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ICD-10-CM 2027 diagnosis code

D89.42Idiopathic mast cell activation syndrome

✓ Billable / specific

D89.42 is a valid, billable ICD-10-CM code for idiopathic mast cell activation syndrome. It can be reported on claims for dates of service from October 1, 2026 – September 30, 2027.

Code last changed in FY2017 (effective October 1, 2016). Source: official FY2027 ICD-10-CM release from CDC/NCHS and CMS. Page updated September 29, 2026. About our data

About D89.42

D89.42 is the ICD-10-CM diagnosis code for idiopathic mast cell activation syndrome. It belongs to category D89 (other disorders involving the immune mechanism, not elsewhere classified), block D80-D89 (certain disorders involving the immune mechanism) and chapter 3 (diseases of the blood and blood-forming organs and certain disorders involving the immune mechanism). It is billable, so it can be reported on its own for dates of service from October 1, 2026 through September 30, 2027.

Within D89.4 (mast cell activation syndrome and related disorders), D89.42 is specifically for idiopathic mast cell activation syndrome. Related codes cover mast cell activation, unspecified (D89.40), monoclonal mast cell activation syndrome (D89.41), secondary mast cell activation (D89.43), hereditary alpha tryptasemia (D89.44) and other mast cell activation disorder (D89.49).

It is not a CC or MCC, so as a secondary diagnosis it doesn't change the MS-DRG severity level.

In MS-DRG v44.0, it is part of the grouping logic for DRG 814 (Reticuloendothelial and Immunity Disorders with MCC, relative weight 2.0729), DRG 815 (Reticuloendothelial and Immunity Disorders with CC, relative weight 1.0444) and DRG 816 (Reticuloendothelial and Immunity Disorders without CC/MCC, relative weight 0.7381), in MDC 16 (Diseases and Disorders of the Blood and Blood Forming Organs and Immunological Disorders), as a principal or secondary diagnosis. The DRG a claim lands in depends on the principal diagnosis, procedures and any CC/MCC secondary diagnoses.

It was added in FY2017, effective October 1, 2016.

Before ICD-10, this condition was coded in ICD-9-CM as 279.8 (other specified disorders involving the immune mechanism).

Notes that apply from higher levels

Instructions written at a parent level also apply to D89.42.

› From D89.4 Mast cell activation syndrome and related disorders
Excludes1
  • aggressive systemic mastocytosis (C96.21)
  • congenital cutaneous mastocytosis (Q82.2)
  • (non-congenital) cutaneous mastocytosis (D47.01)
  • (indolent) systemic mastocytosis (D47.02)
  • malignant mast cell neoplasm (C96.2-)
  • malignant mastocytoma (C96.29)
  • mast cell leukemia (C94.3-)
  • mast cell sarcoma (C96.22)
  • mastocytoma NOS (D47.09)
  • other mast cell neoplasms of uncertain behavior (D47.09)
  • systemic mastocytosis associated with a clonal hematologic non-mast cell lineage disease (SM-AHNMD) (D47.02)
› From D89 Other disorders involving the immune mechanism, not elsewhere classified
Excludes1
  • hyperglobulinemia NOS (R77.1)
  • monoclonal gammopathy (of undetermined significance) (D47.2)
Excludes2
  • transplant failure and rejection (T86.-)

Broader instructions also apply from D80-D89 Certain disorders involving the immune mechanism and Chapter 3: Diseases of the blood and blood-forming organs and certain disorders involving the immune mechanism.

Alphabetic index entries

1 entries

Terms in the official ICD-10-CM index that lead to D89.42.

MS-DRG v44.0 grouping

All DRGs →

Inpatient MS-DRGs this diagnosis can group to, as the principal or a secondary diagnosis, depending on the rest of the claim.

ICD-9-CM equivalent

Converter →
  • 279.8Other specified disorders involving the immune mechanismapproximate

From the CMS 2018 General Equivalence Mappings (GEMs), the final GEMs release.

Code history

  1. 2016
  2. 2017
  3. 2018
  4. 2019
  5. 2020
  6. 2021
  7. 2022
  8. 2023
  9. 2024
  10. 2025
  11. 2026
  12. 2027
  • FY2017 (effective 10/1/2016): Added
  • No changes since FY2017.

Common questions about D89.42

Is D89.42 billable?

Yes. D89.42 is a billable/specific ICD-10-CM code valid for FY2027 (October 1, 2026 – September 30, 2027).

Can D89.42 be used as a principal diagnosis?

Yes. The Medicare Code Editor doesn't restrict D89.42 as a principal diagnosis.

Is D89.42 a CC or MCC?

No. D89.42 is neither a CC nor an MCC under MS-DRG v44.0.

What DRG does D89.42 group to?

D89.42 is used in the MS-DRG v44.0 logic for MS-DRG 814 (Reticuloendothelial and Immunity Disorders with MCC), MS-DRG 815 (Reticuloendothelial and Immunity Disorders with CC) and MS-DRG 816 (Reticuloendothelial and Immunity Disorders without CC/MCC). Which one applies depends on whether it is the principal diagnosis and on the rest of the claim.

What is the ICD-9 code for D89.42?

The CMS General Equivalence Mappings map D89.42 to ICD-9-CM 279.8.