Prussian blue is a common stain used by pathologists to detect the presence of iron in biopsy specimens, such as deposits of storage ferritin in bone marrow biopsy samples.
The original stain formula, known historically (1867) as "Perls' Prussian blue" after its inventor, German pathologist Max Perls (1843-1881), used separate solutions of potassium ferrocyanide and acid to stain tissue (these are now used combined, just before staining). Ferric iron deposits in tissue (present mostly as ferric iron within the storage protein ferritin) then react with the soluble ferrocyanide in the stain, to form insoluble Prussian blue dye (a complex hydrated ferric ferrocyanide substance) in situ (i.e., in place). They are then visualizable microscopically as blue or purple deposits, within cells. The formula is also known as Perls Prussian blue and (incorrectly) as Perl's Prussian blue.