Benzidine stain for the histochemical detection of hemoglobin in splinter hemorrhage (subungual hematoma) and black heel.

Minor nail trauma may cause bluish discoloration of the nail, while tangential skin trauma on the heel can result in a so-called black heel. To rule out melanoma in such clinical situations, a biopsy is needed to reveal homogeneous eosinophilic masses deposited under the nail plate or within it (transepidermal elimination). Most dermatopathologists attempt to demonstrate the presence of hemoglobin in these eosinophilic masses with Prussian blue stain, which typically remains negative. In our experience, these traumatically induced blood deposits are always situated in avascular spaces, devoid of degrading phagocytes. Consequently, a histochemical stain for these deposits should be directed specifically toward hemoglobin, not hemosiderin. In the dermatopathologic literature, the various techniques to detect hemoglobin deposits in tissue sections are not well-known. We would like to emphasize benzidine stain, a highly selective and efficient method to demonstrate the presence of hemoglobin deposits in histologic sections. To date, benzidine stain has not been utilized to characterize splinter hemorrhage (subungual hematoma). Of concern, the use of benzidine in histopathology laboratories is restricted because this agent is a known carcinogen, while the non-mutagenic derivative, 3,3',5,5'-tetramethylbenzidine, does not stain histologic sections. Patent blue V, a completely different and less specific agent, stains hemoglobin an intense blue-green.