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Repeating blood cultures after initial bacteremia: When and how often?

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Repeat cultures are indicated in specific scenarios, but for most patients, frequent and indiscriminate repetition after an initial positive culture is unnecessary and may be associated with excessive use of resources. Prospective studies and practice guidelines are needed to help further define the indications.

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THE TENDENCY TO REPEAT CULTURES

Current literature lacks strong evidence for repeating previously positive blood cultures collected appropriately—ie, 10 mL of blood for aerobic culture and 10 mL for anaerobic culture from 2 different sites, and a positive result from both sets. However, because of the risk of serious complications of bacteremia, particularly in critically ill patients, many clinicians order multiple, repeated sets of blood cultures.

Tabriz et al1 found that one-third of hospitalized patients got repeat cultures after an initial set, regardless of the result of the first set. Most (83.4%) of those cultures yielded no growth, 9.1% grew the same pathogen, and 5.0% were contaminated. Finding a new pathogen was rare, occurring in only 2.5% of repeated cultures.

Wiggers et al2 reported an even higher number of repeat cultures ordered for patients who had an initially positive culture: 38.9%.2 And in another study,3 half of the patients received more than 2 consecutive cultures.

Drawbacks

Unrestrained ordering of repeat blood cultures can increase the risk of a false-positive result, leading to more cultures, echocardiography, other imaging tests, and unnecessary antimicrobial therapy, all of which puts patients at risk of adverse effects of treatment and missed alternative diagnoses and increases the length and cost of hospitalization.4

Advantages

On the other hand, repeat blood cultures may increase the diagnostic yield for conditions such as infective endocarditis and may have implications for the duration of antibiotic therapy.1 The duration of therapy for bacteremia is usually determined from the last negative culture; hence, documenting clearance of bacteremia can determine a precise end-date for antibiotic therapy.

Bacteremia due to Staphylococcus aureus and to endovascular and epidural sources has been found to be independently associated with persistent bacteremia, detected in 6.6% of 1,801 index cases of bacteremia in a retrospective cohort study.2 An endovascular source (adjusted odds ratio [OR] 7.66, 95% confidence interval [CI] 2.30–25.48), an epidural source (adjusted OR 26.99, 95% CI, 1.91–391.08), and S aureus bacteremia (adjusted OR 4.49, 95% CI 1.88–10.73) were independently associated with persistent bacteremia. Escherichia coli (5.1%, P =  .006), viridans group streptococci (1.7%, P =  .035), and beta-hemolytic streptococci (0%, P = .028) were associated with a lower likelihood of persistent bacteremia. Patients with persistent bacteremia were less likely to have achieved source control within 48 hours of the index event (29.7% vs 52.5%, P < .001).2

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Follow-up blood cultures are often needed after bacteremia

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