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ACEP Now: Vol 33 – No 09 – September 2014

Table 1. How to Reduce False-Positive Blood Cultures

Patients with cellulitis. These patients have a pretest probability for bacteremia of 2 percent, so blood cultures in this population are very low yield.⁹ Numerous studies have documented the predominance of strep and staph species in skin and soft tissues infections; thus when antibiotics are indicated, they may be initiated empirically

Patients with uncomplicated pyelonephritis. Urine cultures are higher yield than blood cultures. In the rare cases where blood cultures identify organisms, they rarely differ from those identified in urine and very rarely alter clinical management.¹⁰

Patients being discharged from the ED. Blood cultures are a test for bacteremia, and bacteremia is a serious condition requiring close monitoring—ergo, you should not send blood cultures on patients being discharged from the ED. Outpatient blood cultures expose you to significant medical legal risk as positive results require contacting the patient and determining if further ED or inpatient care is needed. Like any general rule, there are exceptions: nonacute diseases for which blood cultures are the diagnostic test of choice, such as subacute bacterial endocarditis. If you will be sending blood cultures on an outpatient, be clear who will follow up on the results and make sure you have correct contact information.

Patients with normal vital signs or those with isolated fever and otherwise normal vital signs. The presence of systemic inflammatory response syndrome (SIRS) was found to be 96 percent sensitive for bacteremia among patients who had blood cultures drawn on a general medical service, meaning that the absence of SIRS greatly reduces the likelihood of bacteremia.¹¹

What about patients with sepsis? The Surviving Sepsis Campaign recommends blood cultures as part of the three-hour bundle for patients with severe sepsis or septic shock (based on low-strength evidence).¹² This accounts for a small proportion of the patients with the above-mentioned infections.

What Can You Do to Improve the Cost-Effectiveness of Blood Culture Testing?

Do not send blood cultures on patients for whom you have a low clinical suspicion of bacteremia. Talk to your clinical director about removing blood cultures from routine order sets for patients with community-acquired pneumonia, cellulitis, and urine infections including pyelonephritis. Have an educated discussion with the next admitting physician who asks if you sent cultures on the pneumonia patient you are admitting to the floor; the references for this article should be the evidence they need!

When cultures are sent, be sure they are collected appropriately to minimize contamination (see Table 1). Consider a protocol to have cultures drawn by staff trained in blood culture collection technique, and reinforce best practices with your staff.