C91.50
BillableAdult T-cell lymphoma/leukemia (HTLV-1-associated) not having achieved remission
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is C91.50 an HCC code?
Yes. C91.50 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 C91.50
For C91.50 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 C91.50 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
C91.50 is the ICD-10-CM diagnosis code for adult t-cell lymphoma/leukemia (htlv-1-associated) not having achieved remission. A blood cancer (adult T-cell lymphoma/leukemia) caused by HTLV-1 virus infection that has not achieved remission from treatment. C91.50 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, C91.50 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, C91.50 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.
This code is specific to HTLV-1 associated disease - verify viral etiology is documented. Because C91.50 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 C91.50 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This code is specific to HTLV-1 associated disease - verify viral etiology is documented
- •The fifth character '0' indicates no remission achieved - document treatment response
Clinical Significance
Adult T-cell lymphoma/leukemia (ATLL) caused by Human T-cell Lymphotropic Virus type 1 (HTLV-1) is an aggressive malignancy with multiple clinical subtypes (acute, lymphoma, chronic, smoldering). The 'not in remission' status indicates active disease, which in the acute and lymphoma subtypes carries a particularly poor prognosis with median survival of less than one year. HTLV-1 is endemic in specific geographic regions including Japan, Caribbean, Central Africa, and parts of South America.
Documentation Requirements
- ✓Documentation must confirm HTLV-1 positivity by serology and/or viral load testing.
- ✓The clinical subtype (acute, lymphoma, chronic, smoldering) should be specified as it significantly impacts prognosis and treatment.
- ✓Active disease status must be explicitly stated, along with evidence of organ involvement (skin lesions, hypercalcemia, lymphadenopathy, hepatosplenomegaly).
- ✓CD4+CD25+ T-cell immunophenotype should be confirmed.