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Rapid method for determining the antimicrobial susceptibility on chromogenic agar for urine specimen

Journal of Microbiology & Experimentation
Renata Lazo de la Vega Giraud, Estefanía Guadalupe Alvarado Bravo, Gabriel Chávez Giraud, Carmen Giraud Rodríguez

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Background: UTIs are the most common infections. Standard bacterial isolation and antimicrobial testing, usually takes no less than 2-3 days. This can often lead to inadequate empirical therapy. Determining an antibiogram directly from urine sample, may provide preliminary information that improves antimicrobial use and that has a very good correlation with standard procedures. A comparison between rapid method and standardized disk diffusion test was made.
Methods: The samples were preselected for testing. Uncentrifuged and well-mixed urine was Gram stained and examined microscopically (x 1000). Only those specimens containing ≥1 microorganisms/field were selected for the study. The rapid method was performed by directly applying three drops of urine over the surface of a CHROMagar orientation medium, the drops were spreaded with a cotton-tip swab all around the plate. These ones were air dried for 15 min, then the antibiotic disks were pressed on the agar surface and the plates were incubated at 35°C, after 6 hours the interpretation of the inhibition zone could be done. The zone diameters in both methods were measured. Interpretative categories were determined by using CLSI guidelines. The results were compared.
Results: 163 samples were chosen for comparison, the organism isolated was E. coli. Total agreement of category (S-S, I-I, R-R) was high overall at 94.4%. The lowest correlation 87% was found to occur with amoxicillin/clavulanic acid. The highest correlation between methods was with trimethoprim-sulfamethoxazole (99.1%) no errors S-R and R-S. The majority of discrepancies occurred when the difference was from 1 to 4 mm.
Conclusion: The rapid method is equivalent to the standard test when the urine being tested is infected with ≥105org/ml. This procedure allows the earlier reporting in six hours of susceptibility results and improves antimicrobials use.


urine specimen, susceptibility, uncentrifuged, disk diffusion test, blood culture, antimicrobial agent’s