Objective: To examine clinical and laboratory practices associated with contamination of blood culture specimens from adults.
Design and setting: A College of American Pathologists Q-Probes quality improvement study involving prospective evaluation of adult blood culture contamination rates in 640 institutions.
Main outcome measure: Proportion of contaminated blood cultures.
Results: A total of 497134 blood cultures were studied. The median adult inpatient blood culture contamination rate was 2.5% (central 80th percentile=0.9%-5.4%) by laboratory assessment. There was no significant difference in contamination rates between inpatient and outpatient cultures (P=.273). The median contamination rate by clinical assessment (2.1%) was significantly lower (P=.005), primarily because of a lower proportion of cultures with coagulase-negative Staphylococcus that were interpreted as contaminants when only one of multiple specimens was positive. Specimen collection variables associated with significantly lower contamination rates included use of a dedicated phlebotomy service (P=.039), use of tincture of iodine for skin disinfection (P=.036), and application of an antiseptic to the top of the collection device before inoculation (P=.018). Teaching institutions and high numbers of occupied beds were demographic factors associated with higher contamination rates for inpatients but not for outpatients. Culture parameters associated with higher contamination rates included microbial growth from a single specimen, isolation of certain microbial species (eg, coagulase-negative Staphylococcus), and longer time to detect growth in culture. Contamination rates were not significantly affected by the type of blood culture method used, specimen volume, or use of a double-needle collection procedure.
Conclusions: There is wide variation in blood culture contamination rates among institutions. Three specimen collection factors and three culture variables were identified as having a significant effect on blood culture contamination.