Symptomatically
Cellulitis describes a bacterial infection that occurs in the deeper portions of the skin. The first step in diagnosing cellulitis is an examination from a dermatologist or a specialist in infectious diseases. The physician then looks for the symptoms of cellulitis--which include, as Medline explains, an area of skin that looks tight and glossy. The affected area is also painful and tender. For many patients, cellulitis appears to be a rapidly spreading rash, so the doctor asks about the history of the skin problem. He also takes the patient's medical history, because recent trauma or puncturing of skin in or near the affected area suggests cellulitis.
Bacterial Culture
As the National Institute of Allergy and Infectious Diseases says, one test that can be done to diagnose cellulitis is to take a bacterial culture from the affected portion of the skin. This is especially useful if the cellulitis occurs around a wound. Typically, a sample of fluid (such as blood) is taken from the inflamed area, where it can then be cultured using special growth media. Then the resulting bacteria can be examined and potentially identified by looking at them under a microscope. Not only can a wound culture confirm cellulitis, it can alert physicians as to what bacteria are causing the infection, which can help guide treatment.
Blood Clot Exclusion
Symptoms of cellulitis are similar to that of a blood clot, especially when the cellulitis occurs in the legs. Because wound cultures can take several days to be analyzed, physicians often order tests (such as an angiogram) to rule out a blood clot. This is important because blood clots can break up and travel throughout the body, causing distant blockages called emboli to form. A pulmonary embolism--the result of a blood clot traveling to the lungs--is a medical emergency. As a result, it's important to rule out the possibility of a blood clot while the wound culture is being performed. This is also known as a diagnosis of exclusion, in which conditions that cause symptoms similar to cellulitis are ruled out.
