What is it about?
Corynebacterium is a group of bacteria commonly found on human skin. When detected in blood cultures, it can represent either a true bloodstream infection or contamination introduced during blood sampling — a distinction that has major implications for patient management and antibiotic use. Time to positivity (TTP) is the time it takes from the start of blood culture incubation to a positive detection signal. TTP has been used to help differentiate true bacteremia from contamination for other bacteria, but data specific to Corynebacterium have been lacking. In this retrospective study conducted at a tertiary care hospital in Japan, we analyzed 165 cases in which Corynebacterium was isolated from blood cultures between 2014 and 2022. Cases were classified into a true-bacteremia group (n = 77) and a contamination group (n = 88) based on clinical and microbiological criteria. We compared TTP between the two groups and evaluated its diagnostic performance using receiver operating characteristic curve analysis. The median TTP of the true-bacteremia group (26.8 hours) was significantly shorter than that of the contamination group (43.3 hours, P < 0.0001). At a TTP threshold of 31.2 hours, sensitivity and specificity for diagnosing true bacteremia were 70.1% and 81.8%, respectively (AUC = 0.780). A TTP above 69.4 hours showed 96.1% sensitivity for ruling out true bacteremia, while a TTP at or below 25.0 hours showed 95.5% specificity for confirming it. Lipophilic Corynebacterium species had significantly longer TTP than non-lipophilic species.
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Why is it important?
Unnecessary antibiotic treatment for blood culture contamination is a common clinical problem that contributes to antimicrobial resistance and adverse drug effects. This is the first study to specifically characterize TTP for Corynebacterium in blood cultures. The finding that TTP exceeding 69.4 hours makes true bacteremia unlikely — particularly in cases with only one set of positive blood cultures — provides a practical, readily available tool to support clinical decision-making. Importantly, no case of confirmed true bacteremia in the single-positive-set subgroup showed a TTP above 69.4 hours, suggesting this threshold may be especially useful in ambiguous single-set scenarios.
Perspectives
TTP above 69.4 hours can serve as a useful signal to suggest contamination rather than true infection when Corynebacterium is detected in a single blood culture set, potentially reducing unnecessary antibiotic exposure. However, TTP alone is insufficient to confirm true bacteremia, and clinical and microbiological data — including signs and symptoms, number of positive blood culture sets, underlying disease, and presence of medical devices — remain essential for diagnosis. Clinicians and clinical microbiologists should also be aware that lipophilic Corynebacterium species have inherently longer TTP, which should be factored into the interpretation of TTP thresholds. Further multi-center studies are needed to validate these cutoffs in different patient populations and settings.
Dr Naoki Watanabe
Hirosaki University
Read the Original
This page is a summary of: Time to positivity of Corynebacterium in blood culture: Characteristics and diagnostic performance, PLOS One, December 2022, PLOS,
DOI: 10.1371/journal.pone.0278595.
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