2026 ICD-10-CM Diagnosis Code C96.21Aggressive systemic mastocytosis
ICD-10-CM Codes›C00–D49›C81-C96›C96
- Billable — Valid for Submission
- CC — Complication or Comorbidity
- Risk Adjusts — HCC 21
- Chronic Condition
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 11 through 13, 820 through 825, 840 through 842. As a secondary diagnosis, it counts as a complication or comorbidity (CC) and moves an inpatient stay to a higher severity level within its MS-DRG family. It does not count, however, when the principal diagnosis is one of 50 closely related codes. 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.
For Medicare Advantage risk adjustment, C96.21 maps to CMS-HCC Category 21 (Lymphoma and Other Cancers) under the V28 model, adding a risk factor of about 0.671 for a community, non-dual, aged beneficiary in payment year 2026.
Code Identity
Code Classification
Medicare Risk Adjustment (HCC)Billing
C96.21 maps to a payment category in the CMS-HCC model used to risk-adjust Medicare Advantage payments. Weights are the published community factors for payment year 2026.
Source: CMS Payment Year 2026 risk adjustment mappings and model software. Weights are relative factors, not dollar amounts; a beneficiary's total RAF also includes demographics and interactions. Browse all CMS-HCC categories.
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.
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
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.
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
Questions About C96.21Overview
What is the ICD-10 code for aggressive systemic mastocytosis?
The ICD-10-CM code for aggressive systemic mastocytosis is C96.21 (sometimes written as C9621). It is billable on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
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 11, 12, 13, 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.
Is C96.21 a CC or MCC?
CMS lists C96.21 as a CC (complication or comorbidity) for FY 2026. Reported as a secondary diagnosis, it moves the inpatient stay to a higher-weighted DRG within its severity family. It does not count when the principal diagnosis is one of the 50 closely related codes in its exclusion list.
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.
What HCC is C96.21?
C96.21 (aggressive systemic mastocytosis) maps to CMS-HCC Category 21 (Lymphoma and Other Cancers), commonly written as HCC 21, in the CMS-HCC V28 model used for Medicare Advantage risk adjustment in payment year 2026. It mapped to HCC 10 under the retired V24 model. It also maps in the PACE (CMS-HCC V22), ESRD (V21), and ESRD (V24) models. In the Part D prescription drug model it maps to RxHCC 21.
Does C96.21 risk-adjust for Medicare Advantage payment?
Yes. When documented and reported on a Medicare Advantage encounter, C96.21 adds a risk adjustment factor of about 0.671 to the beneficiary's RAF score for a community, non-dual, aged enrollee (published V28 weights range from 0.493 to 0.739 depending on the payment segment). A more severe related category (HCC 17, HCC 18, HCC 19, and HCC 20) supersedes it when both are reported. See the full factor table on the HCC 21 category page.
