Evaluation of calibrated 1 and 10 μl loops and dipslide as compared to pipettes for detection of low count bacteriuria in vitro
Criteria for defining urinary tract infection have changed as the importance of low count bacteriuria in patients with symptoms of infection has been recognized. Little has been published, however, regarding the ability of routine laboratory methods to meet this new challenge. We compared 1 μl and 10 μl calibrated plastic loops (Nunc, Denmark), to a commonly used dipslide method (Uricult, Orion, Finland) using a 0.1 ml pipette as the gold standard. Four typical uropathogens, E. coli, P. aeruginosa, S. saprophyticus and E. faecalis, were mixed in pooled human urine in numbers representing low counts, i.e. 102, 103, and 104, together with 105 colony-forming units (cfu)/ml, and sampling with the four methods was performed 20 times for each with each bacterium and each dilution. Colony counts showed that in many cases the 1 μl loop did not deposit any bacteria on the agar plate when colony counts were lower than 104 cfu/ml. The 10 μl loop constantly deposited 1.5 times higher numbers of bacteria than predicted, which correlated with the results of weighing the amount of water sampled, i.e. 15.24 (1.55) μg (mean (SD)). The dipslide counts correlated with the 10 μl loop although the agar mounts on the slides absorbed water equal to about 200 μl. There were no significant differences in the counts of the four different bacteria. Variation coefficients increased with decreasing volumes sampled, but the 10 μl loop was found sufficiently exact to detect counts from 103 to 104, since only 10-fold multiples are usually reported for urine cultures.