What is ICD-10 code D66?

D66 is the ICD-10-CM diagnosis code for Hereditary factor VIII deficiency. A hereditary bleeding disorder caused by deficiency or dysfunction of clotting factor VIII, commonly known as hemophilia A. D66 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. D66 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. D66 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. D66 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 D66?

CMS phased in the V28 risk adjustment model over payment years 2024 (33%), 2025 (67%), and 2026 (100%), replacing V24. V28 consolidates HCC categories, removes some lower-acuity conditions, and recalibrates RAF weights across the entire model. D66 was a payment HCC under V24 (Severe Hematological Disorders) but the V28 recalibration removed it from the risk adjustment model. Both models can still appear in legacy data and in payer-specific lookback periods.

What should coders know when reporting D66?

Specify the severity level (mild, moderate, severe) if documented Use additional codes for any complications such as inhibitor development Because D66 maps to a payment HCC, the documentation must also satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's risk adjustment score.

D66 ICD-10 Code: Hereditary factor VIII deficiency

ICD-10-CM Code View

HCC Buddy Code Card

Digital ICD-10 code-book layout with official code detail, always-visible risk models, Code Trumping, and Buddy coding guidance.

Code lookupD66Effective periodFY2026 Apr Update (Apr 1-Sep 30)

FY 2026 Apr update / Diseases of the blood and blood-forming organs and certain disorders involving the immune mechanism (D50-D89) / Coagulation defects, purpura and other hemorrhagic conditions (D65-D69)

D66

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

Hereditary factor VIII deficiency

A hereditary bleeding disorder caused by deficiency or dysfunction of clotting factor VIII, commonly known as hemophilia A.

Buddy Insight

Hereditary factor VIII deficiency (Hemophilia A) is the most common severe inherited bleeding disorder, caused by deficient or defective clotting factor VIII.

CMS-HCC V28

Not risk-adjusted for this model/period.

RAF 0

CMS-HCC V24

Mapped

HCC 46

Severe Hematological Disorders

RAF 0.666

ACA/HHS

Not risk-adjusted for this model/period.

RAF 0

ESRD/PACE

Not risk-adjusted for this model/period.

RAF 0

RXHCC

Not risk-adjusted for this model/period.

RAF 0

Code Trumping

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Code Book Path

Official

D6Coagulation defects, purpura and other hemorrhagic conditions (D65-D69)

D66Hereditary factor VIII deficiency

Inclusion Terms

Official

Classical hemophilia
Deficiency factor VIII (with functional defect)
Hemophilia NOS
Hemophilia A

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for D66 in this effective period.

Related Child Codes

Official

ICD-10-CM does not list child codes under D66 for this display context.

Includes

Official

ICD-10-CM does not list Includes notes for D66 in this effective period.

Excludes 1

Official

factor VIII deficiency with vascular defect (D68.0-)

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for D66 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for D66 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for D66 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance

Document the severity level (severe, moderate, mild) based on factor VIII activity levels.

Record current treatment regimen including factor replacement products, dosing frequency, and whether the patient has developed inhibitors (antibodies to factor VIII).

Note any target joints, history of intracranial hemorrhage, and hepatitis/HIV status if applicable from prior blood product exposure.

MEAT Support

HCC Buddy guidance

Document the severity level (severe, moderate, mild) based on factor VIII activity levels.

Record current treatment regimen including factor replacement products, dosing frequency, and whether the patient has developed inhibitors (antibodies to factor VIII).

Note any target joints, history of intracranial hemorrhage, and hepatitis/HIV status if applicable from prior blood product exposure.

Audit Caution

HCC Buddy guidance

Do not confuse hereditary hemophilia A (D66) with acquired hemophilia (D68.311), which has a fundamentally different etiology and treatment approach.

Ensure documentation specifies factor VIII deficiency versus factor IX to distinguish from Hemophilia B (D67).

If the patient has developed inhibitors, also code D68.311 if appropriate.

Common Mistakes

HCC Buddy guidance

D67 (Hereditary factor IX deficiency/Hemophilia B) — clinically similar but different factor deficiency requiring different replacement products

D68.311 (Acquired hemophilia) — autoimmune condition developing later in life with autoantibodies against factor VIII

D68.00-D68.09 (Von Willebrand disease) — may have low factor VIII but primary defect is in von Willebrand factor.

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 D66 an HCC code?

Yes. D66 maps to Severe Hematological Disorders under the V24 model but is not retained in V28.

HCC Category Mapping

V24HCC 46, Severe Hematological Disorders

0.666

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 D66

For D66to 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 D66 during that encounter, not just copy-forwarded from a problem list.

What This Code Means

D66 is the ICD-10-CM diagnosis code for hereditary factor viii deficiency. A hereditary bleeding disorder caused by deficiency or dysfunction of clotting factor VIII, commonly known as hemophilia A. D66 sits in the ICD-10-CM chapter for diseases of the blood and blood-forming organs and certain disorders involving the immune mechanism (d50-d89), within the section covering coagulation defects, purpura and other hemorrhagic conditions (d65-d69).

Under the older CMS-HCC V24 model, D66 maps to Severe Hematological Disorders (HCC 46) with a community, non-dual, aged base RAF weight of 0.666. 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.

Specify the severity level (mild, moderate, severe) if documented. Because D66 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 D66 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•Specify the severity level (mild, moderate, severe) if documented
•Use additional codes for any complications such as inhibitor development

Clinical Significance

Hereditary factor VIII deficiency (Hemophilia A) is the most common severe inherited bleeding disorder, caused by deficient or defective clotting factor VIII. Severity correlates with factor VIII activity levels: severe (<1%), moderate (1-5%), or mild (5-40%). This condition requires lifelong factor replacement therapy and carries significant healthcare resource utilization with a high RAF weight.

Documentation Requirements

✓Document the severity level (severe, moderate, mild) based on factor VIII activity levels.
✓Record current treatment regimen including factor replacement products, dosing frequency, and whether the patient has developed inhibitors (antibodies to factor VIII).
✓Note any target joints, history of intracranial hemorrhage, and hepatitis/HIV status if applicable from prior blood product exposure.

Excludes 1, Do NOT code together

factor VIII deficiency with vascular defect (D68.0-)

Commonly Confused Codes

•D67 (Hereditary factor IX deficiency/Hemophilia B) — clinically similar but different factor deficiency requiring different replacement products
•D68.311 (Acquired hemophilia) — autoimmune condition developing later in life with autoantibodies against factor VIII
•D68.00-D68.09 (Von Willebrand disease) — may have low factor VIII but primary defect is in von Willebrand factor.

Code Hierarchy

└D66Hereditary factor VIII deficiency

D66 ICD-10-CM Code: Hereditary factor VIII deficiency

HCC Buddy Code Card

D66

Is D66 an HCC code?

HCC Category Mapping

MEAT Criteria for D66

What This Code Means

Coding Tips

Clinical Significance

Documentation Requirements

Excludes 1, Do NOT code together

Commonly Confused Codes

Code Hierarchy

Open D66 in the Interactive Encoder