ICD-10 Code C91.1 – Chronic lymphocytic leukemia of B-cell type (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code C91.1 – Chronic lymphocytic leukemia of B-cell type
What it is
C91.1 identifies chronic lymphocytic leukemia involving mature B lymphocytes. Use it for a slow-growing blood cancer documented as CLL or B-cell type CLL.
Clinical signs
Clinical features vary; refer to documentation. Common findings include persistent lymphocytosis, enlarged lymph nodes, splenomegaly, or incidental diagnosis on blood work.
When to use this code
Use C91.1 when the provider documents chronic lymphocytic leukemia, CLL, or B-cell chronic lymphocytic leukemia. It also fits confirmed cases being followed over time, even if the patient is stable or asymptomatic. If the record specifies another leukemia subtype, code that diagnosis instead.
Do not use for
Do not use this code for acute leukemias, non-B-cell lymphoid malignancies, or monoclonal B-cell lymphocytosis. Check documentation when the note only says “lymphocytosis” without a leukemia diagnosis.
Coding tip
Code only when the provider clearly documents CLL or B-cell type; otherwise, check documentation for the exact hematologic diagnosis.