G21.2
BillableSecondary parkinsonism due to other external agents
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is G21.2 an HCC code?
Yes. G21.2 maps to Parkinson's and Huntington's Diseases under the V24 model but is not retained in V28.
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 G21.2
For G21.2 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 G21.2 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
G21.2 is the ICD-10-CM diagnosis code for secondary parkinsonism due to other external agents. Parkinson-like symptoms caused by exposure to external agents such as toxins, chemicals, or environmental substances rather than medications. G21.2 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 older CMS-HCC V24 model, G21.2 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.
Document the specific external agent (e.g., carbon monoxide, manganese, pesticides) in the clinical record. Because G21.2 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 G21.2 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Document the specific external agent (e.g., carbon monoxide, manganese, pesticides) in the clinical record
- •May require coding an additional code for the toxic exposure or environmental cause
Clinical Significance
Secondary parkinsonism due to other external agents captures parkinsonian symptoms caused by environmental toxin exposure such as manganese, carbon monoxide, carbon disulfide, or pesticides. This diagnosis has occupational health and medicolegal implications and guides treatment decisions since removing the exposure may slow or halt progression.
Documentation Requirements
- ✓Specific external agent or toxin identified (manganese, carbon monoxide, pesticides, etc.)
- ✓Documentation of exposure history and circumstances
- ✓Temporal relationship between exposure and symptom development
- ✓Parkinsonian features on neurological examination
- ✓Toxicology testing or exposure biomarkers if available
- ✓Occupational or environmental exposure history
Commonly Confused Codes
- •G21.19 — Other drug induced secondary parkinsonism: for medication-caused parkinsonism, not environmental toxins
- •G21.11 — Neuroleptic induced parkinsonism: specifically for antipsychotic medication effects
- •G20.A1-G20.C — Parkinson's disease: idiopathic with no identified external cause
- •T57.2X1A — Toxic effect of manganese, accidental: captures the poisoning event, not the resulting parkinsonism