The four stages of pressure sores include intact skin with redness, open ulcers, visible fat, and full tissue loss exposing bone and muscle, according to the National Pressure Ulcer Advisory Panel. The presence of dead skin at the base of the ulcer renders a pressure sore unclassifiable, according to MedlinePlus.
Painful, red skin that does not blanch or turn pale when briefly pressed and released represents stage one, as detailed by WebMD. A crater-like opening emerges in stage two with the presence of fluid-filled blisters. Pain begins to diminish at stage three due to tissue damage. At stage four, bone and tendons are exposed and susceptible to infection.