C91.51
BillableAdult T-cell lymphoma/leukemia (HTLV-1-associated), in remission
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is C91.51 an HCC code?
Yes. C91.51 maps to Lymphoma 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 C91.51
For C91.51 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 C91.51 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
C91.51 is the ICD-10-CM diagnosis code for adult t-cell lymphoma/leukemia (htlv-1-associated), in remission. This is a type of blood cancer caused by the HTLV-1 virus that affects T-cells (a type of white blood cell) in adults, and the patient is currently in remission, meaning the cancer is not actively growing or causing symptoms. C91.51 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, C91.51 maps to Lymphoma and Other Cancers (HCC 19) with a community, non-dual, aged base RAF weight of 0.105. Under the older V24 model, C91.51 mapped to the same category but with a base RAF weight of 0.675 — V28 recalibrated weights across the entire model. 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.
Ensure documentation clearly states the patient is in remission; if remission status is unclear, query the physician before coding. Because C91.51 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 C91.51 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Ensure documentation clearly states the patient is in remission; if remission status is unclear, query the physician before coding
- •This code is specific to HTLV-1-associated adult T-cell lymphoma/leukemia; do not use for other T-cell malignancies without HTLV-1 association
Clinical Significance
Adult T-cell lymphoma/leukemia (HTLV-1-associated) in remission indicates that this aggressive viral-associated T-cell malignancy has responded to treatment. Remission in ATLL is uncommon, particularly in acute and lymphoma subtypes, making this status clinically noteworthy. Patients in remission require close surveillance for relapse and continued monitoring for HTLV-1 viral load and other HTLV-1-associated conditions.
Documentation Requirements
- ✓Documentation must confirm the HTLV-1-associated diagnosis and explicitly state remission status, specifying complete versus partial remission.
- ✓The treatment regimen that induced remission (chemotherapy, antiviral therapy, allogeneic transplant) and date of remission achievement should be recorded.
- ✓Ongoing HTLV-1 viral load monitoring and surveillance for relapse must be documented.