In my opinion, a skin biopsy is a very valuable tool in medicine. Unfortunately, it is seldom used by many doctors.
A biopsy is a simple procedure that can be done in most cases with mild sedation or light anesthesia combined with a small dose of a local anesthetic. The procedure usually takes about five minutes, and in most cases stitches are not even necessary. Several small pieces of skin are removed from the pet using tiny incisions or punches with a special tool. The pieces of skin are placed in formalin solution and sent to a pathologist. The pathologist stains the biopsies and examines them microscopically. While not always 100% diagnostic, in most cases a skin biopsy is the best test that can be done to determine the cause of your pet's skin condition.
I recommend a skin biopsy in several instances.
First, if the skin lesions look strange and not typical of common skin disorders. Second, any time a pet has not responded to what I think should be the "correct" therapy. Finally, anytime I see a pet with a chronic skin disease (usually several months old) that has not improved or has worsened.
While underutilized, skin biopsies quickly, easily, and inexpensively diagnose a number of problems that cause dermatitis in dogs and cats. If your pet has a chronic dermatitis that will not get better, or has has strange looking skin lesions, I suggest talking with your pet's doctor about using this important test.