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G20.A1 ICD-10-CM Code: Parkinson's disease without dyskinesia, without mention of fluctuations

G20.A1 maps to CMS-HCC V28 199 (RAF 0.615). Documentation must support MEAT. MEAT criteria · RAF calculator · free HCC coding tools

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

G20.A1

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

Parkinson's disease without dyskinesia, without mention of fluctuations

Parkinson's disease without involuntary movements (dyskinesia) and without motor fluctuations, a progressive neurological disorder affecting movement and muscle control.

Buddy the Bee presenting code insight

Buddy Insight

Parkinson's disease without dyskinesia and without fluctuations represents the earliest or most well-controlled stage of the disease, where motor symptoms respond consistently to medication without complications.

CMS-HCC V28

HCC 199

RAF 0.615

CMS-HCC V24

HCC 78

RAF 0.606

ACA/HHS

HCC 119

Varies by metal level

ESRD/PACE

HCC 78

RAF 0.079

RXHCC

HCC 161

RAF 0.549

Code Book Path

Official
G20Parkinson's disease
G20.AParkinson's disease without dyskinesia
G20.A1Parkinson's disease without dyskinesia, without mention of fluctuations

Inclusion Terms

Official
  • Parkinson's disease NOS
  • Parkinson's disease without dyskinesia, without mention of OFF episodes

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for G20.A1 in this effective period.

Related Child Codes

Official
G20.A2Parkinson's disease without dyskinesia, with fluctuations

Includes

Official

ICD-10-CM does not list Includes notes for G20.A1 in this effective period.

Excludes 1

Official

ICD-10-CM does not list Excludes 1 notes for G20.A1 in this effective period.

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for G20.A1 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for G20.A1 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for G20.A1 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance
Confirmed diagnosis of Parkinson's disease (not parkinsonism or secondary causes)
Explicit documentation that dyskinesia is absent
Explicit documentation that motor fluctuations are absent
Current medication regimen (levodopa, dopamine agonists, etc.)

MEAT Support

HCC Buddy guidance
Confirmed diagnosis of Parkinson's disease (not parkinsonism or secondary causes)
Explicit documentation that dyskinesia is absent
Explicit documentation that motor fluctuations are absent
Current medication regimen (levodopa, dopamine agonists, etc.)

Audit Caution

HCC Buddy guidance
Using the unspecified G20.C when documentation supports a specific dyskinesia/fluctuation status code
Failing to update the code when patients develop dyskinesia or fluctuations over time
Confusing Parkinson's disease with secondary parkinsonism from medications or other causes
Not documenting dyskinesia and fluctuation status at each visit, making specific code assignment impossible

Common Mistakes

HCC Buddy guidance
G20.A2 — Parkinson's disease without dyskinesia, with fluctuations: has motor fluctuations which G20.A1 does not
G20.B1 — Parkinson's disease with dyskinesia, without fluctuations: has dyskinesia which G20.A1 does not
G20.C — Parkinsonism, unspecified: less specific and should not be used when Parkinson's disease is confirmed
G21.11 — Neuroleptic induced parkinsonism: secondary cause, not idiopathic Parkinson's disease

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 G20.A1 an HCC code?

Yes. G20.A1 (Parkinson's disease without dyskinesia, without mention of fluctuations) maps to Parkinson and Other Degenerative Disease of Basal Ganglia under the CMS-HCC V28 risk adjustment model (and Parkinson's and Huntington's Diseases under V24), with a community non-dual aged RAF of 0.615. It is billable for payment year 2026.

Coder answer: G20.A1 is billable and maps to V28 HCC 199, Parkinson and Other Degenerative Disease of Basal Ganglia. Open it in the Code Book for the tabular path, RAF, and MEAT checklist.

Code
G20.A1
Description
Parkinson's disease without dyskinesia, without mention of fluctuations
HCC (V28)
HCC 199 — Parkinson and Other Degenerative Disease of Basal Ganglia
RAF
0.615
Billable
Yes
Payment year
2026

HCC Category Mapping

V28HCC 199, Parkinson and Other Degenerative Disease of Basal Ganglia
0.615
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 G20.A1 in the Code Book — tabular path, V28 RAF, and MEAT checklist →

MEAT Criteria for G20.A1

For G20.A1 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.A1 during that encounter, not just copy-forwarded from a problem list.

Coder workflow notes

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What This Code Means

G20.A1 is the ICD-10-CM diagnosis code for parkinson's disease without dyskinesia, without mention of fluctuations. Parkinson's disease without involuntary movements (dyskinesia) and without motor fluctuations, a progressive neurological disorder affecting movement and muscle control. G20.A1 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.A1 maps to Parkinson and Other Degenerative Disease of Basal Ganglia (HCC 199) with a community, non-dual, aged base RAF weight of 0.615. Under the older CMS-HCC V24 model, G20.A1 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.

Use this code when dyskinesia and fluctuations are explicitly documented as absent. Because G20.A1 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.A1 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

  • •Use this code when dyskinesia and fluctuations are explicitly documented as absent
  • •Distinguish between A1 (no fluctuations) and A2 (with fluctuations) based on clinical documentation

Clinical Significance

Parkinson's disease without dyskinesia and without fluctuations represents the earliest or most well-controlled stage of the disease, where motor symptoms respond consistently to medication without complications. This specific code captures treatment response status, which is essential for accurate risk stratification and tracking disease progression over time.

Documentation Requirements

  • ✓Confirmed diagnosis of Parkinson's disease (not parkinsonism or secondary causes)
  • ✓Explicit documentation that dyskinesia is absent
  • ✓Explicit documentation that motor fluctuations are absent
  • ✓Current medication regimen (levodopa, dopamine agonists, etc.)
  • ✓Motor examination findings (bradykinesia, rigidity, tremor)
  • ✓Functional status and disease stage if documented

Commonly Confused Codes

  • •G20.A2: Parkinson's disease without dyskinesia, with fluctuations: has motor fluctuations which G20.A1 does not
  • •G20.B1: Parkinson's disease with dyskinesia, without fluctuations: has dyskinesia which G20.A1 does not
  • •G20.C: Parkinsonism, unspecified: less specific and should not be used when Parkinson's disease is confirmed
  • •G21.11: Neuroleptic induced parkinsonism: secondary cause, not idiopathic Parkinson's disease

Child Codes

G20.A1Parkinson's disease without dyskinesia, without mention of fluctuations
HCC 199
G20.A2Parkinson's disease without dyskinesia, with fluctuations
HCC 199

Code Hierarchy

└G20Parkinson's disease└G20.AParkinson's disease without dyskinesia└G20.A1Parkinson's disease without dyskinesia, without mention of fluctuations
└G20.A1Parkinson's disease without dyskinesia, without mention of fluctuations

G20.A1 code history

Code setChange
FY2024 (effective Oct 1, 2023)Added to the code set

Source: official CMS ICD-10-CM order and addenda files, FY2016 through FY2027.

Because G20.A1 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.

G20.A1 maps to CMS-HCC V28 category 199, Parkinson and Other Degenerative Disease of Basal Ganglia. See the ICD-10 to HCC mapping hub for how the V28 crosswalk works. Because G20.A1 carries a payment HCC, you can see what it adds to a RAF score and check the documentation the chart needs before it is submitted.

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