Eosinophil fluorescence: a cause of false positive slide tests for herpes simplex virus

Two neonates who had different vesicular eruptions were erroneously diagnosed as having herpes simplex infections because of false positive results of herpes simplex fluorescent antibody slide tests. The infants, who proved to have incontinentia pigmenti and histiocytosis X, respectively, had multiple vesicular lesions containing numerous eosinophils. Eosinophils are known to show nonspecific fluorescence in immunofluorescent techniques because the fluorescein isothiocyanate label of the antiserum binds with the basic proteins in the eosinophilic granules. Clinicians should be aware that nonspecific eosinophil fluorescence may result in false positive results of immunofluorescent slide tests.