Retrospective cohort study comparing two versus three blood culture sets in people who inject drugs

The current standard of drawing two vs three blood culture sets lacks adequate guidance. Because people who inject drugs are at higher risk for bacteraemia and life-threatening infection, consideration of a third blood culture becomes more important.

To investigate the risks and benefits of obtaining two versus three blood culture sets.

Retrospective cohort study of adults who inject drugs at a multicentre catch-net hospital system from 2017-2022.

998 people who inject drugs and 2278 blood culture sets were analysed. There were 1618 episodes with two blood culture sets and 660 episodes with three. A potential benefit of adding a third blood culture was seen in 30 (4.5%) episodes. However, only 13 (2.0%) episodes showed pathogen-identifying benefit, as 17 (2.6%) involved known inadequately treated infections or the same pathogen in another culture. The number of blood culture sets needed to achieve diagnostic benefit was 51. There were more contaminants for three blood culture sets (65, 9.8%) than for two (114, 7.0%) (p < 0.00001). By adding a third blood culture, the risk of a contaminant increased by 39.7%; the number of blood culture sets needed to find a contaminant was 36. Of 122 episodes with only contaminants and available for analysis, 111 (91.0%) experienced at least one complication. 33 (27.0%) patients experienced either prolonged admission, readmission, or unnecessary antibiotic administration.

The benefits of possibly isolating a pathogen from a third blood culture set do not universally outweigh the risks for contaminant growth for people who inject drugs. A third blood culture should be considered in specific clinical scenarios (i.e. inadequately treated endocarditis and osteomyelitis).