Although the diagnosis of mastocytosis may be suspected on the basis of clinical and laboratory findings, it can be established only by tissue biopsy (usually bone marrow biopsy). The diagnostic criteria for systemic mastocytosis are shown in Table 156-2. Laboratory studies that can help support a diagnosis of systemic mastocytosis include measurement of urinary or blood levels of mast cell products such as histamine, histamine metabolites, prostaglandin D₂ (PGD₂) metabolites, or mast cell tryptase. Other studies including bone scan, skeletal survey, and GI contrast studies may be helpful. Other flushing disorders (e.g., carcinoid syndrome, pheochromocytoma) should be excluded.