ICD-10 code A53.9 identifies a confirmed syphilis infection caused by the bacterium Treponema pallidum when the specific stage or type of the disease cannot be determined from the available clinical documentation. Coders apply this code only when stage-specific codes in the A50–A52 categories are not supported by the medical record — it is the last resort in the syphilis coding hierarchy, not the default.
What Does ICD-10 Code A53.9 Mean?
ICD-10 code A53.9, Syphilis, unspecified, represents a confirmed or suspected treponema pallidum infection where the clinical documentation does not establish whether the condition is primary, secondary, latent, or late-stage syphilis. The inclusion term “Syphilis (acquired) NOS” — meaning not otherwise specified — signals that this code should reflect genuine uncertainty documented in the record, not coder convenience.
Key attributes of this code at a glance:
- Applicable setting: Both inpatient and outpatient encounters
- Chapter: Chapter 1 — Certain Infectious and Parasitic Diseases (A00–B99)
- Category: A53 — Other and Unspecified Syphilis
- Not applicable to children under two years of age (see Excludes1 note)
- MS-DRG assignment: 867 (with MCC), 868 (with CC), or 869 (without CC/MCC)
What Conditions and Diagnoses Does A53.9 Cover?
A53.9 applies when syphilis is confirmed — either clinically or serologically — but the record lacks sufficient detail to assign a more specific code from the A50, A51, or A52 categories. In practice, coders encounter this scenario most often in urgent care settings, emergency departments, or cases involving outside lab results without accompanying clinical notes.
Clinical presentations that may appropriately map to A53.9 include:
- A reactive RPR or VDRL result documented without staging information
- A provider note documenting “syphilis” with no indication of symptom duration, lesion characteristics, or prior treatment history
- A patient presenting to a new provider with a prior syphilis diagnosis and incomplete outside records
- An incidental positive treponemal antibody test (e.g., TP-PA or FTA-ABS) where no clinical staging has been performed
What Does A53.9 Specifically Exclude?
The Tabular List carries a Type 1 Excludes note that prohibits assigning A53.9 simultaneously with the following:
- A50.2 — Syphilis in a child under two years of age (congenital syphilis, unspecified); this is the correct code for patients in that age range
- Any code from A51.– (early syphilis) or A52.– (late syphilis) when those conditions are documented — an Excludes1 note is a “never use together” instruction
When Is A53.9 the Right Code to Use?
The most important rule in syphilis coding is that A53.9 is a default-of-last-resort, not a first-choice code. Before assigning it, coders must systematically rule out every more specific option. Follow this decision sequence:
- Confirm the diagnosis is syphilis, not a test for syphilis. Screening encounters are coded to Z11.3 (encounter for screening for infections with a predominantly sexual mode of transmission). A53.9 requires a confirmed or suspected active infection.
- Check whether the patient is under two years old. If yes, the Excludes1 note bars A53.9; use A50.2 or another congenital syphilis code.
- Look for stage documentation. If the record supports primary (A51.0), secondary (A51.1–A51.49), early (A51.9), or latent syphilis with duration specified (A51.4 or A51.5), use that code instead.
- Check for late-stage or systemic involvement. Neurosyphilis, cardiovascular syphilis, and gummatous disease are coded under A52.–; if these are documented, A52 codes take precedence.
- Confirm the documentation is silent on stage. Only after exhausting the above steps does A53.9 accurately reflect the record.
How Does A53.9 Differ From A53.0 and A51.9?
These three codes are the most commonly confused in the “uncertain syphilis” category. Their differences are clinically significant:
| Code | Description | Key Distinguishing Feature |
|---|---|---|
| A53.9 | Syphilis, unspecified | Stage entirely unknown; no serologic duration or clinical staging possible |
| A53.0 | Latent syphilis, unspecified as early or late | Latent infection confirmed (positive serology, no symptoms) but duration unknown |
| A51.9 | Early syphilis, unspecified | Syphilis confirmed as early (within ~2 years) but sub-type not specified |
| A52.9 | Late syphilis, unspecified | Late-stage infection confirmed but specific manifestation not documented |
The critical distinction: A53.0 requires documentation that the infection is latent (asymptomatic with reactive serology), whereas A53.9 is used when you cannot even determine whether the infection is latent, symptomatic, or staged.
What Documentation Is Required to Support A53.9?
Because A53.9 is an unspecified code, payers and auditors evaluate whether the coder made a genuine, record-supported attempt to assign a specific code before landing here. Inadequate documentation of the reasoning process — not just the diagnosis — is the single most common audit trigger for this code.
