Comparison of recovery of organisms from blood cultures diluted 10% (volume/volume) and 20% (volume/volume).

We compared blood cultures that were diluted 1:5 (20%, vol/vol) and 1:10 (10%, vol/vol) and contained specimens from patients with suspected septicemia. Streptococcus pneumoniae was recovered significantly more frequently from blood cultures diluted 20%, whereas gram-negative bacilli, group D streptococci, Staphylococcus aureus, and Candida spp. were recovered significantly sooner and more frequently from blood cultures diluted 10%. Statistically significant differences in isolation rates, however, represented only a small number of patients for whom the positive cultures affected therapy. We conclude that as long as at least two separate sets of blood cultures are obtained per septic episode from each patient, a 1:5 to 1:10 blood/vented (aerobic) medium ratio provides acceptable results. Nevertheless, the results also demonstrate that blood cultures diluted 10% provided greater and faster yields than those provided by blood cultures diluted 20%.