Often a preliminary diagnosis of necrotizing fasciitis is based on the patient's symptoms, including the medical and exposure history. For example, a diabetic patient with a rectal fistula who develops pain, swelling, and scrotal skin changes may be preliminarily diagnosed with Fournier's gangrene, while another diabetic patient with liver failure and skin changes (bullae) and exposure to seawater may be preliminarily diagnosed with a Vibrio vulnificus infection. Initial treatment is often begun based upon a preliminary diagnosis, because waiting for a definitive diagnosis can delay treatment and result in increased morbidity and mortality. Gram staining of exudates (fluid from the infection site) or biopsied tissue may provide the physician clues to determine what organism(s) is causing the infection. For example, the stain can distinguish between Gram-negative and Gram-positive organisms and further distinguish their shapes (coccus or round, rod, or comma-shaped like Vibrio). Definitive diagnosis depends on isolation of the organism(s) from the patient using both aerobic and anaerobic isolation techniques, and rarely, fungal culture methods.

Organisms isolated from necrotizing fasciitis need to have studies done to determine antibiotic resistance, because many organisms causing the disease are multidrug resistant. A surgeon needs to be consulted early to help obtain tissue samples and to be involved with potential treatment protocols (debridement, amputation). Although X-rays occasionally show gas in tissues, investigators suggest doing Doppler, CT, or MRI studies to help show gas in tissues and to help delineate the extent of the infection. Most physicians run additional tests such as white blood cell counts (elevated in necrotizing fasciitis), BUN (blood urea nitrogen), sodium (both decreased in necrotizing fasciitis), and other tests to monitor the patient.

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