G21.8
BillableOther secondary parkinsonism
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is G21.8 an HCC code?
Yes. G21.8 maps to Parkinson and Huntington Diseases under the CMS-HCC V28 risk adjustment model (and Parkinson's and Huntington's Diseases under V24).
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 G21.8
For G21.8 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 G21.8 during that encounter — not just copy-forwarded from a problem list.
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 Huntington Diseases (HCC 199) with a community, non-dual, aged base RAF weight of 0.000. Under the older CMS-HCC V24 model, G21.8 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.
Use this code when a secondary cause is identified but does not fit into G21.0-G21.4 categories. Because G21.8 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 G21.8 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