Evaluation of positive blood cultures. Guidelines for early differentiation of contaminated from valid positive cultures.

Blood culture experience at a general hospital was reviewed for rate of contamination, and to establish criteria differentiating contamination from bacteremia. Only clinical data were used to decide whether or not positive cultures signified bacteremia. Laboratory correlations were then made: multiple positive cultures were found in 69% of bacteremic patients who were cultured more than once, while only 11% of contaminated cases had more than one positive culture. Similarly, 68% of pour-plate cultures were positive in bacteremia, vs 5.8% for contaminated cultures. Some organisms were found only in bacteremia, others only as contaminants. The number of positive cultures, results of simultaneous pour-plate cultures, and identification of the organism are all helpful facts in differentiating positive blood cultures which indicate bacteremia from those representing contamination.