Effect of sample volume on yield of positive blood cultures from adult patients with haematological malignancy.

Six hundred and sixteen blood samples from patients with haematological malignancy were each distributed equally among three identical cells in a Malthus Microbiological Growth Analyser. The mean (SD) volumes inoculated into sets in which one, two, or three of the three bottles were positive were 37.7 (10.1) ml, 37.4 (12.9) ml, and 37.7 (10.5) ml, respectively. Overall, clinically important organisms were isolated from one bottle only with 18 cultures, from two bottles only with 19 cultures, and from all three bottles in a set with 104 cultures. If the yield from a single bottle inoculated with a mean volume of 12.6 ml blood is taken as 100%, the yield from 25.2 ml in two bottles was 110.7% and the yield from 37.7 ml in three bottles was 115.6%. The increased yield from increased volume was considerably lower than that reported from unselected groups of patients, which suggests that the magnitude of bacteriaemia is greater in patients with neutropenia. The isolation of infecting organisms from the blood of patients with neutropenia is, however, particularly important in directing chemotherapy and consequently 45 ml blood samples from these patients continue to be requested.