Q91.5
BillableTrisomy 13, mosaicism (mitotic nondisjunction)
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is Q91.5 an HCC code?
No. Q91.5 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 Q91.5
For Q91.5 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 Q91.5 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
Q91.5 is the ICD-10-CM diagnosis code for trisomy 13, mosaicism (mitotic nondisjunction). Trisomy 13 (Patau syndrome) where some cells have three copies of chromosome 13 and others have two, due to nondisjunction after fertilization; typically results in milder features than complete trisomy 13. Q91.5 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).
Q91.5 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. The condition does not contribute to CMS-HCC risk adjustment despite ongoing complex medical needs and care coordination. Coders reviewing Q91.5 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 Q91.5 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Mosaic trisomy 13 has a better prognosis than complete trisomy 13, so accurate coding is clinically important
- •Document the percentage of affected cells if available from genetic testing
Clinical Significance
Trisomy 13 mosaicism generally presents with less severe features than complete trisomy 13, potentially allowing for longer survival and some developmental progress. However, significant medical complexity and supportive care needs remain substantial throughout the patient's life.
Documentation Requirements
- ✓Karyotype showing mosaic pattern for trisomy 13
- ✓Documentation of mitotic nondisjunction timing
- ✓Percentage of trisomic cells if available
- ✓Clinical phenotype compared to classic Patau syndrome
- ✓Developmental capabilities assessment
- ✓Associated medical complications
- ✓Quality of life and functional status
- ✓Long-term care planning documentation