Question 1

What is ICD-10 code A51.1?

Accepted Answer

A51.1 is the ICD-10-CM diagnosis code for primary anal syphilis. This is a sexually transmitted infection caused by bacteria that affects the anal area during the earliest stage of syphilis. It typically presents as a painless sore or ulcer at the site of infection. A51.1 sits in the ICD-10-CM chapter for certain infectious and parasitic diseases (a00-b99), within the section covering infections with a predominantly sexual mode of transmission (a50-a64).

A51.1 is a billable ICD-10-CM code but does not map to a payment HCC under the CMS-HCC V28, V24, ESRD, or RxHCC risk adjustment models. It can be reported on Medicare Advantage encounter data submissions but it does not contribute to a beneficiary's RAF score and therefore does not affect risk-adjusted payments to the plan.

Ensure documentation clearly indicates primary syphilis (first stage) with anal location to distinguish from secondary or latent syphilis.

Question 2

Is A51.1 an HCC code?

Accepted Answer

No. A51.1 is a billable ICD-10-CM code but does not map to any payment HCC category under the CMS-HCC V28, V24, ESRD, or RxHCC risk adjustment models. It can still be reported on Medicare Advantage encounter data submissions, but it does not contribute to the beneficiary's Risk Adjustment Factor (RAF) score and therefore does not affect the risk-adjusted capitation payment to the plan. V28 reached 100% phase-in for payment year 2026.

Question 3

Is A51.1 billable?

Accepted Answer

Yes. A51.1 is a billable ICD-10-CM code that can be reported on professional and institutional claims as a primary or secondary diagnosis when the provider documentation supports the specific condition described. Billable codes in ICD-10-CM are coded to the highest level of specificity available for the documented diagnosis. A51.1 is current for fiscal year 2026 and remains in the active CMS ICD-10-CM code set published by the National Center for Health Statistics.

Question 4

What is the V28 vs V24 difference for A51.1?

Accepted Answer

A51.1 does not map to a payment HCC under either the CMS-HCC V28 risk adjustment model or the older V24 model. V28 reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition. Because neither model rewards A51.1 with a risk adjustment factor, it contributes zero to a Medicare Advantage beneficiary's RAF score in either version. Coders should check whether provider documentation supports a more specific child code that does map to a payment HCC.

Question 5

What should coders know when reporting A51.1?

Accepted Answer

Ensure documentation clearly indicates primary syphilis (first stage) with anal location to distinguish from secondary or latent syphilis Verify that the patient has been tested and confirmed positive for syphilis, and document any sexual contact history or partner notification attempts as this is a reportable disease

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A51.1

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A51.1 ICD-10-CM Code: Primary anal syphilis

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A51.1

Is A51.1 an HCC code?

What This Code Means

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Child Codes

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