Performance of Gram staining on blood cultures flagged negative by an automated blood culture system

Abstract

Blood is one of the most important specimens sent to a microbiology laboratory for culture. Most blood cultures are incubated for 5-7 days, except in cases where there is a suspicion of infection caused by microorganisms that proliferate slowly, or infections expressed by a small number of bacteria in the bloodstream. Therefore, at the end of incubation, misidentification of positive cultures and false-negative results are a real possibility. The aim of this work was to perform a confirmation by Gram staining of the lack of any microorganisms in blood cultures that were identified as negative by the BACTEC™ FX system at the end of incubation. All bottles defined as negative by the BACTEC FX system were Gram-stained using an automatic device and inoculated on solid growth media. In our work, 15 cultures that were defined as negative by the BACTEC FX system at the end of the incubation were found to contain microorganisms when Gram-stained. The main characteristic of most bacteria and fungi growing in the culture bottles that were defined as negative was slow growth. This finding raises a problematic issue concerning the need to perform Gram staining of all blood cultures, which could overload the routine laboratory work, especially laboratories serving large medical centers and receiving a large number of blood cultures.

Classical Microbiological Diagnostics of Bacteremia: Are the Negative Results Really Negative? What is the Laboratory Result Telling Us About the “Gold Standard”?

“…There are few studies that confirm the possibility of bacterial detection in negative blood culture fluids, with the use of an additional culture on enriched media or using Gram stain, however, due to the lack of a procedure for concentrating or purifying the samples or the use of FISH, the authors obtained lower percentages of bacterial detection compared to our results [37,38]. In the literature, there are no reports on the detection of bacteria directly in the blood using FISH, apart from the studies by our team, so we cannot compare the obtained results with other studies [13,20].…”

Implementation of short incubation MALDI-TOF MS identification from positive blood cultures in routine diagnostics and effects on empiric antimicrobial therapy

“…We are convinced that the 15 min interval observed is a reasonable. Outliers (>1000 min for reporting the microscopy results) were due to cases in which initial microscopy failed to identify bacteria in the Gram stain where they were actually present [20]. …”

Clinical physiological parameters for the prediction of gram-negative bacterial infection in the emergency department

“…Fifth, clinical parameters were obtained only once after arrival at the ED, while initial vital signs may better re ect the original patient status rather than the vital signs obtained after treatment. Sixth, false negative cases may be left out; however, the false negative rate was very low, as Peretz and colleagues discovered that only 0.13% of the initially negative blood cultures were positive after further examination by Gram staining [27]. Although the odds ratio for any combination of the two or three clinical parameters to predict GNB infection were provided in the supplementary table 5, we did not calculate the odds ratio for each possible combinations of the twoparameter or three parameter model (there are six possible two-parameter combinations and four possible three-parameter combinations), therefore, developing a clinical physiology scoring tool to predict GNB infection based on the ndings of this research and comparing the newly established tool with the existing scoring systems, such as qSOFA and SIRS criteria, will be our future goal.…”