In many hospitals, blood cultures have become a reflexive habit for patients presenting to the ER with any suspected infection. This was, in part, due to the inappropriate addition of routine blood cultures as a core measure in 2001 by JCAHO for patient’s admitted to the hospital with community-acquired pneumonia. Then, the strong emphasis by the surviving sepsis campaign to include blood cultures as a part of the sepsis bundle strengthened this habit of early blood cultures for all. Luckily, the Joint Commission has since eliminated blood cultures as a core measure for non-ICU community-acquired pneumonia as of January 1, 2014, but the habit had already been developed in many United States emergency rooms. Now, we have to break this habit since it is inappropriate for most patients with non-severe infections.
There have been numerous studies investigating the utility of blood cultures for community-acquired pneumonia (CAP) and the conclusion is that they are of limited utility. The overall yield of blood cultures in CAP is 6-9% and half of these positive blood cultures are false-positives. Blood cultures only lead to broadening of empiric antibiotics 0.5-1% of the time (including ICU admits). The conclusion from most studies is that blood cultures are only indicated in CAP for severe cases requiring ICU admission, septic shock, intravenous drug use (IVDU), asplenia, cirrhosis, leukopenia, active alcohol abuse, cavitary infiltrates, parapneumonic effusions, and HIV disease.
With regards to cellulitis, the vast majority of cases are due to streptococcus or staphylococcus. The rate of positive blood cultures ranges from 5-15%, many of which are false positive blood cultures. In cases of non-severe cellulitis, positive blood cultures led to augmentation of empiric antibiotics only 0.18% of the time. Most studies suggested that blood cultures are only indicated for cellulitis if IVDU, septic shock, suspected necrotizing fasciitis, cellulitis complicating lymphedema, HIV disease, immunosuppression, neutropenia, organ transplant patients, and those with cancer on chemotherapy.
In summary, blood cultures in immunocompetent patients with non-severe infections from urinary tract infections, cellulitis, and CAP should not be done unless risk factors as described above are present. A study in JAMA in 2012 also identified certain predictors of true positive blood cultures in such infections:
- Shaking chills
- Vasopressor use
- Neutrophil/lymphocyte ratio>10
- SIRS physiology
Finally, if blood cultures are obtained, volume matters. There is a much higher yield if at least 7 mL blood is placed in each blood culture vial.