Blood Cultures Before Antibiotics Matter

Today’s pearl is why it is important to obtain blood cultures prior to antibiotic administration whenever possible.

The difference a few minutes can make.

Does it really matter? If so, how much?

Cheng et al conducted an interesting study where they drew blood cultures on 325 septic patients prior to receiving antibiotics (31.4% positive), and then repeated cultures within 240 minutes after receiving antibiotics (19.4% positive). Median time between antibiotic and second culture was 70 min. Therefore, obtaining cultures after antibiotic initiation decreased sensitivity of blood cultures by 12%, p < 0.001. Sensitivity dropped by 21% if the culture was obtained between 121-240 minutes after antibiotics.

Take-home point:

Antimicrobial therapy can significantly reduce the sensitivity of blood cultures in sepsis patients, even within an hour after antibiotic administration.


Author: Bryan D. Hayes, PharmD

Attending Pharmacist, Emergency Medicine and Toxicology, Massachusetts General Hospital; Assistant Professor of EM, Harvard Medical School

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