[The bactericidal effect of oxybuprocaine. A possible cause of false negative results in bronchoalveolar lavage]

New approaches in the diagnosis of pneumonia, especially in intensive care units, are quantitative cultures of bronchoalveolar (BAL) fluid or the protected specimen brush. The sensitivity of these methods, however, has often been found to be as low as 50-60%. One possible explanation for the low sensitivity of these diagnostic tools is the antimicrobial activity of local anaesthetics used in bronchoscopy. Therefore, we investigated the bactericidal properties of oxybuprocaine, a topical anaesthetic used for bronchoscopy in our clinic, in order to test the reliability of specimens obtained from BAL. METHODS. The bactericidal activity of oxybuprocaine in concentrations of 1%, 0.1%, 0.05% and 0.01% was tested by constructing time-kill curves for Streptococcus pneumoniae, Hemophilus influenzae, Pseudomonas aeruginosa and Escherichia coli. Five stains of each bacterial species were tested. The inoculum size was 10(4) ml, and bacteria were counted after 10, 20, 30, 60 and 120 min. RESULTS. The resulting time-kill curves are demonstrated in Figs. 1-4. The most sensitive bacteria were S. pneumoniae and H. influenzae, in which significant bactericidal activity could be shown even with a 0.01% solution of oxybuprocaine. E. coli and P. aeruginosa were also inhibited, but only at the highest concentration of 1%. CONCLUSIONS. The use of local anaesthetics before material is taken for culture, e.g. from BAL, may give rise to false-negative results and should therefore be avoided or reduced. For each local anaesthetic used in bronchoscopy, the concentrations that can be used without the risk of false-negative results should be determined.