G91.9
BillableHydrocephalus, unspecified
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is G91.9 an HCC code?
Yes. G91.9 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 G91.9
For G91.9 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 G91.9 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
G91.9 is the ICD-10-CM diagnosis code for hydrocephalus, unspecified. A condition where excess fluid accumulates in the brain, but the specific type or cause has not been determined or documented. G91.9 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).
Under the CMS-HCC V28 risk adjustment model, G91.9 maps to Dementia, Mild or Unspecified (HCC 127) with a community, non-dual, aged base RAF weight of 0.464. G91.9 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 non-specific code; attempt to obtain more detailed documentation to assign a more specific G91 code. Because G91.9 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 G91.9 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This is a non-specific code; attempt to obtain more detailed documentation to assign a more specific G91 code
- •Query the provider if additional clinical information is available to clarify the type of hydrocephalus
Clinical Significance
Hydrocephalus, unspecified is used when hydrocephalus is documented but the specific type cannot be determined from the available clinical information. While this code still carries significant risk adjustment value, it represents an opportunity for improved documentation specificity. Coders should query providers when possible to capture a more specific hydrocephalus diagnosis.
Documentation Requirements
- ✓Documentation of hydrocephalus diagnosis, even if type is not further specified
- ✓Imaging evidence of ventricular enlargement
- ✓Clinical symptoms attributable to hydrocephalus
- ✓Any available information about etiology or type that could support a more specific code
- ✓Current management plan including monitoring, medical therapy, or surgical intervention
Commonly Confused Codes
- •G91.0 — Communicating hydrocephalus: use when CSF absorption impairment without obstruction is documented
- •G91.1 — Obstructive hydrocephalus: use when a physical blockage of CSF flow is identified
- •G91.2 — Normal pressure hydrocephalus: use when the classic triad with normal CSF pressure is documented
- •Q03.9 — Congenital hydrocephalus, unspecified: use for hydrocephalus present at birth or due to congenital malformation