Rapid isolation and presumptive diagnosis of uropathogens by using membrane filtration and differential media

Random urine samples from hospitalized patients (n = 550) and seeded sterile filtered urine samples (n = 730) were used to test a membrane filtration technique, Qualture (Future Medical Technologies International, Inc., West Palm Beach, Fla.), for the detection and identification of uropathogens. Results for each sample were compared with those obtained by the calibrated loop (0.01 ml) method to demonstrate the sensitivity of the method as a screening tool and the specificity of the presumptive diagnosis obtained from the pattern of growth on differential media. The medium was supplied as dehydrated nutrient pads (Sartorius AG, Goettingen, Germany) and was activated by rehydration by the addition of the liquid specimen. With a threshold of 10(4) CFU/ml defining a positive culture, the sensitivity of the Qualture was 100%. At lower levels of bacteriuria, the Qualture was more sensitive than the calibrated loop method. Significant infections were presumptively diagnosed at 4 h by filtration rather than at 24 h on agar medium. The specificity of uropathogen identification ranged from 99% for Enterococcus spp. to 83% for Pseudomonas spp. Citrobacter spp. could not be differentiated from Escherichia coli and Providencia spp. could not be differentiated from Proteus spp., which does not create a therapeutic dilemma. Filtration, isolation, quantitation, and presumptive diagnosis are performed in one step, without subculture. Membrane filtration is a sensitive and rapid technique, with the advantage that it can be used as a collection and transport device without the use of growth inhibitors.