Novel screening method for urine cultures using a filter paper dilution system

Abstract

We have developed a novel method for urine culture for office practice based on the use of filter paper as a solid-phase dilution device. Filtration dilutes and spreads the inoculum onto a solid culture surface. Experiments were conducted to determine the optimum inoculum size, microbial permeability through filter papers, and ability to exclude vaginal epithelial cells. The filter paper dilution system was compared to the standard streak method to detect bacteriuria in specimens submitted to the diagnostic laboratory. The sensitivity and specificity of the filter paper dilution system for detection of high-count (≥104 CFU/ml) gram-negative bacteriuria in 487 urine specimens were 98.2 and 97.4%, respectively. The sensitivity and specificity for gram-positive bacteriuria in 404 urine specimens were 91.2 and 99.2%, respectively. Low-count gram-negative bacteriuria (<104 CFU/ml) was detected by the filter paper dilution system in five of nine specimens (55.6%). In addition, the filter paper dilution system was able to detect gram-negative bacteria in 12 of 41 (29.3%) mixed cultures. Lactobacillus and Gardnerella organisms in urine specimens were excluded by the filter paper dilution system. Only three of eight Candida sp. isolates were detected at counts of ≥104 CFU/ml. The system has good storage properties and can be inoculated at the point of source without the need for refrigeration or preservatives. It should be a useful screening method for office practice, where members of the family Enterobacteriaceae and staphylococci cause most infections. Standard culture methods are preferred for hospital diagnostic microbiology laboratories, where there is a need to detect yeasts and fastidious microorganisms and to isolate individual colonies from mixed cultures.