51.9 may be the appropriate code. Check the record for detail that supports a more specific sibling. An unspecified code is for records that do not provide the detail a more specific code needs; a query, not an assumption, is the route to specificity (Guidelines I.A.9.b, I.B.18).

   ReviewA51.0, A51.1, A51.2, A51.3, A51.4, A51.5

   See the relationships section · Guide: Other vs unspecified (NEC vs NOS) →

2. Check the Excludes1 note: if the documentation supports a condition named there, do not simply proceed with A51.9. Excludes1 conditions are not reported together with this code unless the record shows they are unrelated (Guidelines I.A.12.a).

   See the official tabular notes

Choose the right path

1. Does the record document the detail a more specific sibling code needs?
   Yes → Review the specific siblings in this subcategory.
   No → Continue — A51.9 is appropriate when the documentation goes no further.

   ReviewA51.0, A51.1, A51.2, A51.3, A51.4, A51.5

2. Does the documentation support a condition named in A51.9’s Excludes1 note?
   Yes → Do not simply proceed: review the excluded code. Both are reported only when the record shows the two conditions are unrelated.
   No → Continue.

   ReviewN34.1, M02.3

Consider A51.9. Then confirm the code is valid for the date of service in the Verify section.

Documentation check

The provider’s diagnostic statement

Codes are assigned from the provider’s documented diagnosis, not from clinical criteria, test values or a medication list (Guidelines I.A.19).

Any detail beyond this code’s title

What the record states that a more specific sibling code would capture — or its absence, which itself supports the unspecified code.

Official instructions as workflow

• Excludes1 — check before selecting A51.9(2 notes)

  Coding workflow: If the documentation supports a condition named in this note, do not simply proceed with A51.9: the two are not reported together. The one exception is when the record shows the two conditions are unrelated to each other.

  CompareN34.1, M02.3

  See the official tabular notes · Guidelines I.A.12.a

• Excludes2 — not part of A51.9(1 note)

  Coding workflow: The conditions named in this note are not included in A51.9. When the record documents both, both may be reported; the note is a boundary, not a prohibition.

  CompareB20

  See the official tabular notes · Guidelines I.A.12.b

Coding decision scenarios

Pattern scenarios for this code’s structure — decision rules, not clinical cases

Every row is derived from this code’s own record — its title, tabular notes, 7th-character family and same-category siblings — with fixed MedCoder wording; nothing is inferred about a patient. The official notes and guideline text are in the sections each row links to, and they control.

Indexed Clinical Terms (5)

Official source data — entries quoted as published, in the Index’s own lookup phrasing

Clinical term phrases from the official ICD-10-CM Index to Diseases and Injuries that map to this code. These are alphabetic-index entries shown as the Index writes them — lookup phrasing, not necessarily the wording of a final diagnosis.

• Syphilis, syphilitic (acquired), age under 2 years NOS, acquired
• Syphilis, syphilitic (acquired), early
• Syphilis, syphilitic (acquired), early, relapse (treated, untreated)
• Syphilis, syphilitic (acquired), early, symptomatic
• Syphilis, syphilitic (acquired), recently acquired, symptomatic

Don’t confuse A51.9 with…

MedCoder analysis — pairs from this code’s official notes, distinctions written by MedCoder

The codes most likely to be picked instead of A51.9 — named in its own official notes, or sharing its laterality or encounter family — with what separates them and which documentation moves the case. Each links to the full comparison.

• N34.1— Excludes1 conflict (Excludes1 — generally not reported together)

  Not coded here — the tabular list excludes this pair for the same encounter.

  Not reported together with this code, except where the two conditions are documented as unrelated.

  What moves you there: Documentation of nonspecific and nongonococcal urethritis moves the case to N34.1; the two are not reported together unless documented as unrelated.

  Full comparison: A51.9 vs N34.1 →

• M02.3— Excludes1 conflict (Excludes1 — generally not reported together)

  Not coded here — the tabular list excludes this pair for the same encounter.

