A comparative study to determine the recovery rate of microorganisms of bloodstream infections: two versus three blood culture specimens

OBJECTIVE: There has been a development of automated and continuous-monitoring blood culture systems that are more sensitive than conventional systems for the detection of microorganisms. Whether two or three blood culture specimens obtained during a 24-hour period using these automated systems achieving a higher recovery rate of microorganism remains to be determined. The present study was aimed to compare the recovery rates of microorganism of blood-stream infections (BSIs) using two and three blood culture specimens.

MATERIAL AND METHOD: A prospective investigator-blinded study was carried out in patients who needed to have blood cultures in medicine wards and intensive care units as well as an emergency room of King Chulalongkorn Memorial Hospital, Bangkok, Thailand, between October 1, 2010 and March 31, 2011. Three blood culture specimens were obtained from each patient during a 24-hour period. Each specimen was inoculated into an aerobic bottle of blood culture broth (TREK Diagnostics, Cleveland, OH, US), and then incubated at 37 degrees C for seven days.

RESULTS: Of 568 patients, there were 116 (20.4%) unimicrobial episodes with three blood cultures obtained during a 24-hour period. There were 70 (12.3%) and 46 (8.1%) episodes of true pathogen and contaminant, respectively. The recovery rates of true pathogen were 75.7% (53 isolates), 87.1% (61 isolates), and 100% (70 isolates) with the first, second, and third blood culture specimens, respectively (p < 0.05 between the recovery rate with the first two and the third blood culture specimens). There were 25 (35.7%), 38 (58.6%) isolates, and four (5.7%) of Gram-positive, Gram-negative bacteria, and fungi, respectively. Among 25 Gram-positive bacteria, Staphylococcus aureus was the most common isolate (10, 14.3%), followed by Streptococcus pneumoniae (5, 7.1%) and Enterococcus faecalis, Enterococcus faecium, coagulase-negative Staphylococcus (3, 10% each). Among 38 Gram-negative bacteria, Escherichia coli was the most common isolate (13, 18. 6%), followed by Pseudomonas aeruginosa (8, 11.4%), and Klebsiella pneumoniae (6, 8.6%). The sensitivity and specificity of the recovery rate of microorganisms using two blood culture specimens were 85.7% and 92.3%, respectively. The sensitivity and specificity of the recovery rate of microorganisms using three blood culture specimens were 100% and 90.8%, respectively.

CONCLUSION: To the best of the authors'knowledge, the present study is the first prospective study to compare the recovery rate of microorganisms of BSIs between the two and three blood culture specimens using the VersaTREK blood culture system. Three blood culture specimens are required to achieve the recovery rate of more than 99%.