Cellulitis is usually diagnosed based on its typical appearance. Sometimes doctors check a blood count to see if the white blood cells are elevated, meaning the immune system is fighting off an infection. Many times we don't see an elevated white blood cell count even though the immune system is busy fighting the infection. In people who are very sick, blood cultures might be done to see if bacteria have gotten into the blood stream. A cellulitis infection doesn't have pus that could be cultured to see what bacteria are involved.