  Not reported together with this code, except where the two conditions are documented as unrelated.

  What moves you there: Documentation of Reiter's disease moves the case to M02.3; the two are not reported together unless documented as unrelated.

  Full comparison: A51.9 vs M02.3 →

• B20— Excludes2 relationship (Informational)

  Distinct conditions — both may be reported.

  A distinct condition — both may be reported when both are documented.

  What moves you there: Documentation of human immunodeficiency virus [HIV] disease adds B20; it is not part of this code and both may be reported when both are documented.

  Full comparison: A51.9 vs B20 →

Relationships are read from this code's own official notes, its laterality siblings and its encounter family; the wording of each distinction is MedCoder editorial, and the documentation named in each “what moves you there” line is the official note phrase or sibling title itself. Source: CMS ICD-10-CM tabular instructional notes; ICD-10-CM Official Guidelines, Section I.A.12.a.

Verify Before Coding

• No Medicare Code Editor or MS-DRG Definitions Manual restrictions apply to this code.

From the code registry, the Medicare Code Editor, and the MS-DRG Definitions Manual. Check it against a full claim in Claim Check.

Relationships & Classification

MedCoder structured relationships — computed from published CMS and AHRQ datasets

Other codes that name A51.9 or its code family, from the CMS ICD-10-CM tabular instructional notes. Tabular-note edges are stored at the code family level that carries each note.

Referenced by 7 Excludes2 notes across 2 chapters: A65 — Nonvenereal syphilis (via A51.-), A65-A69 — Other spirochetal diseases (A65-A69) (via A51.-), A66 — Yaws (via A51.-), A67 — Pinta [carate] (via A51.-), A68 — Relapsing fevers (via A51.-), A69 — Other spirochetal infections (via A51.-), N34 — Urethritis and urethral syndrome (via A51.-).

These codes’ tabular lists name this diagnosis as distinct — both may be reported when both are documented.

MS-DRG Grouper Relationships (FY2026)

Potential MS-DRG participation — not a DRG assignment.

FY2026 MS-DRG: not on the CMS CC/MCC list — as a secondary diagnosis this code does not change MS-DRG severity for that release.

Named in the grouper logic of 3 MS-DRGs: DRG 867 (MDC 18), DRG 868 (MDC 18), DRG 869 (MDC 18).

From the CMS MS-DRG Definitions Manual (Appendices B and C). Actual DRG assignment depends on the complete claim.

Clinical classification (AHRQ CCSR):INF010 — Sexually transmitted infections (excluding HIV and hepatitis) (default).

Clinical Classifications Software Refined (CCSR) for ICD-10-CM Diagnoses. Healthcare Cost and Utilization Project (HCUP), Agency for Healthcare Research and Quality.

Related Codes

Same clinical category (CCSR)

AHRQ's Clinical Classifications Software groups these diagnoses under the same clinical category (Sexually transmitted infections (excluding HIV and hepatitis)).

A51.32 — Syphilitic alopecia, A51.39 — Other secondary syphilis of skin, A51.41 — Secondary syphilitic meningitis, A51.42 — Secondary syphilitic female pelvic disease, A51.43 — Secondary syphilitic oculopathy, A51.44 — Secondary syphilitic nephritis, A51.45 — Secondary syphilitic hepatitis, A51.46 — Secondary syphilitic osteopathy, A51.49 — Other secondary syphilitic conditions, A51.5 — Early syphilis, latent, A52.00 — Cardiovascular syphilis, unspecified, A52.01 — Syphilitic aneurysm of aorta, A52.02 — Syphilitic aortitis, A52.03 — Syphilitic endocarditis, A52.04 — Syphilitic cerebral arteritis, A52.05 — Other cerebrovascular syphilis, A52.06 — Other syphilitic heart involvement, A52.09 — Other cardiovascular syphilis, A52.10 — Symptomatic neurosyphilis, unspecified, A52.11 — Tabes dorsalis, +139 more

Same Index main term, other category

The ICD-10-CM Index to Diseases and Injuries files this code under the main term “Syphilis, syphilitic”; these codes share that main term but sit in a different category of the Tabular List.

