G21.8 ICD-10-CM Code: Other secondary parkinsonism
G21.8 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 · 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)
G21.8
Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidanceOther secondary parkinsonism
Parkinson-like symptoms caused by a known secondary cause other than those specifically classified in other G21 subcategories.
Buddy Insight
Other secondary parkinsonism captures parkinsonian symptoms from identified causes that do not fit other G21 subcategories, such as normal pressure hydrocephalus-associated parkinsonism, parkinsonism from metabolic disorders, or parkinsonism associated with structural brain lesions.
CMS-HCC V28
MappedHCC 199
Coefficient HCC 199: 0.615 (Community Non-Dual Aged (CNA))
CMS-HCC V24
HistoricalHistorical
Not used for CY2026 payment
ACA/HHS
MappedHCC 119
Code-level coefficient reference
ESRD/PACE
MappedHCC 78
Code-level coefficient reference
RXHCC
MappedHCC 161
Code-level coefficient reference
Code Book Path
Inclusion Terms
OfficialNo inclusion terms are included in this display for G21.8. Check the code and parent instructions in the Code Book.
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
Includes
OfficialNo Includes notes are included in this display for G21.8. Check the code and parent instructions in the Code Book.
Excludes 1
Official- Huntington's disease (G10)Inherited from G21
- neurocognitive disorder with Lewy bodies (G31.83)Inherited from G21
- Shy-Drager syndrome (G90.3)Inherited from G21
- syphilitic Parkinsonism (A52.19)Inherited from G21
Code First
OfficialNo Code First sequencing instructions are included in this display for G21.8. Check the code and parent instructions in the Code Book.
Use Additional
OfficialNo Use Additional Code instructions are included in this display for G21.8. Check the code and parent instructions in the Code Book.
Code Also
OfficialNo Code Also instructions are included in this display for G21.8. Check the code and parent instructions in the Code Book.
Buddy Documentation Tip
MEAT Support
Audit Caution
Common Mistakes
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 G21.8 an HCC code?
Yes. G21.8 (Other secondary parkinsonism) 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: G21.8 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
- G21.8
- Description
- Other secondary parkinsonism
- 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
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 G21.8 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →
MEAT review for G21.8
For G21.8, 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
G21.8 is the ICD-10-CM diagnosis code for other secondary parkinsonism. Parkinson-like symptoms caused by a known secondary cause other than those specifically classified in other G21 subcategories. G21.8 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, G21.8 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. 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 when a secondary cause is identified but does not fit into G21.0-G21.4 categories. For G21.8, 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 G21.8 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
Clinical Significance
Other secondary parkinsonism captures parkinsonian symptoms from identified causes that do not fit other G21 subcategories, such as normal pressure hydrocephalus-associated parkinsonism, parkinsonism from metabolic disorders, or parkinsonism associated with structural brain lesions. Identifying the secondary cause directs treatment toward the underlying condition.
Documentation Requirements
- ✓Specific secondary cause of parkinsonism identified and documented
- ✓Parkinsonian features on neurological examination
- ✓Documentation that the cause does not fit G21.0-G21.4 categories
- ✓Underlying condition coded separately as appropriate
- ✓Causal relationship between the underlying condition and parkinsonism stated by provider
- ✓Response or lack of response to anti-parkinsonian medications
Commonly Confused Codes
- •G21.9: Secondary parkinsonism, unspecified: use only when the secondary cause is truly unknown
- •G21.4: Vascular parkinsonism: use specifically for cerebrovascular disease-related parkinsonism
- •G21.11: Neuroleptic induced parkinsonism: use specifically for antipsychotic-caused parkinsonism
- •G20.C: Parkinsonism, unspecified: does not capture the identified secondary nature