Comparison of the quantitative direct plating method and the BACTEC procedure for rapid diagnosis of Haemophilus influenzae bacteremia in children.

The efficacy and reproducibility of the quantitative direct plating (QDP) method and the semi-automated BACTEC radiometric system (BBL Microbiology Systems, Cockeysville, Md.) were analyzed for the rapid diagnosis of Haemophilus influenzae bacteremia on the basis of 41 positive cultures from 35 patients. The QDP method detected 61% and BACTEC only 19% of the positive cultures within the first 12 h. Similarly, the QDP procedure yielded growth of H. influenzae in 56% of the cultures before a positive growth index reading was obtained with the BACTEC method. The observation that the quantitative procedure recovered H. influenzae in 88% of all positive cultures, using only 1 ml or less of blood, is attributed to the fact that 68% of the cultures had counts in excess of 100 colony-forming units per ml. The reproducibility of the QDP method was documented by the fact that duplicate blood cultures taken within a few hours of each other yielded comparable results on the number of bacteria at low (1 to 100/ml), moderate (100 to 1,000/ml), and high (greater than 1,000/ml) levels of bacteremia. We concluded that the QDP method is a valuable, simple, and inexpensive supplementary technique to the semi-automated BACTEC procedure for the rapid diagnosis of H.influenzae bacteremia in children.