G47.421
BillableNarcolepsy in conditions classified elsewhere with cataplexy
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is G47.421 an HCC code?
No. G47.421 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.
MEAT Criteria for G47.421
For G47.421 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 G47.421 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
G47.421 is the ICD-10-CM diagnosis code for narcolepsy in conditions classified elsewhere with cataplexy. Narcolepsy with muscle weakness episodes that occurs as a symptom of another underlying medical condition or disease. G47.421 sits in the ICD-10-CM chapter for diseases of the nervous system (g00-g99), within the section covering episodic and paroxysmal disorders (g40-g47).
G47.421 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.
Maps to RxHCC 355 (Narcolepsy and Cataplexy) with a RAF weight of 0.000 in the prescription drug model. No mapping to CMS-HCC V28 or V24 payment models, so this code does not directly affect Medicare Advantage capitation payments. The RxHCC mapping reflects pharmacy costs for wake-promoting agents (modafinil, armodafinil, solriamfetol, pitolisant) and sodium oxybate for cataplexy. No hierarchies apply within payment models.
HCC Buddy maintains structured V28 and V24 mapping, RAF weights, and MEAT documentation criteria for G47.421 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Requires dual coding - code the underlying condition first, then this code
- •Use when narcolepsy is secondary to another classified condition (e.g., Niemann-Pick disease type C)
Clinical Significance
Narcolepsy with cataplexy secondary to another medical condition represents symptomatic narcolepsy caused by an identified underlying disorder such as sarcoidosis, multiple sclerosis, tumor, or traumatic brain injury affecting the hypothalamus. This distinction from primary narcolepsy is critical because treating the underlying condition may improve narcolepsy symptoms. The underlying condition must be coded first, making accurate sequencing essential for both clinical care and risk adjustment.
Documentation Requirements
- ✓Underlying causative condition identified and documented (e.g., brain tumor, sarcoidosis, multiple sclerosis, traumatic brain injury)
- ✓The underlying condition code must be sequenced FIRST per ICD-10-CM convention
- ✓Narcolepsy symptoms documented: excessive daytime sleepiness
- ✓Cataplexy episodes documented with emotional triggers
- ✓Temporal relationship between underlying condition and narcolepsy onset
- ✓Sleep study results supporting narcolepsy diagnosis
- ✓Treatment plan addressing both underlying condition and narcolepsy symptoms
Commonly Confused Codes
- •G47.411 (Narcolepsy with cataplexy, primary) — use when narcolepsy is idiopathic, not secondary to another condition
- •G47.429 (Narcolepsy in conditions classified elsewhere, without cataplexy) — secondary narcolepsy but lacks cataplexy
- •G47.14 (Hypersomnia due to a medical condition) — excessive sleepiness from medical condition without full narcolepsy criteria
- •G93.1 (Anoxic brain damage) — may cause secondary narcolepsy but is the underlying condition, not the narcolepsy itself
- •G47.419 (Narcolepsy without cataplexy, primary) — primary form without cataplexy