Quantitative association of blood culture volume with contaminant and pathogen recovery

Blood culture contamination is common and associated with adverse outcomes. The relation between blood culture volume and contamination remains under-evaluated. We assess the relation between blood culture volume and commensal-positive culture rate and characterize the relative abundance of contaminant species in low- and high-volume samples.

Analysis of blood cultures processed between August 2017 and August 2023 in an academic community hospital. Blood culture bottles containing 0-10 mL of fluid were included. Clinical data was not available. Contaminants were defined according to the National Healthcare Safety Network list of common commensals. The association between blood volume categories and the proportion of positive cultures was assessed using a linear logistic regression model. Goodness of fit was evaluated by restricted cubic splines and likelihood ratios.

Of a total of 146,307 blood culture bottles, 8,918 were excluded. A positive relation between blood volume and culture-positivity rate was found for both pathogens and contaminants, with an increase in odds ratio for culture positivity of 1.033 (95% CI: 1.026-1.041, p < 0.001) and 1.030 (95% CI: 1.018-1.041, p < 0.001) for each additional 1 mL for pathogens and contaminants, respectively. This translates to a 10 per 1000 excess cultures growing contaminants comparing 10 to 2 mL bottles. Cutibacterium was over-represented in high-volume samples. Otherwise, differences in relative abundance between the two volume categories were nonsignificant.

The positive association between contaminant growth and blood volume may result from better growth conditions in high volume samples, or suggest that in some cases, contaminant growth may reflect frank bacteremia.