Question 1

What is ICD-10 code G99.8?

Accepted Answer

G99.8 is the ICD-10-CM diagnosis code for other specified disorders of nervous system in diseases classified elsewhere. Other nervous system disorders that occur as a result of another disease classified elsewhere in the medical coding system. G99.8 sits in the ICD-10-CM chapter for diseases of the nervous system (g00-g99), within the section covering other disorders of the nervous system (g89-g99).

G99.8 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 residual code; use only when the nervous system manifestation is not more specifically classified.

Question 2

Is G99.8 an HCC code?

Accepted Answer

No. G99.8 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 G99.8 billable?

Accepted Answer

Yes. G99.8 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. G99.8 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 G99.8?

Accepted Answer

G99.8 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 community model rewards G99.8 with a risk adjustment factor, it contributes zero to a Medicare Advantage beneficiary's V28/V24 community RAF score in either version. Coders should check whether provider documentation supports a more specific child code that does map to a community payment HCC.

Question 5

What should coders know when reporting G99.8?

Accepted Answer

This is a residual code; use only when the nervous system manifestation is not more specifically classified Always code the underlying disease first, then use this code for the nervous system complication

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G99.8

Is G99.8 an HCC code?

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G99.8 ICD-10-CM Code: Other specified disorders of nervous system in diseases classified elsewhere

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G99.8

Is G99.8 an HCC code?

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What This Code Means

Coding Tips

Excludes 1, Do NOT code together

Code First

Child Codes

Code Hierarchy

Work G99.8 in HCC Buddy