Blood cultures are positive in about 4% of all comers with cellulitis. In a prospective, observational, single-center study, researchers assessed the rate of positive blood cultures in admitted patients with skin and soft tissue infections, and in the subsets with injection drug use or fever. Importantly, the investigators did not determine who would undergo blood cultures.
Of 246 admitted patients with skin infection, 86 (35%) had blood cultures, and 7.0% of cultures were positive. Among the 29 blood-cultured patients who were febrile, 3.5% had positive blood cultures. Of 101 admitted patients who were injection drug users, 46% had blood cultures, and of these, 8.7% were positive.
This study is difficult to interpret because the investigators did not determine who underwent blood cultures, and it is likely that sicker patients were the ones who did; moreover, inclusion of only admitted patients enriched the sample for sicker patients. Therefore, the results represent upper limits for blood culture positivity.
Blood cultures should never be done for patients with uncomplicated cellulitis, whether admitted or not, and fever is not an indication. For admitted patients with cellulitis and injection drug use, blood cultures may be a reasonable option. Don't forget that cellulitis complicating lymphedema often results in bacteremia, and cultures are indicated in this group. Of course, blood cultures are indicated for septic patients and those with suspected necrotizing soft-tissue infection.