D89.40 ICD-10-CM Code: Mast cell activation, unspecified

Coding instructions

Official Tabular Instructional Notes: the inclusion, exclusion and sequencing notes published for D89.40 in the official ICD-10-CM tabular list, quoted as published.

Source: CMS/CDC — ICD-10-CM Tabular ListRelease: FY2026Effective: October 1, 2025

Notes without a marker are published on D89.40 itself; “inherited from” names the category or block whose note applies here.

This page already reflects the FY2027 tabular note taking effect October 1, 2026.

Coder workflow for D89.40

MedCoder structured workflow — derived from this code’s own official record

Before you code D89.40

1. Unspecified does not mean incorrect. When the record gives no greater specificity, D89.40 may be the appropriate code. Check the record for detail that supports a more specific sibling. An unspecified code is for records that do not provide the detail a more specific code needs; a query, not an assumption, is the route to specificity (Guidelines I.A.9.b, I.B.18).

   ReviewD89.41, D89.42, D89.43, D89.44, D89.49

   See the relationships section · Guide: Other vs unspecified (NEC vs NOS) →

2. Check the Excludes1 note: if the documentation supports a condition named there, do not simply proceed with D89.40. Excludes1 conditions are not reported together with this code unless the record shows they are unrelated (Guidelines I.A.12.a).

   See the official tabular notes

Choose the right path

1. Does the record document the detail a more specific sibling code needs?
   Yes → Review the specific siblings in this subcategory.
   No → Continue — D89.40 is appropriate when the documentation goes no further.

   ReviewD89.41, D89.42, D89.43, D89.44, D89.49

2. Does the documentation support a condition named in D89.40’s Excludes1 note?
   Yes → Do not simply proceed: review the excluded code. Both are reported only when the record shows the two conditions are unrelated.
   No → Continue.

   ReviewC96.21, Q82.2, D47.01, D47.02, C96.2, C96.29

Consider D89.40. Then confirm the code is valid for the date of service in the Verify section.

Documentation check

The provider’s diagnostic statement

Codes are assigned from the provider’s documented diagnosis, not from clinical criteria, test values or a medication list (Guidelines I.A.19).

Any detail beyond this code’s title

What the record states that a more specific sibling code would capture — or its absence, which itself supports the unspecified code.

Official instructions as workflow

• Excludes1 — check before selecting D89.40(15 notes)

  Coding workflow: If the documentation supports a condition named in this note, do not simply proceed with D89.40: the two are not reported together. The one exception is when the record shows the two conditions are unrelated to each other.

  CompareC96.21, Q82.2, D47.01, D47.02, C96.2, C96.29

  See the official tabular notes · Guidelines I.A.12.a

• Excludes2 — not part of D89.40(2 notes)

  Coding workflow: The conditions named in this note are not included in D89.40. When the record documents both, both may be reported; the note is a boundary, not a prohibition.

  CompareT86, B20

  See the official tabular notes · Guidelines I.A.12.b

Coding decision scenarios

Pattern scenarios for this code’s structure — decision rules, not clinical cases

Every row is derived from this code’s own record — its title, tabular notes, 7th-character family and same-category siblings — with fixed MedCoder wording; nothing is inferred about a patient. The official notes and guideline text are in the sections each row links to, and they control.

Indexed Clinical Terms (1)

Official source data — entries quoted as published, in the Index’s own lookup phrasing

Clinical term phrases from the official ICD-10-CM Index to Diseases and Injuries that map to this code. These are alphabetic-index entries shown as the Index writes them — lookup phrasing, not necessarily the wording of a final diagnosis.

• Activation, mast cell (disorder) (syndrome)

Official Coding Guidelines

No excerpt in the ICD-10-CM Official Guidelines names this code specifically. Its chapter carries only this chapter-wide note:

Don’t confuse D89.40 with…

MedCoder analysis — pairs from this code’s official notes, distinctions written by MedCoder

The codes most likely to be picked instead of D89.40 — named in its own official notes, or sharing its laterality or encounter family — with what separates them and which documentation moves the case. Each links to the full comparison.

• C96.21— Excludes1 conflict (Excludes1 — generally not reported together)

  Not coded here — the tabular list excludes this pair for the same encounter.

  Not reported together with this code, except where the two conditions are documented as unrelated.

  What moves you there: Documentation of aggressive systemic mastocytosis moves the case to C96.21; the two are not reported together unless documented as unrelated.

