G21.9
BillableSecondary parkinsonism, unspecified
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is G21.9 an HCC code?
Yes. G21.9 maps to Parkinson and Huntington Diseases under the CMS-HCC V28 risk adjustment model (and Parkinson's and Huntington's Diseases 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 G21.9
For G21.9 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 G21.9 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
G21.9 is the ICD-10-CM diagnosis code for secondary parkinsonism, unspecified. Parkinson-like symptoms known to be caused by a secondary cause, but the specific cause has not been identified or documented. G21.9 sits in the ICD-10-CM chapter for diseases of the nervous system (g00-g99), within the section covering extrapyramidal and movement disorders (g20-g26).
Under the CMS-HCC V28 risk adjustment model, G21.9 maps to Parkinson and Huntington Diseases (HCC 199) with a community, non-dual, aged base RAF weight of 0.000. Under the older CMS-HCC V24 model, G21.9 maps to Parkinson's and Huntington's Diseases (HCC 78) with a community, non-dual, aged base RAF weight of 0.584. 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 is a less specific code; attempt to identify the underlying cause and use a more specific G21 code if possible. Because G21.9 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 G21.9 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This is a less specific code; attempt to identify the underlying cause and use a more specific G21 code if possible
- •Review clinical documentation and diagnostic workup to determine if a more specific secondary parkinsonism code applies
Clinical Significance
Secondary parkinsonism, unspecified indicates parkinsonian symptoms known to be secondary to some cause, but the specific cause has not been identified or documented. This code is less desirable than specific secondary parkinsonism codes and may indicate an incomplete diagnostic evaluation that warrants provider query.
Documentation Requirements
- ✓Parkinsonian features documented on examination (tremor, bradykinesia, rigidity)
- ✓Documentation that parkinsonism is believed to be secondary (not idiopathic Parkinson's disease)
- ✓Explanation of why the specific secondary cause cannot be determined
- ✓Diagnostic workup performed or planned to identify the cause
- ✓Current functional status and symptom severity
- ✓Treatment approach and response
Commonly Confused Codes
- •G20.C — Parkinsonism, unspecified: does not specify whether primary or secondary
- •G21.8 — Other secondary parkinsonism: use when a specific secondary cause is known
- •G21.4 — Vascular parkinsonism: use when cerebrovascular disease is the identified cause
- •G21.11 — Neuroleptic induced parkinsonism: use when antipsychotic medication is the cause