Abstract

Background: In the present study, it was aimed to identify the microorganism and the factors due to the patients which may cause false positive results in automated blood culture systems, and to evaluate the parameters to be used in interpreting the results correctly.
Materials and methods: The study was carried out between 2016-2017. Fully automated Bact/Alert 3D (BioMérieux, France) system was used as the blood culture method. Blood cell counting was evaluated by fluorescence flow cytometry method. When the signal was received from the blood culture system, the presence of microorganism was first investigated using Gram staining. In spite of the presence of a growth signal, the absence of microorganisms in the acridine oranges, gram preparations and the absence of growth in the inoculated plates was accepted as false positivity. These bottles were also subcultured onto a chocolate agar and a Sabouraud Dextrose Agar media, and were incubated for 14 days. Mann Whitney U test and Chi-Square test were used for statistical analysis. Permission was taken with the number 10.03.20707.
Results: A total of 9216 aerobic blood cultures were included in the study, 1839 (19.9%) of those indicated a positive growth signal. False positivity was detected in 69 (0.75%) of the blood culture bottles. The mean incubation time for positive growth signal was 20 hours in the bottles giving true positive growth signal, whereas the mean incubation time for positive growth signal was 3 hours in the bottles giving false positive growth signal (p<0.001).
Conclusion: Blood culture signal positivity within the first four hours should be taken as false positive growth signal especially in cases of high numbers of leukocytes, neutrophils or immature granulocytes. Thus, we think that unnecessary examinations can be prevented.