What Must the Provider Document in the Clinical Notes?
The following elements are required to support A53.9 in both outpatient and inpatient settings:
- A confirmed or probable syphilis diagnosis stated explicitly in the provider’s assessment or impression — not inferred from lab values alone
- Absence of staging information — the note should not contain language that implies a stage (e.g., “painless ulcer” implies primary; “diffuse rash” implies secondary)
- Reason staging is undetermined, if applicable (e.g., “outside records unavailable,” “patient unable to provide symptom history”)
- Relevant exposure or risk factor documentation to establish medical necessity for serologic testing
- Treatment plan or referral indicating the diagnosis is being acted upon (not simply a reactive screening result awaiting follow-up)
Which Lab Results Support This Code?
Supporting serologic evidence includes any combination of:
- Non-treponemal tests: VDRL (Venereal Disease Research Laboratory) or RPR (Rapid Plasma Reagin) — reactive results confirm infection but do not establish stage
- Treponemal confirmatory tests: FTA-ABS (Fluorescent Treponemal Antibody Absorption) or TP-PA (Treponema pallidum Particle Agglutination) — positive results confirm true infection vs. a false-positive non-treponemal screen
- Titers without historical baseline: If no previous RPR titer is documented, duration cannot be inferred, making A53.9 appropriate when symptoms are also absent
What Is the Documentation Standard for Inpatient vs. Outpatient Settings?
| Setting | Documentation Standard | Key Difference |
|---|---|---|
| Outpatient | Code the confirmed condition or highest degree of certainty supported by the documentation at the time of the encounter | Provider must have documented “syphilis” — coders cannot assign the code based on a positive lab alone |
| Inpatient | Principal diagnosis is the condition found after study to be chiefly responsible for the admission | Coders may query providers to clarify stage before final code assignment; query forms should reference specific staging criteria |
How Does A53.9 Affect Medical Billing and Claims?
From a revenue cycle perspective, A53.9 is a straightforward billable code with no unique LCD (Local Coverage Determination) restrictions at the national level — but payer-specific policies and medical necessity requirements vary. In practice, coders working in STI clinics or infectious disease settings note that claims flagged for A53.9 frequently receive a payer request for medical records, particularly when high-complexity E/M codes are billed alongside it.
Key billing considerations:
- MS-DRG grouping (inpatient): A53.9 as a principal diagnosis maps to MS-DRG 867/868/869, depending on whether a CC (complicating condition) or MCC (major complicating condition) is documented — these DRG weight differences can meaningfully impact inpatient reimbursement
- Medical necessity documentation must reflect why the specific stage could not be determined; a vague “syphilis” notation without clinical context invites downcoding or denial
- Diagnosis coding specificity requirements under the ICD-10-CM Official Coding Guidelines (Section I.B.5) instruct coders to code to the highest level of specificity — use A53.9 only when no more specific code is supportable
What CPT Codes Are Commonly Billed With A53.9?
| CPT Code | Description | Typical Billing Context |
|---|---|---|
| 86592 | Syphilis test, qualitative (RPR, VDRL, ART) | Screening or diagnostic serology |
| 86593 | Syphilis test, quantitative (titer) | Monitoring treatment response |
| 86780 | Treponema pallidum antibody (FTA-ABS, TP-PA) | Confirmatory treponemal testing |
| 99213–99215 | Office/outpatient E/M visit, established patient | Evaluation and management at diagnosis |
| J0490 | Injection, benzathine penicillin G, 600,000 units | Treatment administration at the visit |
| 96372 | Therapeutic/diagnostic injection (IM/SC) | Administration code paired with J0490 |
Are There Any Prior Authorization or Coverage Restrictions?
- Medicare Part B generally covers syphilis serologic testing when medically necessary and documented; no national NCD restricts A53.9 specifically
- Medicaid coverage policies vary by state; many state fee schedules require a specific STI diagnosis for covered treatment injections
- Commercial payers may require documentation of a reactive confirmatory treponemal test before covering treatment injection reimbursement
- FQHC and public health clinic billing often uses encounter-based rates that bundle lab and injection — verify whether individual CPT billing or encounter billing applies in your setting
What Coding Errors Should You Avoid With A53.9?
A53.9 is one of those codes that looks straightforward until an audit surfaces the record. Auditors consistently flag the following patterns on claims carrying this code:
- Using A53.9 when stage is clearly documented. If the provider wrote “patient presents with a chancre — primary syphilis,” coding A53.9 instead of A51.0 is a specificity error that directly conflicts with ICD-10-CM Official Guidelines Section I.B.5.
