Differential diagnosis of the patient with unexplained flushing/anaphylaxis.

In unusual cases of flushing and anaphylaxis, and after the elimination of the more obvious causes of anaphylaxis or those that may be evaluated by readily available techniques, it is possible to confront a limited and difficult differential diagnosis, which includes idiopathic flushing, anaphylaxis, and neoplastic syndromes associated with mastocytosis and carcinoid tumor. Interestingly, there are rather few features that distinguish one of these possibilities from another. However, the presence of allergic signs and symptoms tend to favor the diagnosis of recurrent idiopathic anaphylaxis; and right-sided valvular heart disease, the presence of excessive 5-HIAA in the urine, and a response to somatostatin favor the diagnosis of carcinoid syndrome. The distinguishing features of mastocytosis include the presence of characteristic skin lesions and diagnostic histopathologic findings on bone marrow biopsy. Counts of absolute mast cell numbers in the skin are less helpful. Following such guidelines, it is often possible to focus on the most likely diagnosis, be it idiopathic anaphylaxis, benign cutaneous flushing, mastocytosis, or carcinoid tumor.