Correlation between inoculum volume, positivity rates and microorganisms isolated from blood cultures

The study aimed to assess the volume of blood inoculum in each culture bottle and its effect on the rate and time to positivity, types of microorganisms isolated and contamination rate. During a period of six months, two sets of blood cultures (aerobic and anaerobic: 3006 bottles each) were collected. Of the bottles collected, only 2916 aerobic and 2898 anaerobic culture bottles were analysed by categorising the samples into three different blood volume groups: < 8 ml, 8-10 ml and >10 ml, where ≥8 ml is considered an adequate volume. In the aerobic bottles, there was no significant difference in the rate of positivity among the three groups: 8-10 ml and < 8 ml (p = 0.226) or > 10 ml (p = 0.282). However, there was a significant decrease in positivity in the anaerobic bottles when comparing the > 10 ml blood group with the < 8 ml group (p = 0.032). The contamination rate was lower in the > 10 ml volume category in aerobic blood cultures (p = 0.008) and the isolation rate of Gram-negative bacilli was higher in this category compared with the other two categories (p = 0.05). Our study shows that the volume of blood cultured was a variable that influences the positivity rate and contamination rate of a blood culture sample.