What is ICD-10 code Q21.0?

Q21.0 is the ICD-10-CM diagnosis code for Ventricular septal defect. A hole in the wall separating the two lower chambers of the heart, allowing blood to flow abnormally between them. It belongs to the ICD-10-CM chapter for congenital malformations, deformations and chromosomal abnormalities (q00-qa0). Q21.0 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. Q21.0 is part of the FY2026 ICD-10-CM code set published by the CMS National Center for Health Statistics and used on Medicare, Medicaid, and commercial claims in the United States.

Yes. Q21.0 is a billable ICD-10-CM code that can be reported on professional and institutional claims as a primary or secondary diagnosis when the provider documentation supports the specific condition described. Billable codes in ICD-10-CM are coded to the highest level of specificity available for the documented diagnosis. Q21.0 is current for fiscal year 2026 and remains in the active CMS ICD-10-CM code set published by the National Center for Health Statistics.

What is the V28 vs V24 difference for Q21.0?

Q21.0 does not map to a payment HCC under either the CMS-HCC V28 risk adjustment model or the older V24 model. V28 reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition. Because neither community model rewards Q21.0 with a risk adjustment factor, it contributes zero to a Medicare Advantage beneficiary's V28/V24 community RAF score in either version. It does, however, map under the Part D RxHCC model (Congenital Heart/Circulatory Disorders and Fontan Circulation), which affects the ESRD and/or Part D (RxHCC) risk scores. Coders should check whether provider documentation supports a more specific child code that does map to a community payment HCC.

What should coders know when reporting Q21.0?

Document the size and location of the defect (muscular, membranous, inlet, outlet) when available for more specific coding if needed Note whether the defect is isolated or associated with other cardiac anomalies, as this affects clinical management and coding

2026 ICD-10-CM Q21.0: Ventricular septal defect

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Code lookupQ21.0Effective periodFY2026 Apr Update (Apr 1-Sep 30)

FY 2026 Apr update / Congenital malformations, deformations and chromosomal abnormalities (Q00-QA0) / Congenital malformations of the circulatory system (Q20-Q28)

Q21.0

Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidance

Ventricular septal defect

A hole in the wall separating the two lower chambers of the heart, allowing blood to flow abnormally between them.

Buddy Insight

Ventricular septal defect is the most common congenital heart defect, involving a hole in the wall between the heart's lower chambers.

CMS-HCC V28

N/A

—

Not risk-adjusted for this model/period.

Not mapped

CMS-HCC V24

N/A

—

Not risk-adjusted for this model/period.

Not mapped

ACA/HHS

Mapped

HCC 139

HHS-HCC 139

Varies by metal level

ESRD/PACE

N/A

—

Not risk-adjusted for this model/period.

Not mapped

RXHCC

Mapped

HCC 191

Congenital Heart/Circulatory ...

RAF 0.150

Code Book Path

Official

Q21Congenital malformations of cardiac septa

Q21.0Ventricular septal defect

Inclusion Terms

Official

Roger's disease

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for Q21.0 in this effective period.

Related Child Codes

Official

Q21.1Atrial septal defect

Q21.2Atrioventricular septal defect

Q21.3Tetralogy of Fallot

Q21.4Aortopulmonary septal defect

Q21.8Other congenital malformations of cardiac septa

Includes

Official

ICD-10-CM does not list Includes notes for Q21.0 in this effective period.

Excludes 1

Official

acquired cardiac septal defect (I51.0)

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for Q21.0 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for Q21.0 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for Q21.0 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance

Echocardiographic confirmation of ventricular septal defect

Size and location of the defect

Hemodynamic significance and shunt direction

Associated cardiac anomalies

MEAT Support

HCC Buddy guidance

Echocardiographic confirmation of ventricular septal defect

Size and location of the defect

Hemodynamic significance and shunt direction

Associated cardiac anomalies

Audit Caution

HCC Buddy guidance

Confusing with atrial septal defects

Using post-surgical codes when defect remains open

Missing associated cardiac anomalies

Not documenting hemodynamic significance

Common Mistakes

HCC Buddy guidance

Q21.1 — atrial septal defect

Q21.2 — atrioventricular septal defect

Z95.89 — presence of other cardiac implants (post-surgical)

I25.2 — old myocardial infarction

Last updated: FY2026 ICD-10-CM Apr update, Apr 1, 2026 through Sep 30, 2026. CMS-HCC V28 is 100% phased in for payment year 2026.

Is Q21.0 an HCC code?

Q21.0 is not in the CMS-HCC V28 or V24 community payment model, but it does map to Congenital Heart/Circulatory Disorders and Fontan Circulation under the Part D RxHCC model.

Code: Q21.0
Description: Ventricular septal defect
HCC (V28): No CMS-HCC V28 mapping
RAF: —
Billable: Yes
Payment year: 2026

HCC Category Mapping

RxHCCHCC 191, Congenital Heart/Circulatory Disorders and Fontan Circulation

0.150

Each model's RAF is its CMS base weight for that model's standard population, so weights are not directly comparable across models: CMS-HCC V28 and V24 use Community, Non-Dual, Aged; ESRD uses the dialysis continuing-enrollee model; RxHCC is the Part D continuing-enrollee, non-low-income, aged weight (a larger scale than CMS-HCC). ACA/HHS has no single weight — it varies by metal level. 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.

Work Q21.0 in the Code Book — tabular path, V28 RAF, and MEAT checklist →

MEAT Criteria for Q21.0

For Q21.0to 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 Q21.0 during that encounter, not just copy-forwarded from a problem list.

Coder workflow notes

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What This Code Means

Q21.0 is the ICD-10-CM diagnosis code for ventricular septal defect. A hole in the wall separating the two lower chambers of the heart, allowing blood to flow abnormally between them. Q21.0 sits in the ICD-10-CM chapter for congenital malformations, deformations and chromosomal abnormalities (q00-qa0), within the section covering congenital malformations of the circulatory system (q20-q28).

Q21.0 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.

This code does not map to a CMS-HCC V28 payment category. Capture depends on documentation that supports the diagnosis; verify the HCC assignment against the current CMS mapping for the applicable payment year. Coders reviewing Q21.0 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-impact RAF improvement available within accurate coding.

HCC Buddy maintains structured V28 and V24 mapping, RAF weights, and MEAT documentation criteria for Q21.0 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•Document the size and location of the defect (muscular, membranous, inlet, outlet) when available for more specific coding if needed
•Note whether the defect is isolated or associated with other cardiac anomalies, as this affects clinical management and coding

Clinical Significance

Ventricular septal defect is the most common congenital heart defect, involving a hole in the wall between the heart's lower chambers. While small defects may close spontaneously, larger defects can lead to heart failure and require surgical intervention to prevent long-term complications.