A50.55
BillableLate congenital syphilitic arthropathy
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is A50.55 an HCC code?
Yes. A50.55 maps to Bone/Joint/Muscle Infections/Necrosis under the CMS-HCC V28 risk adjustment model (and Bone/Joint/Muscle Infections/Necrosis under V24).
HCC Category Mapping
RAF weights shown are the community, non-dual, aged base weights from the CMS risk adjustment model file. Actual per-patient RAF contribution depends on member segment, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains in use during the transition and for historical data.
MEAT Criteria for A50.55
For A50.55 to count as a valid HCC diagnosis in a given encounter, the provider's documentation must show MEAT: Monitor, Evaluate, Assess, or Treat. A diagnosis from a prior year does not carry forward automatically — it has to be re-documented and supported each calendar year.
- MMonitor: signs, symptoms, disease progression, or lab trending documented in the note
- EEvaluate: test results, medication response, or physical findings reviewed by the provider
- AAssess: explicit mention in the assessment or plan with acknowledgment of status
- TTreat: medication, referral, procedure, therapy, or counseling tied to the diagnosis
Only one of M/E/A/T is required to support the code, but the documentation must be specific enough to show that the provider actually addressed A50.55 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
A50.55 is the ICD-10-CM diagnosis code for late congenital syphilitic arthropathy. This is a joint disease that develops in children born with syphilis, occurring years after birth as a late complication of the infection passed from mother to child. The condition causes inflammation and damage to the joints, typically affecting the knees and other large joints. A50.55 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).
Under the CMS-HCC V28 risk adjustment model, A50.55 maps to Bone/Joint/Muscle Infections/Necrosis (HCC 92) with a community, non-dual, aged base RAF weight of 0.209. Under the older V24 model, A50.55 mapped to the same category but with a base RAF weight of 0.482 — V28 recalibrated weights across the entire model. V28 is the CMS-HCC risk adjustment model that reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition.
This code requires documentation confirming late congenital syphilis with arthropathy involvement; verify the patient has a history of congenital syphilis and current joint manifestations. Because A50.55 maps to a payment HCC, the provider's documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's Medicare Advantage risk adjustment score. When documentation is ambiguous, coders should issue a provider query rather than assume the highest-specificity variant.
HCC Buddy maintains structured V28 and V24 mapping, RAF weights, and MEAT documentation criteria for A50.55 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This code requires documentation confirming late congenital syphilis with arthropathy involvement; verify the patient has a history of congenital syphilis and current joint manifestations
- •Do not use this code for acquired syphilis or early congenital syphilis; ensure the clinical record specifies this is a late manifestation occurring after age 2 years
Clinical Significance
Late congenital syphilitic arthropathy represents a chronic manifestation of untreated congenital syphilis that develops years after birth, indicating ongoing infectious disease burden requiring long-term management. This condition reflects both the severity of the original congenital infection and the complexity of managing its late sequelae.
Documentation Requirements
- ✓Confirmed history of congenital syphilis (maternal transmission documented)
- ✓Onset after age 2 years (distinguishing late from early congenital syphilis)
- ✓Specific joint(s) involved with clinical findings (swelling, effusion, limitation of motion)
- ✓Serological confirmation (positive treponemal tests such as FTA-ABS)
- ✓Current treatment plan and disease activity status