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

Code

D89.42

Description

Idiopathic mast cell activation syndrome

HCC (V28)

No CMS-HCC V28 mapping

RAF reference coefficient

—

Billable

Yes

Payment year

2026

HCC Category Mapping

These are source-labeled model coefficients, not member totals. A category may still be removed by hierarchy or model cleanup rules. Weights are not directly comparable across models: CMS-HCC V28 and V24 use Community, Non-Dual, Aged; ESRD uses the dialysis continuing-enrollee model; RxHCC is the Part D continuing-enrollee, non-low-income, aged weight (a larger scale than CMS-HCC). ACA/HHS has no single weight — it varies by metal level. Actual per-patient RAF contribution depends on member context, hierarchy and cleanup rules, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains available for historical review.

Work D89.42 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →

MEAT review for D89.42

What This Code Means

HCC Buddy maintains structured V28 and V24 mapping, source-labeled coefficient references, and MEAT documentation criteria for D89.42 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

• •This is a diagnosis of exclusion; ensure secondary causes have been ruled out before coding
• •Document specific symptoms and triggers to support medical necessity

Clinical Significance

Idiopathic mast cell activation syndrome is diagnosed when patients have episodic mast cell mediator release symptoms meeting diagnostic criteria but lack evidence of clonal mast cell disease or an identifiable secondary cause. It is the most common form of mast cell activation syndrome and can cause significant morbidity with recurrent anaphylaxis, flushing, gastrointestinal symptoms, and cardiovascular instability. Management focuses on anti-mediator therapy and trigger avoidance.

Documentation Requirements

• ✓Documentation must demonstrate that the condition meets consensus diagnostic criteria: episodic symptoms consistent with mast cell activation affecting two or more organ systems, laboratory evidence of mast cell activation (elevated tryptase, urinary histamine metabolites), and response to anti-mediator therapy.
• ✓Documentation should confirm exclusion of both clonal mast cell disease (bone marrow biopsy negative for KIT mutations) and identifiable secondary causes.

Commonly Confused Codes

• •D89.41 (Monoclonal mast cell activation syndrome) requires clonal mast cell markers.
• •D89.43 (Secondary mast cell activation) is used when a causative condition is identified.
• •T78.2 (Anaphylactic shock, unspecified) captures acute anaphylactic episodes but not the underlying chronic syndrome.
• •L50.1 (Idiopathic urticaria) may overlap symptomatically but is a distinct and narrower diagnosis.

Child Codes