What is ICD-10 code G20.C?

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. It belongs to the ICD-10-CM chapter for diseases of the nervous system (g00-g99). G20.C is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. G20.C 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. G20.C 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. G20.C 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.

Which HCC does G20.C map to under V28?

G20.C maps to CMS-HCC V28 HCC 199 (Parkinson and Other Degenerative Disease of Basal Ganglia). The category numbers, labels, and conditions come from the CMS-HCC V28 model file for payment year 2026.

2026 ICD-10-CM G20.C: Parkinsonism, unspecified

G20.C maps to CMS-HCC V28 199. A source-labeled RAF reference is available. Confirm the documented diagnosis and applicable coding requirements. MEAT criteria · RAF Calculator · HCC Buddy coding tools

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Code lookupG20.C

Effective periodFY2026 Apr Update (Apr 1-Sep 30)

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

G20.C

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

Parkinsonism, unspecified

A movement disorder characterized by tremors, stiffness, and slow movement, but the specific cause is not identified or documented.

Buddy Insight

Parkinsonism, unspecified is a nonspecific code for parkinsonian symptoms when the cause or specific disease type has not been determined.

Overview Risk Models Code Book Documentation Audit Notes Edition comparison

CMS-HCC V28

Mapped

HCC 199

Parkinson and Other Degenerat...

Code-level coefficient reference

CMS-HCC V24

Historical

Not used for standard CY2026 Medicare Advantage payment. Select a historical period to review V24.

Not used for CY2026 payment

ACA/HHS

Mapped

HCC 119

HHS-HCC 119 adult, RAF varies...

Code-level coefficient reference

ESRD/PACE

Mapped

HCC 78

Parkinsons and Huntingtons Di...

Code-level coefficient reference

RXHCC

Mapped

HCC 161

Parkinson Disease

Code-level coefficient reference

Code Book Path

Official

G20Parkinson's disease G20.CParkinsonism, unspecified

Inclusion Terms

Official

Parkinsonism, NOS
HemiparkinsonismInherited from G20
Idiopathic Parkinsonism or Parkinson's diseaseInherited from G20
Paralysis agitansInherited from G20
Primary Parkinsonism or Parkinson's diseaseInherited from G20

Excludes 2

Official

certain conditions originating in the perinatal period (P04-P96)Inherited from G00-G99
certain infectious and parasitic diseases (A00-B99)Inherited from G00-G99
complications of pregnancy, childbirth and the puerperium (O00-O9A)Inherited from G00-G99
congenital malformations, deformations, and chromosomal abnormalities (Q00-Q99)Inherited from G00-G99
endocrine, nutritional and metabolic diseases (E00-E88)Inherited from G00-G99
injury, poisoning and certain other consequences of external causes (S00-T88)Inherited from G00-G99
neoplasms (C00-D49)Inherited from G00-G99
symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified (R00-R94)Inherited from G00-G99

Related Codes

Official

G20.AParkinson's disease without dyskinesia G20.BParkinson's disease with dyskinesia

Includes

Official

No Includes notes are included in this display for G20.C. Check the code and parent instructions in the Code Book.

Excludes 1

Official

Parkinson's disease NOS (G20.A1)
Parkinson's disease with dyskinesia (G20.B-)
Parkinson's disease without dyskinesia (G20.A-)
secondary parkinsonism (G21.-)

Code First

Official

No Code First sequencing instructions are included in this display for G20.C. Check the code and parent instructions in the Code Book.

Use Additional

Official

code, if applicable, to identify:Inherited from G20
dementia with anxiety (F02.84, F02.A4, F02.B4, F02.C4)Inherited from G20
dementia with behavioral disturbance (F02.81-, F02.A1-, F02.B1-, F02.C1-)Inherited from G20
dementia with mood disturbance (F02.83, F02.A3, F02.B3, F02.C3)Inherited from G20
dementia with psychotic disturbance (F02.82, F02.A2, F02.B2, F02.C2)Inherited from G20
dementia without behavioral disturbance (F02.80, F02.A0, F02.B0, F02.C0)Inherited from G20
mild neurocognitive disorder due to known physiological condition (F06.7-)Inherited from G20

Code Also

Official

No Code Also instructions are included in this display for G20.C. Check the code and parent instructions in the Code Book.

