What is a Periodic Acid Schiff (PAS) reaction?
A PAS reaction is a laboratory test whereby, through a series of oxidation-reduction reactions, polysaccharides, basement membrane material, and mucosubstances are stained bright red. Its utility in dermatopathology is widespread, as it allows for the visualization of the lamina densa of the basement membrane zone (type IV collagen) in diseases where its alteration is of significance such as lupus erythematosus. In addition, because many normally colorless fungi possess carbohydrates within their cell walls or capsules, PAS reaction is an effective means to allow for their identification in tissue. PAS reaction may be performed on cutaneous biopsies, nail plate biopsies (Fig. 1), or biopsies from the deep tissue.
