ICD-10 Code C91.5 – Adult T-cell lymphoma/leukemia (HTLV-1-associated) (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code C91.5 – Adult T-cell lymphoma/leukemia (HTLV-1-associated)
What it is
C91.5 identifies adult T-cell lymphoma/leukemia linked to human T-cell lymphotropic virus type 1 (HTLV-1). Use it for this specific mature T-cell malignancy, which may present as leukemia, lymphoma, or both.
Clinical signs
Common findings include lymphadenopathy, skin lesions, hepatosplenomegaly, and abnormal lymphocytes in blood or tissue. Clinical features vary; refer to documentation.
When to use this code
Use C91.5 when the provider documents adult T-cell lymphoma, adult T-cell leukemia, or ATL with HTLV-1 association. It also fits cases described as combined lymphoma/leukemia when the diagnosis is clearly this entity. Check documentation if the record only says T-cell lymphoma without HTLV-1 linkage.
Do not use for
Do not use this code for other non-HTLV-1 T-cell lymphomas, acute lymphoblastic leukemia, or unspecified leukemia. If the chart does not confirm adult T-cell lymphoma/leukemia, code the documented condition instead.
Coding tip
Look for explicit HTLV-1 association in pathology, oncology, or discharge documentation before assigning C91.5.