A50.52 — Hutchinson's teeth (mulberry molars), A50.53 — Hutchinson's triad (congenital, late, or 2 years or more after birth NEC, symptomatic or with manifestations NEC, Hutchinson's triad), A50.54 — Late congenital cardiovascular syphilis (aneurysm, congenital), A50.55 — Late congenital syphilitic arthropathy (congenital, late, or 2 years or more after birth NEC, symptomatic or with manifestations NEC, arthropathy), A50.56 — Late congenital syphilitic osteochondropathy (congenital, late, or 2 years or more after birth NEC, symptomatic or with manifestations NEC, osteochondropathy), A50.57 — Syphilitic saddle nose (nose, saddle back deformity), A50.59 — Other late congenital syphilis, symptomatic (stigmata), A50.6 — Late congenital syphilis, latent (congenital, late, or 2 years or more after birth NEC, latent), A50.7 — Late congenital syphilis, unspecified (congenital, late, or 2 years or more after birth NEC), A50.9 — Congenital syphilis, unspecified (congenital), A52.00 — Cardiovascular syphilis, unspecified (vascular), A52.01 — Syphilitic aneurysm of aorta (aneurysm), A52.02 — Syphilitic aortitis (aorta), A52.03 — Syphilitic endocarditis (aortic), A52.04 — Syphilitic cerebral arteritis (mesarteritis, brain), A52.05 — Other cerebrovascular syphilis (cerebrovascular), A52.06 — Other syphilitic heart involvement (heart), A52.09 — Other cardiovascular syphilis (portal vein), A52.11 — Tabes dorsalis (ataxia), A52.12 — Other cerebrospinal syphilis (spinal), +40 more

Lab tests where this diagnosis supports Medicare coverage (NCD)

Medicare's National Coverage Determination (NCD) program lists this diagnosis as medical justification for these lab tests.

HIV Screening Test

Contextual Map

Every relationship of A51.9 in one view: hierarchy, official tabular instructions in both directions, clinical classification, risk adjustment, MS-DRG participation, index terms and change history — each edge carrying the CMS source it derives from.

Run A51.9 with these 7 related codes in Claim Check

Hierarchy

• A00-B99 — Chapter 1: Certain Infectious and Parasitic Diseases (A00-B99) (A00-B99)[Hierarchy]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• A50-A64 — Infections with a predominantly sexual mode of transmission[Hierarchy]— CMS ICD-10-CM tabular structure · icd10cm-fy2026

Referenced by Excludes2 notes

• A65 — Nonvenereal syphilis[Excludes2](via A51.-): “syphilis (A50-A53)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026
• A65-A69 — Other spirochetal diseases (A65-A69)[Excludes2](via A51.-): “syphilis (A50-A53)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026
• A66 — Yaws[Excludes2](via A51.-): “syphilis (A50-A53)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026
• A67 — Pinta [carate][Excludes2](via A51.-): “syphilis (A50-A53)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026
• A68 — Relapsing fevers[Excludes2](via A51.-): “syphilis (A50-A53)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026
• A69 — Other spirochetal infections[Excludes2](via A51.-): “syphilis (A50-A53)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026
• N34 — Urethritis and urethral syndrome[Excludes2](via A51.-): “urethritis in diseases with a predominantly sexual mode of transmission (A50-A64)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026

Clinical classification (CCSR)

• INF010 — Sexually transmitted infections (excluding HIV and hepatitis)[CCSR]— AHRQ CCSR for ICD-10-CM Diagnoses (HCUP)

