D84.1
BillableDefects in the complement system
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is D84.1 an HCC code?
Yes. D84.1 maps to Disorders of Immunity, Neutropenia under the CMS-HCC V28 risk adjustment model (and Other Significant Endocrine and Metabolic Disorders under V24).
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 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.
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 Disorders of Immunity, Neutropenia (HCC 115) with a community, non-dual, aged base RAF weight of 0.369. Under the older CMS-HCC V24 model, D84.1 maps to Other Significant Endocrine and Metabolic Disorders (HCC 23) with a community, non-dual, aged base RAF weight of 0.230. 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 which complement component is deficient (C1, C2, C3, C4, etc.) if documented. Because D84.1 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 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.