Study: Wait for preantibiotic culture results before ordering additional cultures

By Mary Quirk,
icanNEWS staff

icanNEWS – In patients who are already receiving antibiotics, repeat or follow-up blood cultures performed during the first 72 hours of antibiotic therapy rarely isolate new pathogens, report Christopher Grace, MD, and colleagues in the June 1 issue of Clinical Infectious Diseases.

They say that their study is the first to measure the yield of antibiotic blood cultures for patients who are admitted to the hospital for community-acquired infections or fever. "We found that the results of antibiotic blood cultures could be predicted on the basis of preantibiotic blood culture results and that a low number of pathogens were isolated from antibiotic blood cultures after having been missed in the preantibiotic blood cultures." In only one study patient was a pathogen isolated from culture done during the initial 72 hours of therapy that had not been isolated from preantibiotic culture.

Grace wrote in an e-mail response to icanNEWS that repeat blood cultures can be useful sometimes: "If someone has Staphylococcus aureus bacteremia repeating the blood cultures may help define a continuous bacteremia that would mandate a longer (4 to 6 weeks) of therapy. Also for Staphylococcus epidermidis isolated from the blood culture, repeating the cultures may help differentiate contaminants from a true bacteremia."

A total of 139 patients met eligibility criteria for inclusion in the retrospective study. Reasons for exclusion included: inadequate number of blood cultures, antibiotic use prior to hospital stay, no antibiotic therapy, admission to the intensive care unit, or neutropenia. Most of the patients had comorbid conditions.

Among the 139 patients, 83 (59.7%) had preantibiotic cultures that were either contaminated or negative for pathogens and 56 (40.3%) had preantibiotic cultures that were positive for pathogens (ie, 25 [45%] for staphylococci, 14 [25%] for streptococci, and 17 [30%] for aerobic gram-negative bacilli). None of the 83 patients with contaminated or negative preantibiotic blood cultures had pathogens isolated from antibiotic blood cultures, whereas 26 (46%) of the 56 bacteremic patients had pathogens isolated from the antibiotic blood cultures. "Patients who had staphylococci, streptococci, or aerobic gram-negative bacilli isolated from preantibiotic blood cultures had the same pathogen isolated from antibiotic blood cultures," the article says. S aureus was the most common pathogen isolated from antibiotic blood cultures (18 of 26 cultures).

Only isolation of S aureus from preantibiotic blood cultures and presence of endocarditis predicted pathogen isolation from antibiotic blood cultures, according to the multivariate analysis.

The authors note that current guidelines do not address the utility of obtaining blood cultures once antibiotic therapy has been initiated.

"I think clinical guidelines would be useful, though it will take more (and prospective) studies to help develop these guidelines," Grace wrote in his reply.

First published (May 14, 2001) on icanPREVENT.com. Copyright 2001, ican INC.

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