Comparison of Direct and Standardized Disk Diffusion Susceptibility Testing of Urine

Abstract

Direct antimicrobial susceptibility testing [DST] of urine specimens remains controversial largely because of concerns regarding its accuracy, particularly with mixed cultures. To evaluate the performance of DST in the setting of acute urinary tract infection [UTI], DST was performed using 19 traditional and contemporary antimicrobial agents on urine specimens from 122 women with suspected acute UTI, and these results were compared with the results of standardized disc diffusion susceptibility tests done on the same specimens. Direct tests were interpretable; for 96.8% of the specimens met the criteria of UTI. Of the 1,178 antibiotic comparisons between the direct and the standard tests, 0.2% represented both very major [VM] and major [M] errors while 99.6% were in agreement. Errors were more common in association with older antimicrobial agents, non-Escherichia coli strains, low urine bacterial concentration and the presence of mixed growth in the direct test. Urine culture was negative in 45% of urine samples obtained from symptomatic women and DST was unnecessarily performed for such samples wasting both effort and money. In order to overcome such a problem, the present study evaluated concomitant wet mount examination of unstained centrifuged urine samples looking for bacteria, where finding =50 organisms/ml urine was associated with 98.8% sensitivity and a negative predictive value of 97%, which means that urine microscopy, can be used as a good negative screen. It is concluded that when used selectively and interpreted carefully, DST of urine specimens offers an efficient, rapid, and accurate method for antimicrobial susceptibility determination for acute UTI, providing that it is guided by the underused underestimated urine microscopy looking for the presence or absence of bacteria.