  Full comparison: D89.40 vs C96.21 →

• Q82.2— Excludes1 conflict (Excludes1 — generally not reported together)

  Not coded here — the tabular list excludes this pair for the same encounter.

  Not reported together with this code, except where the two conditions are documented as unrelated.

  What moves you there: Documentation of congenital cutaneous mastocytosis moves the case to Q82.2; the two are not reported together unless documented as unrelated.

  Full comparison: D89.40 vs Q82.2 →

• D47.01— Excludes1 conflict (Excludes1 — generally not reported together)

  Not coded here — the tabular list excludes this pair for the same encounter.

  Not reported together with this code, except where the two conditions are documented as unrelated.

  What moves you there: Documentation of (non-congenital) cutaneous mastocytosis moves the case to D47.01; the two are not reported together unless documented as unrelated.

  Full comparison: D89.40 vs D47.01 →

• D47.02— Excludes1 conflict (Excludes1 — generally not reported together)

  Not coded here — the tabular list excludes this pair for the same encounter.

  Not reported together with this code, except where the two conditions are documented as unrelated.

  What moves you there: Documentation of (indolent) systemic mastocytosis moves the case to D47.02; the two are not reported together unless documented as unrelated.

  Full comparison: D89.40 vs D47.02 →

• C96.2— Excludes1 conflict (Excludes1 — generally not reported together)

  Not coded here — the tabular list excludes this pair for the same encounter.

  Not reported together with this code, except where the two conditions are documented as unrelated.

  What moves you there: Documentation of aggressive systemic mastocytosis moves the case to C96.2; the two are not reported together unless documented as unrelated.

  Full comparison: D89.40 vs C96.2 →

• C96.29— Excludes1 conflict (Excludes1 — generally not reported together)

  Not coded here — the tabular list excludes this pair for the same encounter.

  Not reported together with this code, except where the two conditions are documented as unrelated.

  What moves you there: Documentation of malignant mastocytoma moves the case to C96.29; the two are not reported together unless documented as unrelated.

  Full comparison: D89.40 vs C96.29 →

Relationships are read from this code's own official notes, its laterality siblings and its encounter family; the wording of each distinction is MedCoder editorial, and the documentation named in each “what moves you there” line is the official note phrase or sibling title itself. Source: CMS ICD-10-CM tabular instructional notes; ICD-10-CM Official Guidelines, Section I.A.12.a.

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 in Claim Check.

Relationships & Classification

MedCoder structured relationships — computed from published CMS and AHRQ datasets

Other codes that name D89.40 or its code family, from the CMS ICD-10-CM tabular instructional notes. Tabular-note edges are stored at the code family level that carries each note.

Referenced by 1 Excludes1 note: D72 — Other disorders of white blood cells (via D89.-).

These codes’ tabular lists mark this diagnosis as mutually exclusive — not reported together unless the conditions are documented as unrelated.

MS-DRG Grouper Relationships (FY2026)

Potential MS-DRG participation — not a DRG assignment.

FY2026 MS-DRG: not on the CMS CC/MCC list — as a secondary diagnosis this code does not change MS-DRG severity for that release.

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.

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.

Related Codes

Same clinical category (CCSR)

AHRQ's Clinical Classifications Software groups these diagnoses under the same clinical category (Immunity disorders).

D86.84 — Sarcoid pyelonephritis, D86.85 — Sarcoid myocarditis, D86.86 — Sarcoid arthropathy, D86.87 — Sarcoid myositis, D86.89 — Sarcoidosis of other sites, D86.9 — Sarcoidosis, unspecified, D89.0 — Polyclonal hypergammaglobulinemia, D89.1 — Cryoglobulinemia, D89.2 — Hypergammaglobulinemia, unspecified, D89.3 — Immune reconstitution syndrome, D89.41 — Monoclonal mast cell activation syndrome, D89.42 — Idiopathic mast cell activation syndrome, D89.43 — Secondary mast cell activation, D89.44 — Hereditary alpha tryptasemia, D89.49 — Other mast cell activation disorder, D89.810 — Acute graft-versus-host disease, D89.811 — Chronic graft-versus-host disease, D89.812 — Acute on chronic graft-versus-host disease, D89.813 — Graft-versus-host disease, unspecified, D89.82 — Autoimmune lymphoproliferative syndrome [ALPS], +64 more

Contextual Map

Every relationship of D89.40 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.

