Blood cultures ineffective for management of uncomplicated SSTIs

C. Buddy Creech, MD, MPH

It's typically easier to get somewhere if you know where you're going. This is particularly true in the management of infectious diseases. Knowing the causative organism and its susceptibilities is an essential piece of the puzzle when making antibiotic selections.  But more often than we wish, our cultures remain negative and we are left to choose based on broad, local epidemiology.

In this study, Malone and colleagues sought to define the utility of blood cultures in patients with uncomplicated skin and soft tissue infections. They reason (and rightly so) that if blood cultures are unlikely to yield a pathogen, or if the likelihood of contaminant growth is greater than true pathogen yield, then these should be performed only in those with a greater likelihood of complicated SSTI. Through medical record review they identified an extremely low frequency of positive blood cultures in children with uncomplicated SSTI – less than 1%. This is in contrast to those with complicated SSTI, in which >10% revealed a pathogen. These data are likely to result in considerable cost savings, particularly when one considers the increased inpatient lengths of stay incurred while monitoring cultures.

The data are provocative, and are likely to reassure providers faced with an uncomplicated buttock abscess or thigh boil. In these situations, abscess cultures are the gold standard and blood cultures may be unlikely to add additional information. However, two important caveats remain. The first is that clinical acumen and general patient characteristics should always govern our laboratory testing - such is the basis for pre-test probabilities. Therefore, an ill appearing child with a buttock abscess is still likely to have a blood culture obtained, and these data would not yet draw this practice into question. The second, and maybe most methodologically important, is that over half of all children in this study had received at least one dose of systemic antimicrobials at the time of culture. 

The authors address this as a limitation of the study, and we should recognize that this may, in some situations, be sufficient to render a blood culture negative. Nevertheless, the yield from blood cultures in patients with uncomplicated SSTI appears to be remarkably low; therefore, one approach that many of us follow is to limit blood cultures to those with complicated SSTI and those in whom a clinical suspicion of bacteremia exists. This may be exactly the roadmap needed to arrive at a microbiologic diagnosis.