G31.86
BillableAlexander disease
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is G31.86 an HCC code?
Yes. G31.86 maps to Dementia, Mild or Unspecified under the CMS-HCC V28 risk adjustment model.
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 G31.86
For G31.86 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 G31.86 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
G31.86 is the ICD-10-CM diagnosis code for alexander disease. Alexander disease is a rare inherited neurological disorder that affects the white matter of the brain, causing progressive weakness, loss of coordination, and cognitive decline. G31.86 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).
Under the CMS-HCC V28 risk adjustment model, G31.86 maps to Dementia, Mild or Unspecified (HCC 127) with a community, non-dual, aged base RAF weight of 0.464. G31.86 was not retained as a payment HCC under the older V24 model, so V28 introduced or recategorized it during the 2024–2026 phase-in. 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.
This is a rare genetic disorder; ensure documentation clearly identifies Alexander disease rather than other leukodystrophies. Because G31.86 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 G31.86 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This is a rare genetic disorder; ensure documentation clearly identifies Alexander disease rather than other leukodystrophies
- •May require additional codes for associated symptoms such as seizures, spasticity, or developmental delays
Clinical Significance
Alexander disease is a rare progressive leukodystrophy caused by mutations in the GFAP gene, primarily affecting white matter of the brain. It causes progressive spasticity, seizures, and cognitive decline, with infantile forms being most severe. Accurate coding ensures proper risk adjustment for these patients with high-intensity care needs including neurology, genetics, and rehabilitation services.
Documentation Requirements
- ✓Provider documentation specifically identifying Alexander disease
- ✓Genetic testing confirming GFAP gene mutation
- ✓Neuroimaging (MRI) showing characteristic frontal-predominant white matter abnormalities
- ✓Documentation of clinical features: macrocephaly (infantile form), spasticity, seizures, developmental regression
- ✓Classification of disease form (infantile, juvenile, or adult onset)
- ✓Current treatment plan and multidisciplinary care team involvement