Mastocytosis, as the name implies, is a group of disorders caused by excessive mast cell production, and urticaria pigmentosa is a systemic condition. On skin exam, lesions composed of mast cells urticate/flush when rubbed (Darier's sign). Loss of weight and splenomegaly suggest systemic involvement. The bone marrow is the most commonly involved organ (besides the skin).

The infantile type is usually confined to the skin, whereas the adult type affects visceral organs as well. Bone marrow biopsy should be considered in adults with urticaria pigmentosa, elevated serum tryptase (>20ng/ml), or signs and symptoms of systemic involvement. If skin biopsy is performed, metachomatic staining with toluidine blue is used for visualization of mast cells in tissue sections. Other findings include elevated urinary histamine or a history of itching triggered by aspirin or alcohol ingestion.