2026 ICD-10-CM Diagnosis Code C96.21Aggressive systemic mastocytosis

ICD-10-CM Codes›C00–D49›C81-C96›C96

ICD-10-CM C96.21
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

C96.21 is a billable ICD-10-CM diagnosis code for aggressive systemic mastocytosis. 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. Coders also document this condition as aggressive lymphadenopathic mastocytosis with eosinophilia. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Non-Hodgkin lymphoma.

Code Identity

ICD-10-CM Code
C96.21
Billable Status
Yes — Valid for Submission
Code Describes
Aggressive systemic mastocytosis
Short Description
Aggressive systemic mastocytosis
Same as the full description in the CMS dataset.
Parent Code
C96.2
Malignant mast cell neoplasm

Code Classification

ChapterC00–D49Neoplasms
SectionC81-C96Malignant neoplasms of lymphoid, hematopoietic and related tissue
CategoryC96Other and unspecified malignant neoplasms of lymphoid, hematopoietic and related tissue
This CodeC96.21Aggressive systemic mastocytosis

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Aggressive lymphadenopathic mastocytosis with eosinophilia
  • Aggressive systemic mastocytosis
  • Malignant mastocytosis
  • Systemic mast cell disease

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

External Cause of Injuries IndexGuidance

References for this code in the External Cause of Injuries Index.

    • Mastocytosis
      • aggressive systemic

Clinical ClassificationClinical

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

CCSR NEO058
Non-Hodgkin lymphoma
Default principal diagnosis: inpatient Yes · outpatient Yes

Patient EducationClinical

Systemic mastocytosis

Systemic mastocytosis is a blood disorder that can affect many different body systems. Individuals with the condition can develop signs and symptoms at any age, but it usually appears after adolescence.

Read the full article at MedlinePlus

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

Convert C96.21 to ICD-9-CMHistory

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

ICD-9-CM
202.60 Mlg mast unsp xtrndl org
Approximate The match is approximate rather than exact.
ICD-9-CM
202.61 Mal mastocytosis head
Approximate The match is approximate rather than exact.
ICD-9-CM
202.62 Mal mastocytosis thorax
Approximate The match is approximate rather than exact.
ICD-9-CM
202.63 Mal mastocytosis abdom
Approximate The match is approximate rather than exact.
ICD-9-CM
202.64 Mal mastocytosis axilla
Approximate The match is approximate rather than exact.
ICD-9-CM
202.65 Mal mastocytosis inguin
Approximate The match is approximate rather than exact.
ICD-9-CM
202.66 Mal mastocytosis pelvic
Approximate The match is approximate rather than exact.
ICD-9-CM
202.67 Mal mastocytosis spleen
Approximate The match is approximate rather than exact.
ICD-9-CM
202.68 Mal mastocytosis mult
Approximate The match is approximate rather than exact.

Code History & ChangesHistory

Replacement C96.21 replaces the following previously assigned code(s):

  • C96.2 - Malignant mast cell neoplasm
  • C96.2 - Malignant mast cell tumor
FY 2018AddedAdded to the ICD-10-CM code setEffective October 1, 2017.
FY 2019–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About C96.21Overview

Is C96.21 (Malignant mast cell neoplasm) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report aggressive systemic mastocytosis on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does C96.21 group to?

When aggressive systemic mastocytosis 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 C96.21?

Under the General Equivalence Mappings, aggressive systemic mastocytosis converts to ICD-9-CM 202.60 (mlg mast unsp xtrndl org), 202.61 (mal mastocytosis head), and 202.62 (mal mastocytosis thorax). 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.