C91.42
BillableHairy cell leukemia, in relapse
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is C91.42 an HCC code?
Yes. C91.42 maps to Colorectal, Bladder, and Other Cancers under the CMS-HCC V28 risk adjustment model (and Lymphoma and Other Cancers 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 C91.42
For C91.42 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 C91.42 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
C91.42 is the ICD-10-CM diagnosis code for hairy cell leukemia, in relapse. A rare blood cancer (hairy cell leukemia) with hair-like abnormal cells that has returned after a period of remission. C91.42 sits in the ICD-10-CM chapter for neoplasms (c00-d49), within the section covering malignant neoplasms of lymphoid, hematopoietic and related tissue (c81-c96).
Under the CMS-HCC V28 risk adjustment model, C91.42 maps to Colorectal, Bladder, and Other Cancers (HCC 22) with a community, non-dual, aged base RAF weight of 0.000. Under the older CMS-HCC V24 model, C91.42 maps to Lymphoma and Other Cancers (HCC 10) with a community, non-dual, aged base RAF weight of 0.675. 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.
The fifth character '2' indicates relapse - ensure documentation shows cancer has recurred. Because C91.42 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 C91.42 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •The fifth character '2' indicates relapse - ensure documentation shows cancer has recurred
- •Document timing and circumstances of relapse when available
Clinical Significance
Hairy cell leukemia in relapse indicates recurrence of disease after a prior period of documented remission. Relapsed HCL may present with recurrent cytopenias, re-enlargement of the spleen, or bone marrow re-infiltration. Second remissions are achievable with the same purine analog or alternative agents (rituximab, vemurafenib for BRAF V600E-mutated cases), though duration of subsequent remissions may be shorter.
Documentation Requirements
- ✓Documentation must establish prior remission and current evidence of disease recurrence, including rising hairy cells in blood or bone marrow, returning cytopenias, or splenic re-enlargement.
- ✓The interval since last remission, BRAF V600E mutation status (if tested), and planned treatment approach should be recorded.
- ✓Updated immunophenotyping at relapse helps confirm diagnosis and rule out other entities.
Commonly Confused Codes
- •C91.40 (HCL not in remission) is for disease that never achieved remission.
- •C91.41 (HCL in remission) applies when disease is controlled.
- •C91.12 (B-CLL in relapse) should not be used for relapsed HCL.
- •C91.Z2 (other lymphoid leukemia in relapse) is less specific and should be avoided when HCL is confirmed.