G20.C
BillableParkinsonism, unspecified
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is G20.C an HCC code?
Yes. G20.C 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 G20.C
For G20.C 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 G20.C during that encounter — not just copy-forwarded from a problem list.
What This Code Means
G20.C is the ICD-10-CM diagnosis code for parkinsonism, unspecified. A movement disorder characterized by tremors, stiffness, and slow movement, but the specific cause is not identified or documented. G20.C 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, G20.C 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, G20.C 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.
Use this code only when the type of parkinsonism cannot be determined or is not specified in the clinical documentation. Because G20.C 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 G20.C sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
Clinical Significance
Parkinsonism, unspecified is a nonspecific code for parkinsonian symptoms when the cause or specific disease type has not been determined. This code should ideally be temporary, used during the diagnostic workup phase before a more specific diagnosis is established. It still signals significant neurological disease requiring specialist evaluation.
Documentation Requirements
- ✓Parkinsonian features documented (tremor, bradykinesia, rigidity, postural instability)
- ✓Statement that specific parkinsonism type has not been determined
- ✓Diagnostic workup in progress or explanation of why diagnosis is uncertain
- ✓Current symptom severity and functional impact
- ✓Medication trials and responses if applicable
- ✓Planned diagnostic testing or specialist referral
Excludes 1 — Do NOT code together
Commonly Confused Codes
- •G20.A1 — Parkinson's disease without dyskinesia, without fluctuations: use when idiopathic Parkinson's is confirmed
- •G21.11 — Neuroleptic induced parkinsonism: use when drug-induced cause is identified
- •G21.4 — Vascular parkinsonism: use when cerebrovascular disease is the identified cause
- •R25.1 — Tremor, unspecified: symptom code, even less specific than parkinsonism diagnosis