Benign idiopathic hypereosinophilia: a feeble masquerader or a smouldering form of the hypereosinophilic syndrome?

In a patient with long-standing idiopathic hypereosinophilia with no apparent organ damage we measured serum eosinophil cationic protein (ECP) and eosinophil protein X (EPX) titers, activated circulating eosinophil rates (by means of monoclonal antibodies EG1 and EG2), and the release of ECP and EPX in vitro by leukocytes at different cultures stages in order to detect possible functional abnormalities associated with hypereosinophilia. Our patient had elevated serum levels of both ECP and EPX, together with a high EG2 count, which would suggest eosinophil activation. However, serum levels of ECP and EPX were not significantly high in relation to the total number of eosinophil cells, although they were more numerous than in healthy controls. Moreover, the release of intracytoplasmic basic proteins by the patient's eosinophils was poor even after in vitro stimulation. Since hypereosinophilic syndrome (HES) with organ damage can appear as long as 8-9 years after the presence of a hypereosinophilic state, the absolutely benign nature of our patient's condition still cannot be defined. Thus, there is the possibility it could be slow-onset or smoldering HES.