Frequency of low level bacteremia in infants from birth to two months of age

Background. The frequency of low level bacteremia (less than or equal to 10 colony-forming units/ml) in infants from birth to 2 months of age and the optimal volume of blood and number of blood cultures to be collected have not been well-documented. During 1991 guidelines at this hospital for collection of blood for culture from these infants were revised. Methods. Blood from each infant with suspected bacteremia was usually inoculated into an Isolator 1.5 Microbial Tube(R) (1.5 mi of blood) and a bottle of anaerobic broth (0.5 to 3.0 mi of blood). The use of a second Isolator tube and the total blood volume recommended for culture (2 to 6 mi) depended on the weight and total blood volume of each infant. Results. Forty-four bacterial pathogens were recovered from the blood of 40 (2.5%) of 1589 infants. Of 34 infants from whose blood the concentration of pathogens could be determined, 23 (68%) had low level bacteremia. Of 50 isolates of pathogens recovered from Isolator cultures, 32 (64%) were detected in counts of less than or equal to 10 colony-forming units/ml. When 2 or 3 blood culture devices were inoculated with a total of 2 to 6 mi of blood from each infant, significantly more cases of bacteremia were detected (34 (3.0%) of 1126 infants had positive blood cultures) than when only one culture device containing less than or equal to 1.5 mi of blood was used (2 (0.5%) of 398 infants had positive blood cultures; P = 0.008). However, when 4 or more culture devices were inoculated with a total of >6 mi of blood from each infant (5 (7.7%) of 65 infants had positive blood cultures), the difference in recovery of pathogens compared with the culturing of from 2 to 6 mi of blood per infant was not significant (P = 0.089). Conclusions. Low level bacteremia was common in our infants' patient population. The culturing of up to 6 mi of blood which represented up to 4.5% of an infant's total blood volume was required for detection of the pathogens.