Lack of requirement for blind subcultures of BACTEC blood culture media

Abstract

To determine the need for blind subculturing of BACTEC (Johnston Laboratories, Cockeysville, Md.) blood culture media, we compared results of radiometric readings, visual inspection, and blind subculturing for nearly 7,500 blood specimens. Visual inspection and radiometric testing were performed on day 1 through 7, and blind subcultures were made on day 3. In the first phase of the study, 402 of 3,896 aerobic bottles were positive by radiometric testing (growth index, greater than 25), visual inspection, or subculturing. Only six bottles were radiometrically negative but subculture positive on day 3. The second phase of the study was designed to determine if aerobic bottles eventually became radiometrically positive in those cases in which they were radiometrically negative but subculture positive on day 3. Two bottles were subculture positive but never gave a growth index of greater than or equal to 25 by day 7. One yielded Staphylococcus epidermidis, and one yielded viridans, Streptococcus sp. A total of 35 anaerobic organisms were isolated from 3,896 blood specimens. All of these anaerobes were detected by both radiometric testing and subculturing. We examined a total of 14,972 blood culture bottles. Twenty-nine bottles considered negative by visual inspection or radiometric readings were found to be positive by subculturing. Fifteen of these were shown, by chart review, to contain contaminants. Organisms in the other negative bottles would not have gone undetected because companion bottles from the same patients were radiometrically or visually positive. We concluded that it is necessary to perform blind subcultures of BACTEC 7B and 8B blood culture bottles.