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Language matters: Mast cell terminology

This is by no means a comprehensive list – just a review on definitions of some commonly confused terms.

Acute: This word gets used a lot when people mean “severe.” Acute does not mean severe. It means sudden onset or having a short, limited course. For example, stage III anaphylaxis is an acute complication of mast cell disease. Its symptoms come on suddenly, require immediate treatment, and once treated, resolves. (I am not referring to the after effects of anaphylaxis – just the emergency and treatment.) In a medical sense, acute is the opposite of chronic.

Chronic: Long term, occurs all the time, is expected to occur forever. I have mast cell disease and am chronically ill. I have acute anaphylactic emergencies.

Progressive: Getting worse or will get worse. This term gets used loosely by patients to mean that their symptoms get worse. Medically speaking, this generally refers to progression of disease from one stage to the next, like SM to ASM. SM and MCAS are not inherently progressive diseases. People who have progressed from SM to SSM or ASM have progressive disease.


Systemic symptoms: Any symptoms that do not involve the skin. Can be present in cutaneous mastocytosis or MCAS. So diarrhea is a systemic symptom. Tachycardia is a systemic symptom. Systemic symptoms do not mean you have SM.

Systemic mastocytosis: the diagnosis you receive if you meet either the major criterion listed subsequently and at least 1 of the 4 minor criteria, or at least 3 minor criteria if the major criterion is not met:

Major criterion

Multifocal, dense infiltrates of mast cells (≥15 mast cells in aggregates) detected in sections of bone marrow and/or other extracutaneous organ(s)

Minor criteria

In biopsy sections of bone marrow or other extracutaneous organs, >25% of the mast cells in the infiltrate are spindle-shaped or have atypical morphology, or, of all mast cells in bone marrow aspirate smears, >25% are immature or atypical

Detection of an activating point mutation at codon 816 of KIT in bone marrow, blood, or other extracutaneous organ

Mast cells in bone marrow, blood, or other extracutaneous organ express CD2 and/or CD25 in addition to normal mast cell markers

Serum total tryptase persistently exceeds 20 ng/mL (unless there is an associated clonal myeloid disorder, in which case this parameter is not valid)

The diagnosis of SM is unrelated to the symptoms the patient experiences. Some SM patients have no symptoms. Some have severe symptoms.

Systemic symptoms ≠ systemic disease (SM)


Aggressive symptoms: Frequent or severe symptoms, which may be life threatening.

Aggressive disease: Doctors sometimes use this term to mean a quick progression of symptoms or rapid change in quality of life.

Aggressive systemic mastocytosis: A diagnosis that indicates multiple organ infiltration and damage by mast cells. Lifespan is significantly shortened in many patients. It is diagnosed by already meeting the criteria for SM and then also having at least one C finding, listed here:

Bone marrow dysfunction manifested by one or more cytopenia (ANC < 1.0 × 109/l, Hb < 10 g/dl, or platelets < 100 × 109/l), but no frank non-mast cell haematopoietic malignancy

Palpable hepatomegaly with impairment of liver function, ascites and/or portal hypertension

Skeletal involvement with large-sized osteolysis and/or pathological fract

Palpable splenomegaly with hypersplenism

Malabsorption with weight loss due to GI mast cell infiltrates

Aggressive symptoms and aggressive disease ≠ aggressive systemic mastocytosis (ASM)


Smoldering systemic mastocytosis (SSM): A progression from SM with markers that indicate likelihood of developing ASM. Diagnosed if two or more of the following B findings are present with previous diagnosis of SM:

Bone marrow biopsy showing > 30% infiltration by mast cells (focal, dense aggregates) and/or

serum total tryptase level > 200 ng/ml


Signs of dysplasia or myeloproliferaion in non-mast cell lineage, but insufficient criteria

for definitive diagnosis of a haematopoietic neoplasm by WHO, with normal or only slightly

abnormal blood counts


Hepatomegaly without impairment of liver function, and/or palpable splenomegaly without

hypersplenism, and/or palpable or visceral lymphadenopathy