By Stan Deresinski, MD, FACP, FIDSA
Clinical Professor of Medicine, Stanford University
SYNOPSIS: The yield from repeat blood cultures is very low, suggesting that their general use in most patients should be discouraged.
SOURCE: Zanella MC, de Lorenzi-Tognon M, Fischer A, et al. Bacteremia detection in second or subsequent blood cultures among hospitalized patients in a tertiary care hospital. JAMA Netw Open 2022;5:e228065.
Zanella and colleagues retrospectively examined the clinical value of repeat blood cultures. They analyzed all blood cultures in patients at the Geneva University Hospitals over 12 months ending Jan. 1, 2020. Those included had at least two blood culture sets obtained, with the first obtained within 24 hours after admission and a second set at least 24 hours later. Patients with hematopoietic stem cell transplants (HSCT) were excluded from the analysis.
A total of 23,088 blood culture bottles were obtained during 3,214 care episodes in 2,863 unique patients. Among these, 8.34% of bottles were positive but with 0.29% representing contaminants. The mean time to positivity (TTP) was ≤ 24 hours in 76.8% of the positives and, of the 446 sets for which the mean TTP was > 24 hours, 71.1% were first sets and these had a 14.5% contamination rate. When contaminated blood cultures and those performed on patients with endovascular infection were excluded, the probability of detecting true bacteremia with a second blood culture obtained at least 24 hours after the first was only 2.6%. Of note is that most patients were receiving antibiotics at the time blood cultures were drawn, but the proportion was similar in those with positive and negative results. There also was no significant difference in the frequency of antibiotic receipt in comparing those whose mean TTP was ≤ 24 hours or > 24 hours.
Although HSCT recipients were excluded from the study, those with solid organ transplants were included, and their results did not differ significantly from the larger group.
COMMENTARY
In some settings, blood cultures may be underused, but more generally, in U.S. hospitals they may be performed unnecessarily, and this is especially true regarding repeat blood cultures. This phenomenon is one of the major drivers of the call to implement diagnostic stewardship in the management of infectious diseases.
In this study, obtaining additional blood cultures while the first set was still incubating for > 24 hours resulted in only 2.6% probability of detecting bacteremia when excluding those obtained in patients with endovascular infection and those representing contamination. Results such as these should be taken into account in devising guidelines for the use of blood cultures.
The yield from repeat blood cultures is low, suggesting their general use in most patients should be discouraged.
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