Run D89.40 with this related code in Claim Check

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]— CMS ICD-10-CM tabular structure · icd10cm-fy2026

Referenced by Excludes1 notes

• D72 — Other disorders of white blood cells[Excludes1](via D89.-): “immunity disorders (D80-D89)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026

Clinical classification (CCSR)

• BLD008 — Immunity disorders[CCSR]— AHRQ CCSR for ICD-10-CM Diagnoses (HCUP)

MS-DRG Grouper

• DRG 814 — RETICULOENDOTHELIAL AND IMMUNITY DISORDERS WITH MCC[MS-DRG]: “RETICULOENDOTHELIAL AND IMMUNITY DISORDERS WITH MCC (MDC 16)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026
• DRG 815 — RETICULOENDOTHELIAL AND IMMUNITY DISORDERS WITH CC[MS-DRG]: “RETICULOENDOTHELIAL AND IMMUNITY DISORDERS WITH CC (MDC 16)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026
• DRG 816 — RETICULOENDOTHELIAL AND IMMUNITY DISORDERS WITHOUT CC/MCC[MS-DRG]: “RETICULOENDOTHELIAL AND IMMUNITY DISORDERS WITHOUT CC/MCC (MDC 16)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026

MDC crossing

• 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. 1,614 same-MDC procedures group here; browse them on the MDC page.”— CMS MS-DRG Definitions Manual · FY2026

Index entries

• Activation, mast cell (disorder) (syndrome)[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026

Nearest codes (28)

• D89 — Other disorders involving the immune mechanism, not elsewhere classified[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• D89.0 — Polyclonal hypergammaglobulinemia[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• D89.1 — Cryoglobulinemia[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• D89.2 — Hypergammaglobulinemia, unspecified[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• D89.3 — Immune reconstitution syndrome[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• D89.4 — Mast cell activation syndrome and related disorders[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• D89.41 — Monoclonal mast cell activation syndrome[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• D89.42 — Idiopathic mast cell activation syndrome[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• and 20 more

Change history (3)

• FY2027 — Excludes2 note will be added[Change history]— CMS release files (code change ledger) · icd10cm-fy2027
• and 2 more

Sources for this page

Codes, titles, notes, index terms and mappings on this page are transcribed from the datasets below. Relationships MedCoder computed and text MedCoder wrote are labelled where they appear.

Code, title, tabular notes and index terms Official source data

CMS/CDC ICD-10-CM FY2026 tabular list, index and tables, effective October 1, 2025 Release, file and checksum · Publisher’s page

Coding guidelines Official source data

ICD-10-CM Official Guidelines for Coding and Reporting (FY2026), quoted by section Release, file and checksum · Publisher’s page

Claim edits Official source data

CMS Definitions of Medicare Code Edits — v43.1 (April 2026) Release, file and checksum · Publisher’s page

Inpatient payment groups Official source data

CMS MS-DRG Definitions Manual (incl. Appendix B diagnosis index, Appendix C CC/MCC list, Appendix E procedure index) and IPPS Final Rule tables — v43 Release, file and checksum · Publisher’s page

Change history and date-of-service validity Official source data

CMS ICD-10-CM release addenda, ingested release by release into the change ledger Release, file and checksum · Publisher’s page

Comparisons, relationships and the contextual map MedCoder structured relationship

Computed by MedCoder from the tabular notes and tables above; every derived relationship is marked as derived where it appears

Summary and FAQ answers MedCoder explanation

Written by MedCoder to explain the sources above: drafted with AI assistance, checked by a person against the release files, and labelled as MedCoder text where it appears. Not official text.

Labels on this page: Official source data · MedCoder structured relationship · MedCoder explanation. How to read the labels · All data sources and release dates

Nearest Codes in This Family

Official ICD-10-CM classifications closest to D89.40 in its code family, with their registry titles.

• D89.0 — Polyclonal hypergammaglobulinemia
• D89.1 — Cryoglobulinemia
• D89.2 — Hypergammaglobulinemia, unspecified
• D89.3 — Immune reconstitution syndrome
• D89.4 — Mast cell activation syndrome and related disorders
• D89.41 — Monoclonal mast cell activation syndrome
• D89.42 — Idiopathic mast cell activation syndrome
• D89.43 — Secondary mast cell activation
• D89.44 — Hereditary alpha tryptasemia
• D89.49 — Other mast cell activation disorder

View all codes in the D89 family