- Coding A53.9 for a history of syphilis. A resolved infection with no current disease activity should be coded to Z86.13 (personal history of syphilis), not A53.9 — this is one of the most frequently cited errors in STI clinic coding audits.
- Coding A53.9 from a positive lab result alone. Per outpatient coding guidelines, diagnosis codes must be supported by a provider’s clinical statement. A coder cannot assign A53.9 solely because an RPR result is reactive without a corresponding provider diagnosis.
- Omitting the confirmatory treponemal test when both were ordered. When a non-treponemal screen (RPR) and a treponemal confirmatory test (FTA-ABS) are both ordered and reported, both lab CPT codes should be reported; billing only one understates the encounter.
- Applying A53.9 to congenital cases. For patients under two years, the Excludes1 note is absolute; A50.– codes are mandatory.
What Do Auditors Look for When Reviewing Claims With A53.9?
- Mismatch between diagnosis and treatment intensity — a high-complexity E/M visit for an “unspecified” diagnosis without documented rationale raises scrutiny
- Pattern of persistent A53.9 use across multiple encounters for the same patient — if staging remains undetermined after two encounters, auditors may question whether the provider has documented appropriately
- Absent confirmatory lab documentation — if only a non-treponemal test is documented, the specificity of the diagnosis is harder to defend
- A53.9 on an inpatient claim as the principal diagnosis without a query trail showing the coder attempted to clarify stage
How Does A53.9 Relate to Other ICD-10 Codes?
A53.9 sits within a structured coding hierarchy for syphilis that follows the disease’s clinical staging. Understanding where it fits relative to adjacent codes prevents the most common specificity errors.
| Related Code | Description | Relationship to A53.9 |
|---|---|---|
| A50.2 | Congenital syphilis, unspecified (≤2 years) | Excludes1 — never use with A53.9 for patients under 2 |
| A51.0 | Primary genital syphilis | More specific — use when chancre is documented |
| A51.9 | Early syphilis, unspecified | More specific — use when infection is within ~2 years |
| A52.3 | Neurosyphilis, unspecified | More specific — use when neurological involvement documented |
| A53.0 | Latent syphilis, unspecified (unknown duration) | Sibling code — use when infection is confirmed latent |
| Z86.13 | Personal history of syphilis | Use for resolved/treated syphilis — not current infection |
| Z11.3 | Encounter for STI screening | Use for screening visit, not active diagnosis |
| B20 | HIV disease | Code additionally when HIV co-infection is documented |
Per the ICD-10-CM Official Coding Guidelines, syphilis codes do not carry a “use additional code” instruction for HIV at the A53 category level, but coders should follow institutional guidelines and payer rules for sequencing when HIV is a concurrent active diagnosis.
What Is the Correct Code Sequencing When A53.9 Appears With Other Diagnoses?
- Outpatient, syphilis is the reason for the encounter: A53.9 is sequenced as the first-listed diagnosis.
- Inpatient, syphilis discovered incidentally: Sequence the condition chiefly responsible for the admission first; A53.9 is an additional diagnosis.
- HIV co-infection present: Follow ICD-10-CM guidelines for HIV sequencing (B20 is generally sequenced first when the patient has confirmed HIV disease, per Section I.C.1.a).
- Congenital syphilis suspected but not confirmed in an infant under two: Do not assign A53.9; query the provider for confirmation or assign the appropriate A50.– code.
Real-World Coding Scenario — How A53.9 Is Applied in Practice
Scenario: Maria, a 34-year-old established patient, presents to an outpatient infectious disease clinic. She was recently referred from an urgent care center after a routine RPR screen came back reactive at 1:4. She reports no current symptoms and has no documentation of any prior syphilis testing or treatment. The ID physician documents: “Reactive RPR confirmed by TP-PA. Patient has no sores, rash, or neurological symptoms. No history of prior syphilis treatment. Syphilis, stage undetermined — likely latent but duration unknown. Will treat with benzathine penicillin G 2.4 million units IM today.”
Correct Code Application
- A53.0 — Latent syphilis, unspecified as early or late (the provider explicitly documented “likely latent, duration unknown” — this is a latent presentation, not merely “unspecified syphilis”)
- 86592 — RPR, qualitative
- 86780 — Treponema pallidum antibody (TP-PA)
- J0490 — Benzathine penicillin G injection, per unit billed
- 96372 — Administration of IM injection
Common Mistake in This Scenario
- Incorrect code: A53.9 — A coder who defaults to A53.9 because the stage is called “undetermined” overlooks the provider’s statement “likely latent.” Latent with unknown duration = A53.0, not A53.9. This specificity error may trigger a claim adjustment or audit flag.
