G21.0
BillableMalignant neuroleptic syndrome
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is G21.0 an HCC code?
No. G21.0 is a billable ICD-10-CM code but does not map to any HCC category in V28, V24, ESRD, or RxHCC.
This code does not map to an HCC category in any model (V28, V24, ESRD, RxHCC).
What This Code Means
G21.0 is the ICD-10-CM diagnosis code for malignant neuroleptic syndrome. A serious, potentially life-threatening reaction to antipsychotic medications characterized by high fever, muscle rigidity, altered mental status, and autonomic instability. G21.0 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).
G21.0 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.
This is a medical emergency requiring immediate documentation of the offending medication and clinical presentation.
HCC Buddy maintains structured V28 and V24 mapping, RAF weights, and MEAT documentation criteria for G21.0 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This is a medical emergency requiring immediate documentation of the offending medication and clinical presentation
- •Always code the causative drug separately using the appropriate T-code from the poisoning/adverse effect section
Excludes 1 — Do NOT code together
- neuroleptic induced parkinsonism (G21.11)