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How reliable is blood tryptase as a marker of systemic disease in an infant with cutaneous mastocytomas?

A healthy boy presented at 12 weeks with multiple nodular lesions on the buttocks, right ankle, left wrist (Fig. 1) and face, which had been developing since birth. The nodules urticated with gentle stroking (Darier s sign). He developed chronic diarrhoea from the age of 10 months but otherwise remained well. There was no lymphadenopathy or hepatosplenomegaly. Development was normal. There was no family history of mastocytosis or neurofibromas. Skin biopsy under general anaesthetic confirmed the diagnosis of multiple cutaneous mastocytomas at the time of presentation. The common Kit D816V mutation was not detected on lesional skin (estimated sensitivity of the method is 10%). Serum tryptase in blood taken immediately before anaesthesia was raised at 49.2 lg ⁄ L (adult threshold level suggestive of systemic mastocytosis ‡ 20). Full blood count and liver function tests were normal. Faecal microbiology, reducing substances and tissue transglutaminases were normal. Apart from an unexplained transient peak of alkaline phosphatase of predominantly bone origin (2887 U ⁄ L; normal 110–440) at the 1-year follow-up, which returned to normal within 3 months, the patient remained healthy 18 months after presentation. The blood tryptase has reduced to 16.1 lg ⁄ L, 2 years later. Mastocytomas are benign mast-cell tumours in the skin. They urticate spontaneously or after rubbing (Darier s sign). They are the most common cutaneous manifestation of mastocytosis in infancy. Children present with mastocytomas, urticaria pigmentosa or diffuse cutaneous mastocytosis. They usually have low tryptase levels and are negative for the codon D816V mutation in the tyrosine kinase receptor Kit. Mastocytomas carry a good prognosis with spontaneous remission usually occurring before puberty. Systemic mastocytosis generally presents in adults. The signs and symptoms of systemic disease depend on the organ involved and are due to either mast-cell degranulation or infiltrative effects. They include flushing, hypotension, abdominal pain, diarrhoea, dyspepsia, weight loss, pathological fractures, hepatosplenomegaly and bone-marrow failure. The condition is associated with myelodysplasia and leukaemia. Therefore, correct diagnosis and classification of the disease are important for an accurate prognosis. Systemic mastocytosis is defined in the 2001 World Health Organization classification by one major criterion (aggregates of at least 15 mast cells in tissue other than skin) or three or four minor criteria (> 25% spindle-shaped mast cells, D816V mutation, blood tryptase level > 20 ng ⁄ mL, CD25 and CD2 immunophenotype). Sampling of bone marrow is widely performed in adults to diagnose systemic mastocytosis but only performed in children when there is suspicion of aggressive systemic disease or an associated blood disorder due to the invasive nature of the investigation and the need for general anaesthesia. Clinical features of systemic mastocytosis may include developing skin lesions after 2 years of age, hepatosplenomegaly, unexplained lymphadenopathy, peripheral blood abnormalities (including eosinophilia) or persistence of blood tryptase elevation into adulthood. However, it is not known whether a serum tryptase level somewhat exceeding 20 lg ⁄ L is a good indicator of systemic mastocytosis in very young children. The contribution of skin to systemic mast-cell load may become lower with age as the body volume increases proportionately more than surface area. The reduction of blood tryptase in our patient may reflect this change in conjunction with spontaneous resolution of the mastocytomas. The patient s continuing good health suggests that spontaneous recovery is likely, although close follow-up will be essential. This case provides a reminder that bone marrow biopsy in children with cutaneous lesions of mastocytosis should only be undertaken on the strength of a full clinical assessment, rather than on blood tryptase measurements alone.