Increased urinary methylimidazoleacetic acid (MelmAA) as an indicator of systemic mastocytosis

The urinary excretion of histamine and its main metabolite, methylimidazoleacetic acid (MelmAA), was determined in 25 adult patients with the clinical diagnosis of urticaria pigmentosa (UP). Extensive clinical and laboratory investigation, including skin histology, bone marrow examination and scintigraphy of skeleton, liver and spleen, implied systemic manifestations in 16 cases. All patients with systemic mastocytosis (SM) excreted abnormal amounts of MelmAA (greater than 4.1 mg/24 h) and most of them 8.0 mg or more per day, while histamine excretion was increased in only nine (greater than 40 microgram/24 h). Thus, the urine content of MelmAA, but not histamine, could differentiate between UP and SM. Severe pruritus was found concomitant with increased urinary MelmAA and indicated systemic mastocytosis.