ICD-10 Code C91.42 – Hairy cell leukemia, in relapse (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code C91.42 – Hairy cell leukemia, in relapse
What it is
C91.42 identifies hairy cell leukemia that has returned after a prior remission. Use it when the record clearly states relapse or recurrent disease.
Clinical signs
Clinical features vary; refer to documentation. Commonly reported findings include recurrent leukemia with abnormal B-cell lymphocytes and renewed evidence of disease after prior treatment.
When to use this code
Use this code when the provider documents hairy cell leukemia as relapsed, recurrent, or returned after remission. It applies when the current encounter is for active disease again, not for history alone. Check documentation if the note only mentions monitoring or prior treatment.
Do not use for
Do not use C91.42 for hairy cell leukemia in remission, history of leukemia, or an unspecified leukemia diagnosis. If the chart does not state relapse, check documentation.
Coding tip
Confirm that the provider links the current disease to relapse, because recurrence wording alone may not be enough.