What is ICD-10 code D84.1?

D84.1 is the ICD-10-CM diagnosis code for Defects in the complement system. A condition where the complement system, a group of proteins that help the immune system fight infections, is not working properly. D84.1 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. D84.1 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. D84.1 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. D84.1 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.

Which HCC does D84.1 map to under V28?

D84.1 maps to CMS-HCC V28 HCC 115 (Specified Immunodeficiencies and White Blood Cell Disorders). The category numbers, labels, and conditions come from the CMS-HCC V28 model file for payment year 2026.

2026 ICD-10-CM D84.1: Defects in the complement

D84.1 maps to CMS-HCC V28 115. A source-labeled RAF reference is available. Documentation must support MEAT. MEAT criteria · RAF Calculator · HCC coding software

ICD-10-CM Code View

HCC Buddy Code Card

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Code lookupD84.1Effective 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) / Certain disorders involving the immune mechanism (D80-D89)

D84.1

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

Defects in the complement system

A condition where the complement system, a group of proteins that help the immune system fight infections, is not working properly.

Buddy Insight

CMS-HCC V28

Mapped

HCC 115

Specified Immunodeficiencies ...

Code-level coefficient reference

CMS-HCC V24

Historical

Not used for standard CY2026 Medicare Advantage payment. Select a historical period to review V24.

Not used for CY2026 payment

ACA/HHS

Mapped

HCC 029

HHS-HCC 029 adult, RAF varies...

Code-level coefficient reference

ESRD/PACE

Mapped

HCC 23

Other Significant Endocrine a...

Code-level coefficient reference

RXHCC

Mapped

HCC 98

Hereditary Angioedema and Oth...

Code-level coefficient reference

Code Book Path

Official

D84Other immunodeficiencies

D84.1Defects in the complement system

Inclusion Terms

Official

C1 esterase inhibitor [C1-INH] deficiency

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for D84.1 in this effective period.

Related Child Codes

Official

D84.0Lymphocyte function antigen-1 [LFA-1] defect

D84.8Other specified immunodeficiencies

D84.9Immunodeficiency, unspecified

Includes

Official

ICD-10-CM does not list Includes notes for D84.1 in this effective period.

Excludes 1

Official

ICD-10-CM does not list Excludes 1 notes for D84.1 in this effective period.

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for D84.1 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for D84.1 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for D84.1 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance

Documentation must identify which complement component is deficient (C1 through C9, factor B, factor D, factor H, properdin), include CH50 and AH50 functional assay results and individual complement component levels, clinical manifestations (recurrent Neisseria infections, lupus-like autoimmune disease, or other sequelae), and treatment plan including vaccination against Neisseria meningitidis.

MEAT Support

HCC Buddy guidance

Audit Caution

HCC Buddy guidance

Complement deficiency spans a wide spectrum of severity

early component deficiencies often present with autoimmune disease rather than infections. Do not code complement consumption from active infection or autoimmune disease flare as a primary complement deficiency

those are secondary findings. Ensure the documented deficiency is a primary genetic or chronic condition rather than a transient laboratory finding.

Common Mistakes

HCC Buddy guidance

D84.0 (lymphocyte function antigen-1 defect, a cellular adhesion defect rather than complement), D84.89 (other immunodeficiencies not specifically related to complement), M32.9 (systemic lupus erythematosus which may be associated with early complement deficiency but is a separate diagnosis), D84.9 (immunodeficiency, unspecified).

Current with CMS: FY2026 ICD-10-CM Apr 1 update (effective Apr 1 – Sep 30, 2026) · CMS-HCC V28, 100% phased in for payment year 2026. FY2027 code set already staged for October 1, 2026. How HCC Buddy stays current →

Is D84.1 an HCC code?

Yes. D84.1 (Defects in the complement system) maps to HCC 115, Specified Immunodeficiencies and White Blood Cell Disorders under the CMS-HCC V28 risk adjustment model, with a source-labeled community non-dual aged reference coefficient of 0.565. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes. It is billable for payment year 2026.

Coder answer: D84.1 is billable and maps to V28 HCC 115, Specified Immunodeficiencies and White Blood Cell Disorders. Open it in the Code Book for the tabular path, RAF, and MEAT checklist.

Code: D84.1
Description: Defects in the complement system
HCC (V28): HCC 115 — Specified Immunodeficiencies and White Blood Cell Disorders
RAF reference coefficient: 0.565
Billable: Yes
Payment year: 2026

HCC Category Mapping

V28HCC 115, Specified Immunodeficiencies and White Blood Cell Disorders

0.565

ESRDHCC 23, Other Significant Endocrine and Metabolic Disorders

Not separately weighted

RxHCCHCC 98, Hereditary Angioedema and Other Defects in the Complement System

Not separately weighted

These are source-labeled model coefficients, not member totals. A category may still be removed by hierarchy or model cleanup rules. 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 context, hierarchy and cleanup rules, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains available for historical review.

Work D84.1 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →

MEAT Criteria for D84.1

For D84.1 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 D84.1 during that encounter, not just copy-forwarded from a problem list.

Coder workflow notes

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What This Code Means

D84.1 is the ICD-10-CM diagnosis code for defects in the complement system. A condition where the complement system, a group of proteins that help the immune system fight infections, is not working properly. D84.1 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 certain disorders involving the immune mechanism (d80-d89).

Under the CMS-HCC V28 risk adjustment model, D84.1 maps to Specified Immunodeficiencies and White Blood Cell Disorders (HCC 115) with a source-labeled community, non-dual, aged reference coefficient of 0.565. No V24 mapping is shown for D84.1; use the applicable model and payment year when reviewing the V28 mapping. 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. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes.

Specify which complement component is deficient (C1, C2, C3, C4, etc.) if documented. Because D84.1 maps to an HCC category, the provider's documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) before the diagnosis is used for risk adjustment. 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, source-labeled coefficient references, and MEAT documentation criteria for D84.1 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•Specify which complement component is deficient (C1, C2, C3, C4, etc.) if documented
•Link to any recurrent infections or autoimmune conditions that may result from the deficiency

Clinical Significance

Defects in the complement system represent a group of immunodeficiencies affecting the complement cascade, a critical component of innate immunity involved in pathogen opsonization, inflammation, and membrane attack complex formation. Different complement component deficiencies predispose to different infections: early complement deficiencies (C1-C4) increase autoimmune disease risk, while terminal complement deficiencies (C5-C9) increase susceptibility to Neisseria infections.

Documentation Requirements

✓Documentation must identify which complement component is deficient (C1 through C9, factor B, factor D, factor H, properdin), include CH50 and AH50 functional assay results and individual complement component levels, clinical manifestations (recurrent Neisseria infections, lupus-like autoimmune disease, or other sequelae), and treatment plan including vaccination against Neisseria meningitidis.

Commonly Confused Codes

•D84.0 (lymphocyte function antigen-1 defect, a cellular adhesion defect rather than complement), D84.89 (other immunodeficiencies not specifically related to complement), M32.9 (systemic lupus erythematosus which may be associated with early complement deficiency but is a separate diagnosis), D84.9 (immunodeficiency, unspecified).