Comparison of Direct and Standardized Disk Diffusion Susceptibility Testing of Urine Cultures

Abstract

A comparison between direct and standardized disk diffusion tests was made on a total of 300 urine specimens containing 105 organisms/ml. Of these, 246 represented pure cultures and 54 represented mixed cultures. The number of major discrepancies per organism tested in pure culture was 18 (7.3%) and in mixed cultures it was 23 (42.6%). The percentage of major discrepancies per total number of antimicrobial drug comparisons made was 1.4%. Although this procedure may be of value in selected cases with pure cultures of organisms present in quantities 2105/ml, its use on a routine basis is not recommended.It has recently been recommended by Kunin (7) that direct susceptibility tests be performed on urine specimens that contain bacteria in Gram-stained smears of the uncentrifuged specimen or in wet-mount preparations of centrifuged urinary sediments. Since two published reports (1, 9) of evaluations of direct susceptibility testing of urine have presented seemingly conflicting data about the procedure's reliability and accuracy, we performed a comparison between it and the standardized disk diffusion method in an attempt to reconcile these differences. MATERIALS AND METHODSClean catch, midstream urine specimens collected by a urine collection service were used throughout the study. Initially, urine specimens were randomly selected for testing; however, due to their low rate of positivity, urine specimens were screened on receipt in the laboratory. Uncentrifuged, well-mixed urine was Gram stained and examined microscopically (x 1,000). Only those specimens containing >2 organisms/field were selected for further study. Processing of urine specimens and identification procedures were performed as described by Washington (11).Susceptibility test procedures. Direct susceptibility tests were performed on undiluted urine by swabbing the surface of Mueller-Hinton agar plates (BBL) with a sterile, cotton-tipped swab. Excess urine was expressed by pressing the swab against the side of the collection container. The plates were air dried for a maximum of 15 min, and high-content antimicrobial disks were distributed by means of a multidisk dispenser (BBL). Individual disks were pressed firmly on the agar surface with alcoholflamed forceps. Plates were incubated at 37 C, and zone diameters were measured with calipers after 16 to 18 h of incubation.The standard disk diffusion method, as described by Bauer et al. Zone diameter interpretations obtained with each method were compared, and discrepancies were defined as follows: very major discrepancies represented by resistance by the standard method and susceptibility by the direct method, major discrepancies represented by susceptibility by the standard method and resistance by the direct method, and minor discrepancies represented by intermediate susceptibility by one method and susceptibility or resistance by the other.Ampicillin (10 ug), carbenicillin (100 ,ug), cephalothin (30 ILg), gentamicin (10 ,ug), kanamycin (30 ,ug), nitrofurantoin (300 Ag), nalidixic acid (30 ug), and tetr...

Comparison of Direct and Standardized Antibiotic Susceptibility Testing in Bacteriuria

“…The results in our study show close cor respondence with the report by Hollick and Washington [1976], who found 95.5% con cordance, and by Perez and Gillenwater [1973], whose figure was 96.8%. Perez and Gillenwater concluded that direct testing is of great value in clinical practice.…”

“…As a rule these tests are based on a disc diffusion procedure with standardized inoculum. The direct disc diffusion method, be used in a clinical microbiological without standardized inoculum, has been evaluated in clinical practice [Perez and Gillenwater, 1973;Hollick and Washington, 1976;Dornbusch et al, 1979], with con flicting judgements concerning its usefulness in routine work. The purpose of the present study, therefore, was to evaluate the results of direct susceptibility tests further by com parisons with standardized tests on urine cultures in a clinical laboratory.…”

Comparison of Direct and Standardized Antimicrobial Susceptibility Testing of Positive Blood Cultures

“…However, a study in our laboratory comparing direct and standardized disk diffusion susceptibility tests on urine specimens produced results similar to those obtained in this study (5).…”

Oral doxycycline for non-systemic urinary tract infections (UTIs) due to P. aeruginosa and other Gram negative uropathogens

“…However, clinicians still ponder in disbelief when an oral antibiotic, e.g., doxycycline is used for formidable uropathogen, e.g., P. aeruginosa. Because tetracyclines are not routinely tested against Gram negative uropathogens many physicians do not even consider doxycycline a therapeutic option [11,12,17,18,20]. I continue to use oral antibiotics successfully based on the estimated urinary concentrations to eliminate a variety of aerobic Gram negative uropathogens from bladder urine.…”

“…Clinicians often forget that in vitro susceptibility testing is based on achievable serum concentrations for rapidly growing aerobic organisms and primarily pertains to bloodstream infections. In vitro susceptibility testing, by definition, is at best an extrapolation of conditions at the site of infection, i.e., with lower UTIs since susceptibility testing is performed in broth (not serum), at serum pH07.4 (not urinary pH) and at 35°C (not body temperature) [19,20]. For lower tract UTIs, some have tried to more closely predict in vivo effectiveness by performing susceptibility tests on Gram negative uropathogens in human urine, at urinary pH, and at achievable urinary concentrations (Table 3) [21].…”