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D89.42 ICD-10-CM Code: Idiopathic mast cell activation syndrome

Billing Status: YES. This is a valid, specific, and billable ICD-10-CM reference.

Inpatient Payment Groups (MS-DRG)

MS-DRGs this diagnosis helps define, as principal or secondary, per the CMS ICD-10-CM/PCS MS-DRG Definitions Manual v43.0, Appendix B.

  • MS-DRG 814 — RETICULOENDOTHELIAL AND IMMUNITY DISORDERS WITH MCC (MDC 16)
  • MS-DRG 815 — RETICULOENDOTHELIAL AND IMMUNITY DISORDERS WITH CC (MDC 16)
  • MS-DRG 816 — RETICULOENDOTHELIAL AND IMMUNITY DISORDERS WITHOUT CC/MCC (MDC 16)

A diagnosis appearing in a group's logic does not by itself determine the DRG assigned to a stay; the grouper uses the full claim.

Official Registry Overview & Definition

Idiopathic mast cell activation syndrome is a billable ICD-10-CM diagnosis code (D89.42).

Official Tabular Instructional Notes

Sequencing, inclusion, and exclusion notes published for D89.42 in the official ICD-10-CM tabular list.

Includes

Conditions the official ICD-10-CM tabular list includes under this code.

  • defects in the complement system
  • immunodeficiency disorders, except human immunodeficiency virus [HIV] disease
  • sarcoidosis

Excludes1 — Not Coded Here

Conditions generally not reported together with this code (Excludes1) -- an error unless the two conditions are documented as unrelated to each other (ICD-10-CM Official Guidelines, Section I.A.12.a).

Excludes2 — Not Included Here

Conditions not covered by this code, but which may be reported alongside it when both are present.

Indexed Clinical Terms (1)

Clinical term phrases from the official ICD-10-CM Index to Diseases and Injuries that map to this code.

  • Activation, mast cell (disorder) (syndrome), idiopathic

Frequently Compared Codes

The official Excludes notes on D89.42 name these codes. Each comparison page covers when the two can — or must not — be reported together.

Change history

  • FY2027 — 2026-10-01
    Excludes2 note added
    human immunodeficiency virus [HIV] disease (B20)
    FY2027 changes
  • FY2027 — 2026-10-01
    Excludes1 note removed
    human immunodeficiency virus [HIV] disease (B20)
    FY2027 changes
  • FY2017 — 2016-10-01
    Added to the code set
    Idiopathic mast cell activation syndrome
    FY2017 changes

Verify Before Coding

  • No Medicare Code Editor or MS-DRG Definitions Manual restrictions apply to this code.

From the code registry, the Medicare Code Editor, and the MS-DRG Definitions Manual. Check it against a full claim.

Relationships & Classification

Clinical classification (AHRQ CCSR):BLD008 — Immunity disorders (default).

Clinical Classifications Software Refined (CCSR) for ICD-10-CM Diagnoses. Healthcare Cost and Utilization Project (HCUP), Agency for Healthcare Research and Quality.

Potential MS-DRG Relationships (FY2026)

Not on the CMS CC/MCC list — as a secondary diagnosis this code does not change MS-DRG severity.

Named in the grouper logic of 3 MS-DRGs: DRG 814 (MDC 16), DRG 815 (MDC 16), DRG 816 (MDC 16).

From the CMS MS-DRG Definitions Manual (Appendices B and C). Actual DRG assignment depends on the complete claim.

Contextual Map

Every relationship of D89.42 in one view: hierarchy, official tabular instructions in both directions, clinical classification, risk adjustment, MS-DRG participation, index terms and change history — each edge carrying the CMS source it derives from.

