Question 1

What is ICD-10 code G21.0?

Accepted Answer

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.

Question 2

Is G21.0 an HCC code?

Accepted Answer

No. G21.0 is a billable ICD-10-CM code but does not map to any payment HCC category under the CMS-HCC V28, V24, ESRD, or RxHCC risk adjustment models. It can still be reported on Medicare Advantage encounter data submissions, but it does not contribute to the beneficiary's Risk Adjustment Factor (RAF) score and therefore does not affect the risk-adjusted capitation payment to the plan. V28 reached 100% phase-in for payment year 2026.

Question 3

Is G21.0 billable?

Accepted Answer

Yes. G21.0 is a billable ICD-10-CM code that can be reported on professional and institutional claims as a primary or secondary diagnosis when the provider documentation supports the specific condition described. Billable codes in ICD-10-CM are coded to the highest level of specificity available for the documented diagnosis. G21.0 is current for fiscal year 2026 and remains in the active CMS ICD-10-CM code set published by the National Center for Health Statistics.

Question 4

What is the V28 vs V24 difference for G21.0?

Accepted Answer

G21.0 does not map to a payment HCC under either the CMS-HCC V28 risk adjustment model or the older V24 model. V28 reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition. Because neither model rewards G21.0 with a risk adjustment factor, it contributes zero to a Medicare Advantage beneficiary's RAF score in either version. Coders should check whether provider documentation supports a more specific child code that does map to a payment HCC.

Question 5

What should coders know when reporting G21.0?

Accepted Answer

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

HCC Buddy Code Card

G21.0

Is G21.0 an HCC code?

What This Code Means

Coding Tips

Excludes 1, Do NOT code together

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Child Codes

Code Hierarchy

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G21.0 ICD-10-CM Code: Malignant neuroleptic syndrome

HCC Buddy Code Card

G21.0

Is G21.0 an HCC code?

What This Code Means

Coding Tips

Excludes 1, Do NOT code together

Use Additional Code

Child Codes

Code Hierarchy

Work G21.0 in HCC Buddy