Petechial reaction following patch testing with cobalt

A total of 132 patch tested patients reacted with petechial reactions to cobalt chloride 1% in petrolatum; 23 were retested with various concentrations of cobalt. In about 60% of those retested the petechial reaction could be reproduced. Histopathological examination showed slight perivascular lymphocytic infiltration, swollen endothelium and extravasation of erythrocytes but no signs of vasculitis. It is suggested that in predisposed patients the petechial reaction following patch testing could be the result of primary irritation.