MS-DRG Grouper

• DRG 867 — OTHER INFECTIOUS AND PARASITIC DISEASES DIAGNOSES WITH MCC[MS-DRG]: “OTHER INFECTIOUS AND PARASITIC DISEASES DIAGNOSES WITH MCC (MDC 18)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026
• DRG 868 — OTHER INFECTIOUS AND PARASITIC DISEASES DIAGNOSES WITH CC[MS-DRG]: “OTHER INFECTIOUS AND PARASITIC DISEASES DIAGNOSES WITH CC (MDC 18)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026
• DRG 869 — OTHER INFECTIOUS AND PARASITIC DISEASES DIAGNOSES WITHOUT CC/MCC[MS-DRG]: “OTHER INFECTIOUS AND PARASITIC DISEASES DIAGNOSES WITHOUT CC/MCC (MDC 18)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026

MDC crossing

• MDC 18 — Infectious and Parasitic Diseases, Systemic or Unspecified Sites[MDC crossing]: “Infectious and Parasitic Diseases, Systemic or Unspecified Sites — the grouper's crossing between diagnoses and procedures: a principal diagnosis sets the MDC, and same-MDC procedures move the stay to its surgical DRGs. 1 same-MDC procedures group here; browse them on the MDC page.”— CMS MS-DRG Definitions Manual · FY2026

Index entries

• Syphilis, syphilitic (acquired), age under 2 years NOS, acquired[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
• Syphilis, syphilitic (acquired), early[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
• Syphilis, syphilitic (acquired), early, relapse (treated, untreated)[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
• Syphilis, syphilitic (acquired), early, symptomatic[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
• Syphilis, syphilitic (acquired), recently acquired, symptomatic[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026

Nearest codes (17)

• A51 — Early syphilis[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• A51.0 — Primary genital syphilis[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• A51.1 — Primary anal syphilis[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• A51.2 — Primary syphilis of other sites[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• A51.3 — Secondary syphilis of skin and mucous membranes[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• A51.31 — Condyloma latum[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• A51.32 — Syphilitic alopecia[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• A51.39 — Other secondary syphilis of skin[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• and 9 more

Change history

• FY2016 — In the code set at ICD-10-CM adoption [Change history]— CMS release files (code change ledger) · icd10cm-fy2016

Sources for this page

Codes, titles, notes, index terms and mappings on this page are transcribed from the datasets below. Relationships MedCoder computed and text MedCoder wrote are labelled where they appear.

Code, title, tabular notes and index terms Official source data

CMS/CDC ICD-10-CM FY2026 tabular list, index and tables, effective October 1, 2025 Release, file and checksum · Publisher’s page

Claim edits Official source data

CMS Definitions of Medicare Code Edits — v43.1 (April 2026) Release, file and checksum · Publisher’s page

Inpatient payment groups Official source data

CMS MS-DRG Definitions Manual (incl. Appendix B diagnosis index, Appendix C CC/MCC list, Appendix E procedure index) and IPPS Final Rule tables — v43 Release, file and checksum · Publisher’s page

Change history and date-of-service validity Official source data

CMS ICD-10-CM release addenda, ingested release by release into the change ledger Release, file and checksum · Publisher’s page

Comparisons, relationships and the contextual map MedCoder structured relationship

Computed by MedCoder from the tabular notes and tables above; every derived relationship is marked as derived where it appears

Summary and FAQ answers MedCoder explanation

Written by MedCoder to explain the sources above: drafted with AI assistance, checked by a person against the release files, and labelled as MedCoder text where it appears. Not official text.

Labels on this page: Official source data · MedCoder structured relationship · MedCoder explanation. How to read the labels · All data sources and release dates

Nearest Codes in This Family

Official ICD-10-CM classifications closest to A51.9 in its code family, with their registry titles.

• A51.39 — Other secondary syphilis of skin
• A51.4 — Other secondary syphilis
• A51.41 — Secondary syphilitic meningitis
• A51.42 — Secondary syphilitic female pelvic disease
• A51.43 — Secondary syphilitic oculopathy
• A51.44 — Secondary syphilitic nephritis
• A51.45 — Secondary syphilitic hepatitis
• A51.46 — Secondary syphilitic osteopathy
• A51.49 — Other secondary syphilitic conditions
• A51.5 — Early syphilis, latent

View all codes in the A51 family