Q91.0
BillableTrisomy 18, nonmosaicism (meiotic nondisjunction)
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is Q91.0 an HCC code?
No. Q91.0 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 Q91.0
For Q91.0to 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 Q91.0 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
Q91.0 is the ICD-10-CM diagnosis code for trisomy 18, nonmosaicism (meiotic nondisjunction). Trisomy 18 (Edwards syndrome) caused by complete nondisjunction during meiosis, resulting in three copies of chromosome 18; characterized by severe developmental abnormalities and intellectual disability. Q91.0 sits in the ICD-10-CM chapter for congenital malformations, deformations, chromosomal abnormalities, and genetic disorders (q00-qa0), within the section covering chromosomal abnormalities, not elsewhere classified (q90-q99).
Q91.0 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.
Maps to RxHCC 148 (Mild Intellectual Disabilities) with RAF weight 0.0 in the RxHCC model only. Despite the severe nature and intensive care requirements, this condition does not contribute to CMS-HCC risk adjustment. Coders reviewing Q91.0 should check whether additional documentation would support a more specific child code in the same hierarchy that does map to a payment HCC — capturing the correct specificity is the highest-leverage RAF improvement available within accurate coding.
HCC Buddy maintains structured V28 and V24 mapping, RAF weights, and MEAT documentation criteria for Q91.0 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This code specifies the genetic mechanism (meiotic nondisjunction) which is important for genetic counseling and recurrence risk assessment
- •Document whether the condition is confirmed through karyotype analysis or other genetic testing
Clinical Significance
Trisomy 18 nonmosaicism represents the most severe form of Edwards syndrome with profound developmental disabilities and high mortality risk. This diagnosis indicates need for palliative care planning, intensive medical support, and family counseling regarding prognosis and care options.
Documentation Requirements
- ✓Karyotype analysis showing 47,XX or XY,+18
- ✓Documentation of meiotic nondisjunction mechanism
- ✓Clinical features consistent with Edwards syndrome
- ✓Growth and developmental assessments
- ✓Cardiac evaluation findings
- ✓Neurologic assessment results
- ✓Feeding and respiratory status documentation
- ✓Genetic counseling documentation