Buddy Documentation Tip

HCC Buddy guidance

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

MEAT Support

HCC Buddy guidance

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

Audit Caution

HCC Buddy guidance

Continuing to use this unspecified code after a specific Parkinson's disease or secondary parkinsonism diagnosis is established

Not querying the provider about which specific parkinsonism code is appropriate based on diagnostic results

Using this code for essential tremor which is a different condition entirely

Failing to update the code once the dyskinesia and fluctuation status is documented for Parkinson's disease

Common Mistakes

HCC Buddy guidance

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

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.C an HCC code?

Yes. G20.C (Parkinsonism, unspecified) maps to HCC 199, Parkinson and Other Degenerative Disease of Basal Ganglia under the CMS-HCC V28 risk adjustment model, with a source-labeled community non-dual aged reference coefficient of 0.615. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes. It is billable for payment year 2026.

Coder answer: G20.C 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.C
Description: Parkinsonism, unspecified
HCC (V28): HCC 199 — Parkinson and Other Degenerative Disease of Basal Ganglia
RAF reference coefficient: 0.615
Billable: Yes
Payment year: 2026

HCC Category Mapping

V28HCC 199, Parkinson and Other Degenerative Disease of Basal Ganglia

0.615

ESRDHCC 78, Parkinsons and Huntingtons Diseases

Not separately weighted

RxHCCHCC 161, Parkinson Disease

Not separately weighted

These are source-labeled model coefficients, not member totals. A category may still be removed by hierarchy or model cleanup rules. 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 context, hierarchy and cleanup rules, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains available for historical review.

Work G20.C in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →

MEAT review for G20.C

For G20.C, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule.

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

Coder workflow notes

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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 Other Degenerative Disease of Basal Ganglia (HCC 199) with a source-labeled community, non-dual, aged reference coefficient of 0.615. No V24 mapping is shown for G20.C; use the applicable model and payment year when reviewing the V28 mapping. For CY2026 non-PACE Medicare Advantage, CMS uses 100% of the 2024 CMS-HCC model (V28). PACE uses a separate model blend. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes.

Use this code only when the type of parkinsonism cannot be determined or is not specified in the clinical documentation. For G20.C, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule. 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, source-labeled coefficient references, 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

•Use this code only when the type of parkinsonism cannot be determined or is not specified in the clinical documentation
•Review documentation carefully to see if a more specific parkinsonism code (G20 or G21 subcategories) can be assigned instead

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

Parkinson's disease NOS (G20.A1)
Parkinson's disease with dyskinesia (G20.B-)
Parkinson's disease without dyskinesia (G20.A-)
secondary parkinsonism (G21.-)

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

Child Codes

G20.AParkinson's disease without dyskinesia

G20.BParkinson's disease with dyskinesia

G20.CParkinsonism, unspecified

HCC 199

Code Hierarchy

└G20Parkinson's disease └G20.CParkinsonism, unspecified

└G20.CParkinsonism, unspecified

HCC Buddy Code Card

G20.C

Is G20.C an HCC code?

HCC Category Mapping

MEAT review for G20.C

Get the V28 mapping + MEAT cheat sheet

What This Code Means

Coding Tips

Clinical Significance

Documentation Requirements

Excludes 1, Do NOT code together

Commonly Confused Codes

Child Codes

Code Hierarchy

G20.C code history

Work G20.C in HCC Buddy

G20.C ICD-10-CM Code: Parkinsonism, unspecified

HCC Buddy Code Card

G20.C

Is G20.C an HCC code?

HCC Category Mapping

MEAT review for G20.C

Get the V28 mapping + MEAT cheat sheet

What This Code Means

Coding Tips

Clinical Significance

Documentation Requirements

Excludes 1, Do NOT code together

Commonly Confused Codes

Child Codes

Code Hierarchy

G20.C code history

Work G20.C in HCC Buddy