Q23.4
BillableHypoplastic left heart syndrome
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is Q23.4 an HCC code?
No. Q23.4 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 Q23.4
For Q23.4 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 Q23.4 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
Q23.4 is the ICD-10-CM diagnosis code for hypoplastic left heart syndrome. A serious birth defect where the left side of the heart is severely underdeveloped, including a small left ventricle and aortic valve abnormalities. Q23.4 sits in the ICD-10-CM chapter for congenital malformations, deformations, chromosomal abnormalities, and genetic disorders (q00-qa0), within the section covering congenital malformations of the circulatory system (q20-q28).
Q23.4 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 only to RxHCC 191 (Obesity) with RAF 0.0, which dramatically undervalues this most severe congenital heart syndrome requiring multiple high-risk surgeries and lifelong intensive cardiac monitoring. This condition has the highest resource utilization among congenital heart defects. Coders reviewing Q23.4 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 Q23.4 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This is a critical congenital condition requiring immediate documentation of associated defects and surgical interventions
- •Document any palliative procedures performed (Norwood procedure, Glenn procedure, Fontan procedure)
Clinical Significance
Hypoplastic left heart syndrome is one of the most severe congenital heart defects where the left side of the heart is critically underdeveloped, requiring immediate intervention for survival. This condition necessitates staged surgical palliation (Norwood sequence) and represents one of the highest-risk congenital heart conditions.
Documentation Requirements
- ✓Echocardiographic confirmation of hypoplastic left ventricle
- ✓Assessment of mitral and aortic valve atresia or stenosis
- ✓Evaluation of ascending aorta hypoplasia
- ✓Staged surgical intervention history (Norwood, Glenn, Fontan)
- ✓Current single ventricle function and hemodynamics
- ✓Arrhythmia monitoring and anticoagulation status
- ✓Exercise tolerance and functional assessment
- ✓Long-term complications and transplant considerations