Diagnosis

The diagnosis of cellulitis is made by inspection. Laboratory studies including a WBC count, blood culture, and aspirate culture are obtained for specific indications: immunocompromise, fever, severe local infection, facial involvement, and failure to respond to therapy.

The WBC count is normal in most cases of infection due to S. aureus or S. pyogenes,, which are locally invasive. On the other hand, cellulitis due to H. influenzae results from bacteremia and is usually accompanied by a polymorphonuclear leukocytosis. In one study of children with cellulitis, the WBC count was over 15,000/pL in 3 of 4 children infected with H. influenzae, and 0 of 19 infected with S. aureus or S. pyogenes.6 Among 194 patients with H. influenzae cellulitis reported in the literature as reviewed in 1983, the WBC count was greater than 15,000/pL in 84 percent, with a mean of 20,850/pL. 7

The blood culture is usually negative in infections due to S. aureus and S. pyogenes. On the other hand, H. influenzae as a rule causes a bacteremic infection.

Aspirate cultures are best obtained close to the center of an infected lesion, as the periphery may consist primarily of edema fluid devoid of organisms. The needle should be sufficiently large to permit the evacuation of purulent material—22 gauge for the face and 19 gauge for the trunk and extremities. Using a 5- or 10-mL syringe prefilled with 1 mL of sterile, nonbacteriostatic saline, the needle is directed into the subcutaneous tissue to a depth of approximately 0.5 to 1.0 cm, and aspiration is attempted. If there is no return, the saline is injected and reaspirated. The material obtained is used for culture and Gram stain.

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