Hierarchy

  • D50-D89 — Chapter 3: Diseases of the Blood and Blood-forming Organs and Certain Disorders Involving the Immune Mechanism (D50-D89) (D50-D89) [Hierarchy]

Clinical classification (CCSR)

  • BLD008 — Immunity disorders [CCSR]

Potential MS-DRG

  • DRG 814 — RETICULOENDOTHELIAL AND IMMUNITY DISORDERS WITH MCC [MS-DRG]: “RETICULOENDOTHELIAL AND IMMUNITY DISORDERS WITH MCC (MDC 16)”
  • DRG 815 — RETICULOENDOTHELIAL AND IMMUNITY DISORDERS WITH CC [MS-DRG]: “RETICULOENDOTHELIAL AND IMMUNITY DISORDERS WITH CC (MDC 16)”
  • DRG 816 — RETICULOENDOTHELIAL AND IMMUNITY DISORDERS WITHOUT CC/MCC [MS-DRG]: “RETICULOENDOTHELIAL AND IMMUNITY DISORDERS WITHOUT CC/MCC (MDC 16)”

MDC crossing · procedures (1614)

  • MDC 16 — Diseases and Disorders of Blood, Blood Forming Organs and Immunologic Disorders [MDC crossing]: “Diseases and Disorders of Blood, Blood Forming Organs and Immunologic Disorders — the grouper's crossing between diagnoses and procedures: a principal diagnosis sets the MDC, and same-MDC procedures move the stay to its surgical DRGs.”
  • 02HV02Z — Insertion of Monitoring Device into Superior Vena Cava, Open Approach [Same-MDC procedure]: “Insertion of Monitoring Device into Superior Vena Cava, Open Approach — grouped in MDC 16, the procedure side of this code's crossing.” · check together
  • 02HV0DZ — Insertion of Intraluminal Device into Superior Vena Cava, Open Approach [Same-MDC procedure]: “Insertion of Intraluminal Device into Superior Vena Cava, Open Approach — grouped in MDC 16, the procedure side of this code's crossing.” · check together
  • 02HV3DZ — Insertion of Intraluminal Device into Superior Vena Cava, Percutaneous Approach [Same-MDC procedure]: “Insertion of Intraluminal Device into Superior Vena Cava, Percutaneous Approach — grouped in MDC 16, the procedure side of this code's crossing.” · check together
  • 02HV42Z — Insertion of Monitoring Device into Superior Vena Cava, Percutaneous Endoscopic Approach [Same-MDC procedure]: “Insertion of Monitoring Device into Superior Vena Cava, Percutaneous Endoscopic Approach — grouped in MDC 16, the procedure side of this code's crossing.” · check together
  • 02HV4DZ — Insertion of Intraluminal Device into Superior Vena Cava, Percutaneous Endoscopic Approach [Same-MDC procedure]: “Insertion of Intraluminal Device into Superior Vena Cava, Percutaneous Endoscopic Approach — grouped in MDC 16, the procedure side of this code's crossing.” · check together
  • and 1609 more

Index entries

  • Activation, mast cell (disorder) (syndrome), idiopathic[Index term]

Nearest codes (28)

  • D89 — Other disorders involving the immune mechanism, not elsewhere classified [Sibling]
  • D89.0 — Polyclonal hypergammaglobulinemia [Sibling]
  • D89.1 — Cryoglobulinemia [Sibling]
  • D89.2 — Hypergammaglobulinemia, unspecified [Sibling]
  • D89.3 — Immune reconstitution syndrome [Sibling]
  • D89.4 — Mast cell activation syndrome and related disorders [Sibling]
  • D89.40 — Mast cell activation, unspecified [Sibling]
  • D89.41 — Monoclonal mast cell activation syndrome [Sibling]
  • and 20 more

Change history (3)

  • FY2027 — Excludes2 note added [Change history]
  • and 2 more

Nearest Codes in This Family

Official ICD-10-CM classifications closest to D89.42 in its code family, with their registry titles.

  • D89.2 — Hypergammaglobulinemia, unspecified
  • D89.3 — Immune reconstitution syndrome
  • D89.4 — Mast cell activation syndrome and related disorders
  • D89.40 — Mast cell activation, unspecified
  • D89.41 — Monoclonal mast cell activation syndrome
  • D89.43 — Secondary mast cell activation
  • D89.44 — Hereditary alpha tryptasemia
  • D89.49 — Other mast cell activation disorder
  • D89.8 — Other specified disorders involving the immune mechanism, not elsewhere classified
  • D89.81 — Graft-versus-host disease

View all codes in the D89 family