THEORETICAL ASPECTS OF DETECTION OF BACTEREMIA AS A FUNCTION OF THE VOLUME OF BLOOD CULTURED

Clinically significant bacteraemia in adults is characterized by low n umbers of circulating bacteria. Assuming a Poisson or a binomial distr ibution 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% a t a given mean number of colony-forming units (cfu) per sample unit. T hese theoretical assumptions have been used on an empirical population of patients with proven bacteraemia. Results: 18% of Staphylococcus a ureus bacteraemias and 29% of Escherichia coli bacteraemias have circu lating bacteria with an estimated spread of less than 0.04 cfu/ml. Wit h a 95% probability of detection of a bacteraemia, a concentration in blood corresponding to 3 cfu/sampling unit is necessary. In our empiri cal material, where 30 ml was cultured, the probability of detection o f E. coli bacteraemias would have decreased by 11% if 20 ml had been c ultured, and 27% if only 10 ml had been cultured. The corresponding fi gures for S. aureus were 6% and 15%, respectively. For low grade E. co li bacteraemias (<0.04 cfu/ml) the decrease would have been 33% and 67 %, respectively.