Is staging a pressure ulcer making a medical dx?

I work at a hospital that is saying that nurses are not allowed to stage a pressure ulcer because this is making a medical diagnosis. We are allowed to describe the wound and even go so far as to state we "presume" it to be a stage ? based on the description, but cannot actually stage the pressure ulcer. Because most physicians do not know how to stage a pressure ulcer either-there is no official diagnosis of a pressure ulcer on the medical record in many cases, but our hospital is apparently ok with this as long as the nurses are not staging them. Our hospital says this is according to CMS guidelines, but all I can find from my research is that the physician has to have the pressure ulcer location and stage in the list of diagnosis, not that the nurse cannot stage it. Can anyone confirm if this is actually a CMS guideline or not?