D81.9
BillableCombined immunodeficiency, unspecified
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is D81.9 an HCC code?
Yes. D81.9 maps to Common Variable and Combined Immunodeficiencies under the CMS-HCC V28 risk adjustment model (and Disorders of Immunity 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 D81.9
For D81.9to 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 D81.9 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
D81.9 is the ICD-10-CM diagnosis code for combined immunodeficiency, unspecified. A rare condition where the immune system is weakened due to multiple genetic defects, but the specific type of defect is not identified or documented. D81.9 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, D81.9 maps to Common Variable and Combined Immunodeficiencies (HCC 114) with a community, non-dual, aged base RAF weight of 2.262. Under the older CMS-HCC V24 model, D81.9 maps to Disorders of Immunity (HCC 47) with a community, non-dual, aged base RAF weight of 0.665. 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.
This is the least specific code for combined immunodeficiency; use only when no more specific D81 code applies. Because D81.9 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 D81.9 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This is the least specific code for combined immunodeficiency; use only when no more specific D81 code applies
- •Query the provider to determine if a more specific diagnosis code can be assigned based on testing results
Clinical Significance
Combined immunodeficiency, unspecified, indicates that a patient has both cellular (T-cell) and humoral (B-cell/antibody) immune deficiency, but the specific genetic or molecular defect has not been characterized. This code represents patients undergoing diagnostic evaluation or those whose immunodeficiency cannot be further classified despite workup.
Documentation Requirements
- ✓Documentation should include evidence of both T-cell and B-cell dysfunction (lymphocyte subset analysis, immunoglobulin levels, functional testing), the reason why a more specific diagnosis cannot be assigned, clinical manifestations including infection history, and the plan for further immunological workup.
- ✓Query providers for additional specificity whenever possible.
Commonly Confused Codes
- •D81.89 (other combined immunodeficiencies when a specific type is identified but lacks its own code), D84.9 (immunodeficiency, unspecified, which does not specify combined cellular and humoral defects), D80.9 (unspecified antibody deficiency for primarily humoral defects), D83.9 (common variable immunodeficiency, unspecified, which has specific diagnostic criteria).