Category 2 (FIG. 1)
The skin has 2 surface sections, the dermis and epidermis.
The epidermis is not part of the dermis and actually not connected. The two are held together by finger like projections. Friction (such as rubbing the heel or sacrum on bed or chair through movement) can pull these two fragile areas apart. If the outer layer stays intact, fluid floods between the two sections creating a blister. If the two sections are torn apart (as in friction) then the lower layer (dermis) is exposed. Nerve endings will be open to the air, causing pain.
A blister may contain some blood if the injury is severe and this could turn the blister black. This is one reason why a pressure ulcer should never be categorised until the wound bed can be visualised. In this case, the blister roof will naturally debride and the wound would be healed beneath. However, while black, it is possible that it could be reported as a possible category 3 or 4. This would be an error. It should be ungradable.
Category 2 will not generally contain slough or dead tissue. If it does contain slough and the wound bed cannot be visualised, then it should be categorised as ungradable.
Category 2 may heal spontaneously under the blister. Do not remove the blister roof (which may turn black).
Remember this stage is more painful than the deeper pressure ulcers as the nerves are exposed.