What is ICD-10 code G21.2?

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. It belongs to the ICD-10-CM chapter for diseases of the nervous system (g00-g99). G21.2 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. G21.2 is part of the FY2026 ICD-10-CM code set published by the CMS National Center for Health Statistics and used on Medicare, Medicaid, and commercial claims in the United States.

Yes. G21.2 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. G21.2 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.

What is the V28 vs V24 difference for G21.2?

CMS phased in the V28 risk adjustment model over payment years 2024 (33%), 2025 (67%), and 2026 (100%), replacing V24. V28 consolidates HCC categories, removes some lower-acuity conditions, and recalibrates RAF weights across the entire model. G21.2 was a payment HCC under V24 (Parkinson's and Huntington's Diseases) but the V28 recalibration removed it from the risk adjustment model. Both models can still appear in legacy data and in payer-specific lookback periods.

What should coders know when reporting G21.2?

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 Because G21.2 maps to a payment HCC, the documentation must also satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's risk adjustment score.

2026 ICD-10-CM G21.2: Secondary parkinsonism due

ICD-10-CM Code View

HCC Buddy Code Card

Digital ICD-10 code-book layout with official code detail, always-visible risk models, Code Trumping, and Buddy coding guidance.

Code lookupG21.2Effective periodFY2026 Apr Update (Apr 1-Sep 30)

FY 2026 Apr update / Diseases of the nervous system (G00-G99) / Extrapyramidal and movement disorders (G20-G26)

G21.2

Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidance

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.

Buddy Insight

Secondary parkinsonism due to other external agents captures parkinsonian symptoms caused by environmental toxin exposure such as manganese, carbon monoxide, carbon disulfide, or pesticides.

CMS-HCC V28

N/A

—

Not risk-adjusted for this model/period.

Not mapped

CMS-HCC V24

Mapped

HCC 78

Parkinson's and Huntington's ...

RAF 0.606

ACA/HHS

Mapped

HCC 119

HHS-HCC 119

Varies by metal level

ESRD/PACE

Mapped

HCC 78

Parkinson's and Huntington's ...

RAF 0.079

RXHCC

Mapped

HCC 161

Parkinson's Disease and Other...

RAF 0.549

Code Book Path

Official

G21Secondary parkinsonism

G21.2Secondary parkinsonism due to other external agents

Inclusion Terms

Official

ICD-10-CM does not list inclusion terms for G21.2 in this effective period.

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for G21.2 in this effective period.

Related Child Codes

Official

G21.0Malignant neuroleptic syndrome

G21.1Other drug-induced secondary parkinsonism

G21.3Postencephalitic parkinsonism

G21.4Vascular parkinsonism

G21.8Other secondary parkinsonism

Includes

Official

ICD-10-CM does not list Includes notes for G21.2 in this effective period.

Excludes 1

Official

Huntington's disease (G10)
neurocognitive disorder with Lewy bodies (G31.83)
Shy-Drager syndrome (G90.3)
syphilitic Parkinsonism (A52.19)

Code First

Official

(T51-T65) to identify external agent

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for G21.2 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for G21.2 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance

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

MEAT Support

HCC Buddy guidance

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

Audit Caution

HCC Buddy guidance

Using idiopathic Parkinson's disease codes when an environmental exposure has been identified as the cause

Confusing drug-induced parkinsonism (G21.11/G21.19) with toxin-induced parkinsonism (G21.2)

Not coding the external cause or toxic effect code alongside the parkinsonism

Failing to obtain occupational exposure history that could identify the causative agent

Common Mistakes

HCC Buddy guidance

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

Current with CMS: FY2026 ICD-10-CM Apr 1 update (effective Apr 1 – Sep 30, 2026) · CMS-HCC V28, 100% phased in for payment year 2026. FY2027 code set already staged for October 1, 2026. How HCC Buddy stays current →

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.

Code: G21.2
Description: Secondary parkinsonism due to other external agents
HCC (V28): No CMS-HCC V28 mapping
RAF: —
Billable: Yes
Payment year: 2026

HCC Category Mapping

V24HCC 78, Parkinson's and Huntington's Diseases

0.606

ESRDHCC 78, Parkinson's and Huntington's Diseases

0.079

RxHCCHCC 161, Parkinson's Disease and Other Movement Disorders

0.549

Each model's RAF is its CMS base weight for that model's standard population, so weights are not directly comparable across models: CMS-HCC V28 and V24 use Community, Non-Dual, Aged; ESRD uses the dialysis continuing-enrollee model; RxHCC is the Part D continuing-enrollee, non-low-income, aged weight (a larger scale than CMS-HCC). ACA/HHS has no single weight — it varies by metal level. 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.

Work G21.2 in the Code Book — tabular path, V28 RAF, and MEAT checklist →

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.

Coder workflow notes

Get the V28 RAF + MEAT cheat sheet

One printable page: confirm a code's V28 HCC status, its RAF weight, and the MEAT your note needs to make it stick. Free, no card.

Free PDF. No card. Unsubscribe anytime.

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 0.606. 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

Code First

(T51-T65) to identify external agent

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