Predictive value of isolating Pseudomonas aeruginosa from aerobic and anaerobic blood culture bottles

Pseudomonas aeruginosa is a particularly virulent pathogen when it causes bacteraemia. and early diagnosis is essential to reduce morbidity and mortality. It is an aerobe and is thought by many to be almost exclusively isolated from the aerobic blood culture bottle in cases of bacteraemia. This study analysed 277 Gram-negative bacteraemic episodes over 1 year at a single institution in order to assess the predictive value of this finding. In 39 of 44 episodes of P. aeruginosa bacteraemia, the organism was isolated from the aerobic bottle only, which gave a sensitivity of 88.6% for this 'test' and a specificity of 73.8%. However, for all episodes of Gram-negative bacteraemia, the likelihood of a Gram-negative bacillus occurring in the aerobic bottle first being P. aeruginosa was only 39%. The converse finding of a Gram-negative bacillus isolated first in the anaerobic bottle or from both bottles together was clinically helpful, having a negative predictive value of 97.2% (i.e. that the organism was not P. aeruginosa).