Rationale:

• Determining the specific etiologic agent is important to guide antibiotic choice; antimicrobial sensitivity studies may allow narrower antibiotic coverage or may dictate changing antibiotics if resistance is detected.
• The changing spectrum of antibiotic resistance patterns of both gram-negative and gram-positive organisms makes empiric treatment difficult, but underscores the need for definitive cultures and sensitivities.

Evidence:

• Superficial skin cultures may be misleading because they may not represent the organisms in the underlying infectious process (27) especially for diabetic foot infections (13).
• A review reports that aspiration of the leading edege of erythema or the most intense areas of induration, or even punch biopsy of cellulitis, yields positive cultures in 2% to 40% of cases (33).
• Streptococcal anti-DNAase B and ASO titers are positive in up to 83% of patients with streptococcal pyoderma (34). The former test is considered more sensitive that the latter (35; 36).
• Blood cultures are positive in <5% of cases (18; 33). Among patients with streptococcal toxic shock syndrome, 60% have positive blood cultures (37).
• Streptococcal cellulitis may develop lymphangitic streaking, a raised indurated border, a peau d'orange appearance, as well as vesicles and bullae filled with serous fluid. Cultures of fluid are usually negative (18).
• Streptococcal necrotizing fasciitis may also develop bullae. Cultures of fluid within the bullae will usually be positive.
• S. aureus cellulitis often develops an abscess with purulent fluid. Cultures will be positive.
• Renal impairment and thrombocytopenia can precede vascular collapse and necrotic changes in the skin in necrotizing fasciitis caused by group A streptococcus. Thus, elevated creatinine and CPK, and decreased platelets in a patient with soft tissue infection are early clinical clues that a more severe infection, such as streptococcal toxic shock syndrome with or without necrotizing fasciitis or myonecrosis, is developing (18; 27).
• Plain radiographs of infected areas may be used to detect gas in tissue, but CT scans are more sensitive and specific. MRI scans are more sensitive than CT scans for detecting edema, but they may be less useful in differentiating between cellulitis and necrotizing fasciitis due to low specificity (38; 29; 39; 40).
• The detection of gas in tissue suggests a severe infection that usually requires urgent surgical intervention (18). Radiographic imaging aids in the detection of gas in tissue as well as the depth of involvement, but may not reliably distinguish between cellulitis and necrotizing fasciitis (38).

Comments:

• Although less frequent than streptococcal necrotizing fasciitis, S. aureus may also cause this disease (41).
• In the absence of a defined portal of entry, and without material to culture, determining the exact etiologic agent is difficult. This is likely because streptococcal infection, although highly inflammatory, is paucibacterial. Infection by streptococci produces toxins that can cause a robust inflammatory response.