- Why it fails: A53.9 is reserved for situations where even the latent vs. symptomatic distinction cannot be made. Here, the provider explicitly characterized the presentation as latent.
Frequently Asked Questions About ICD-10 Code A53.9
Is ICD-10 Code A53.9 Still Valid for Use in 2026?
ICD-10 code A53.9 is a valid, billable diagnosis code for FY 2026, effective October 1, 2025 through September 30, 2026, with no changes to its code description or validity status since its introduction in FY 2016. Coders should confirm validity annually against the CMS ICD-10-CM Official Coding Guidelines release, as codes in the syphilis category have historically been stable.
What Is the Difference Between A53.9 and A53.0?
A53.9 is used when syphilis is confirmed but the type, stage, and latency status cannot be determined from the clinical documentation. A53.0 specifically designates latent syphilis — a serologically reactive infection with no clinical symptoms — when the duration of infection is unknown. The critical distinction is that A53.0 requires the provider to have characterized the infection as latent; A53.9 is used when even that determination has not been made.
Can A53.9 Be Assigned Based on a Positive Lab Result Alone?
A53.9 cannot be assigned solely on the basis of a reactive serology result. Per outpatient ICD-10-CM Official Coding Guidelines Section IV.H, diagnosis codes are assigned based on the provider’s documented diagnosis, not independently interpreted by the coder. A reactive RPR must be accompanied by a provider’s clinical statement diagnosing syphilis before this code may be applied.
When Should A53.9 Be Used Instead of Z86.13?
A53.9 is appropriate for an active, current syphilis infection — confirmed or probable — where the stage is undetermined. Z86.13 (personal history of syphilis) applies when the infection has been treated and resolved, with no current active disease. Using A53.9 for a patient with a documented history of treated syphilis and no current infection is a coding error that misrepresents the clinical picture and could support inappropriate medical necessity claims.
Does A53.9 Apply to Congenital Syphilis?
A53.9 does not apply to congenital syphilis or to any syphilis diagnosis in a patient under two years of age. The Tabular List carries a Type 1 Excludes note barring simultaneous use of A53.9 and A50.2, and the applicable A50.– codes must be used for congenital cases. This exclusion is absolute — it cannot be overridden by clinical context.
What CPT Codes Are Typically Paired With A53.9 for Billing?
The most commonly paired CPT codes are 86592 (RPR, qualitative), 86593 (RPR, quantitative titer), and 86780 (treponemal antibody confirmatory test). When treatment is administered at the visit, J0490 (benzathine penicillin G) and 96372 (IM injection administration) are also commonly billed. Evaluation and management codes (99213–99215 for established patients) reflect the clinical encounter and are supported when documented separately from the procedure.
Key Takeaways
Every coder working in an STI clinic, infectious disease practice, or any setting where syphilis diagnoses appear should keep these points front of mind for A53.9:
- A53.9 is a specificity last resort — it is only appropriate when stage, latency, and type are all undetermined from the clinical documentation
- Before assigning A53.9, systematically rule out A51.– (early), A52.– (late), A53.0 (latent, unknown duration), and A50.– (congenital) codes
- A53.9 ≠ history of syphilis — use Z86.13 for resolved, treated infections; using A53.9 for historical cases is a documented audit risk
- The Type 1 Excludes note prohibits A53.9 for patients under two years of age — A50.– codes are mandatory in that population
- Stage documentation in the provider’s note is the controlling factor; when the provider labels the infection as “latent,” A53.0 is correct even if duration is unknown
- Coders should review the ICD-10-CM Official Coding Guidelines annually and consult CDC surveillance guidelines when treating settings see high-volume syphilis coding — both are updated on a regular cycle
- In 2023, over 209,000 total syphilis cases were reported in the U.S. — the highest count since 1950 — making precise syphilis coding a meaningful contributor to public health surveillance data accuracy, not just a billing technicality
For authoritative coding guidance, reference the CMS ICD-10-CM Official Coding Guidelines published annually by the Centers for Medicare & Medicaid Services, the CDC STI Surveillance Report for epidemiological context, and the AHA Coding Clinic for any Coding Clinic advisories related to syphilis or STI coding.