G31.9 ICD-10-CM Code: Degenerative disease of nervous system, unspecified
HCC Buddy Code Card
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FY 2026 Apr update / Diseases of the nervous system (G00-G99) / Other degenerative diseases of the nervous system (G30-G32)
G31.9
Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidanceDegenerative disease of nervous system, unspecified
This code is used when a patient has a degenerative disease of the nervous system but the specific type has not been determined or documented.
Buddy Insight
Degenerative disease of the nervous system, unspecified, is used when a patient has a confirmed degenerative neurological disease but the specific type has not been determined.
CMS-HCC V28
00
RAF 0
CMS-HCC V24
00
RAF 0
ACA/HHS
00
RAF 0
ESRD/PACE
00
RAF 0
RXHCC
MappedHCC 112
RAF 0.0
Code Trumping
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Code Book Path
Inclusion Terms
OfficialICD-10-CM does not list inclusion terms for G31.9 in this effective period.
Excludes 2
Official- Reye's syndrome (G93.7)
Related Child Codes
Includes
OfficialICD-10-CM does not list Includes notes for G31.9 in this effective period.
Excludes 1
OfficialICD-10-CM does not list Excludes 1 notes for G31.9 in this effective period.
Code First
OfficialICD-10-CM does not list Code First sequencing instructions for G31.9 in this effective period.
Use Additional
Official- code, if applicable, for codes G31.0-G31.83, G31.85-G31.9, to identify:
- dementia with anxiety (F02.84, F02.A4, F02.B4, F02.C4)
- dementia with behavioral disturbance (F02.81-, F02.A1-, F02.B1-, F02.C1-)
- dementia with mood disturbance (F02.83, F02.A3, F02.B3, F02.C3)
- dementia with psychotic disturbance (F02.82, F02.A2, F02.B2, F02.C2)
Code Also
OfficialICD-10-CM does not list Code Also instructions for G31.9 in this effective period.
Buddy Documentation Tip
MEAT Support
Audit Caution
Common Mistakes
Last updated: FY2026 ICD-10-CM Apr update, Apr 1, 2026 through Sep 30, 2026. CMS-HCC V28 is 100% phased in for payment year 2026.
Is G31.9 an HCC code?
No. G31.9 is a billable ICD-10-CM code but does not map to any HCC category in V28, V24, ESRD, or RxHCC.
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.
Work G31.9 in the Code Book — tabular path, V28 RAF, and MEAT checklist →
MEAT Criteria for G31.9
For G31.9to 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 G31.9 during that encounter, not just copy-forwarded from a problem list.
Coder workflow notes
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What This Code Means
G31.9 is the ICD-10-CM diagnosis code for degenerative disease of nervous system, unspecified. This code is used when a patient has a degenerative disease of the nervous system but the specific type has not been determined or documented. G31.9 sits in the ICD-10-CM chapter for diseases of the nervous system (g00-g99), within the section covering other degenerative diseases of the nervous system (g30-g32).
G31.9 is a billable ICD-10-CM code but does not map to a payment HCC under the CMS-HCC V28, V24, ESRD, or RxHCC risk adjustment models. It can be reported on Medicare Advantage encounter data submissions but it does not contribute to a beneficiary's RAF score and therefore does not affect risk-adjusted payments to the plan.
Does not map to any HCC in V28 or V24 models. Only maps to RxHCC 112 with 0.0 RAF weight. This unspecified code has zero risk adjustment value. Strong motivation exists to query providers for specificity, as many specific degenerative neurological conditions (Alzheimer's, Lewy body, Pick's disease) carry significant HCC value.
HCC Buddy maintains structured V28 and V24 mapping, RAF weights, and MEAT documentation criteria for G31.9 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This is an unspecified code; use only when the specific degenerative disease cannot be identified from documentation
- •Query the provider if additional testing or clinical information could specify the exact degenerative condition
Clinical Significance
Degenerative disease of the nervous system, unspecified, is used when a patient has a confirmed degenerative neurological disease but the specific type has not been determined. This code represents the lowest level of specificity in this category and should prompt a query for more specific diagnostic information. These patients still have progressive neurological conditions requiring ongoing care.
Documentation Requirements
- ✓Provider documentation confirming a degenerative disease of the nervous system is present
- ✓Clinical evidence of progressive neurological deterioration
- ✓Documentation that the specific type of degeneration could not be determined
- ✓Neurological examination findings consistent with degenerative disease
- ✓Plan for further diagnostic workup if the condition has not been fully evaluated
Commonly Confused Codes
- •G31.89: Other specified degenerative diseases of nervous system; use when a specific condition is identified but has no more precise code
- •G31.1: Senile degeneration of brain; more specific when age-related brain degeneration is documented
- •G31.9 vs. R41.81: Age-related cognitive decline is normal aging, not a degenerative disease
- •G30.9: Alzheimer's disease, unspecified; should be used instead if Alzheimer's is diagnosed