Theoretical aspects of detection of bacteraemia as a function of the volume of blood cultured

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Abstract
Clinically significant bacteraemia in adults is characterized by low numbers of circulating bacteria. Assuming a Poisson or a binomial distribution we have calculated the probability of detecting bacteria as a function of the concentration in blood, estimated the concentration of bacteria in blood from a given test result, and calculated the blood volume required to detect bacterial growth with a probability of 95% at a given mean number of colony-forming units (cfu) per sample unit. These theoretical assumptions have been used on an empirical population of patients with proven bacteraemia. 18% of Staphylococcus aureus bacteraemias and 29% of Escherichia coli bacteraemias have circulating bacteria with an estimated spread of less than 0.04 cfu/ml. With a 95% probability of detection of a bacteraemia, a concentration in blood corresponding to 3 cfu/sampling unit is necessary. In our empirical material, where 30 ml was cultured, the probability of detection of E. coli bacteraemias would have decreased by 11% if 20 ml had been cultured, and 27% if only 10 ml had been cultured. The corresponding figures for S. aureus were 6% and 15%, respectively. For low grade E. coli bacteraemias (< 0.04 cfu/ml) the decrease would have been 33% and 67%, respectively.