2026 ICD-10-CM Diagnosis Code C91.42Hairy cell leukemia, in relapse

ICD-10-CM Codes›C00–D49›C81-C96›C91

ICD-10-CM C91.42
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

C91.42 is a billable ICD-10-CM diagnosis code for hairy cell leukemia, in relapse. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 820 through 825, 840 through 842. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Leukemia - hairy cell.

Code Identity

ICD-10-CM Code
C91.42
Billable Status
Yes — Valid for Submission
Code Describes
Hairy cell leukemia, in relapse
Short Description
Hairy cell leukemia, in relapse
Same as the full description in the CMS dataset.
Parent Code
C91.4
Hairy cell leukemia

Code Classification

ChapterC00–D49Neoplasms
SectionC81-C96Malignant neoplasms of lymphoid, hematopoietic and related tissue
CategoryC91Lymphoid leukemia
This CodeC91.42Hairy cell leukemia, in relapse

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR NEO063
Leukemia - hairy cell
Default principal diagnosis: inpatient Yes · outpatient Yes

Patient EducationClinical

Leukemia

Leukemia is a term for cancers of the blood cells. Leukemia starts in blood-forming tissues such as the bone marrow. Your bone marrow makes the cells which will develop into white blood cells, red blood cells, and platelets. Each type of cell has a different job:

The full article covers:

  • What is leukemia?
  • What are the types of leukemia?
  • What causes leukemia?
  • Who is at risk for leukemia?
  • What are the symptoms of leukemia?
  • How is leukemia diagnosed?
  • What are the treatments for leukemia?

Read the full article at MedlinePlus

Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.

Convert C91.42 to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
202.40 Lk rtctl unsp xtrndl org
Approximate The match is approximate rather than exact.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About C91.42Overview

Is C91.42 (Hairy cell leukemia) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report hairy cell leukemia, in relapse on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does C91.42 group to?

When hairy cell leukemia, in relapse is the principal diagnosis on an inpatient stay, it groups to MS-DRG 820, 821, 822, 823, 824, 825, 840, 841, 842, with relative weights from 1.0104 to 5.8648 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of C91.42?

Under the General Equivalence Mappings, hairy cell leukemia, in relapse converts to ICD-9-CM 202.40 (lk rtctl unsp xtrndl org). The mapping is approximate, so confirm the match fits the documentation.

Footnotes

[1] Chronic - a chronic condition code indicates a condition lasting 12 months or longer and its effect on the patient based on one or both of the following criteria:

  • The condition results in the need for ongoing intervention with medical products,treatment, services, and special equipment
  • The condition places limitations on self-care, independent living, and social interactions.