Improving the diagnosis of urinary tract infections by the use of enriched media and a 48-hour incubation period

Abstract

Introduction. The absence of a gold-standard methodology for the microbiological diagnosis of urinary tract infections (UTI) has led to insufficient standardization of criteria for the interpretation of results and processing methods, particularly incubation time and culture media. Hypothesis. 48-hour incubation time period and use of blood agar enhances the sensitivity of microorganisms isolated significantly. Aim. To determine the sensitivity of blood agar and Brilliance UTI chromogenic agar, incubating for different periods (24–48 hours), for the detection of positive urine cultures. Methodoloy. Comparisons were made between all possible combinations of media and incubation times. As the gold-standard reference, we used the routine methodology of our laboratory, which involves prior screening with available clinical data, flow cytometry, sediment analysis and/or Gram staining. Screened samples were then cultured on blood agar and chromogenic agar and incubated for 48 hours. Also, based on the results of Gram staining, additional media were added in selected cases. Results. The most significant difference was found between chromogenic agar incubated for 24 hours and blood agar incubated for 48 hours, with the latter method allowing the recovery of 10.14 % more microorganisms (P < 0.0001). Furthermore, the value of performing Gram staining to guide processing was demonstrated, as it avoided the loss of at least 5.14 % of isolates. Conclusions. At least in urological and nephrological patients it is essential to include enriched culture media (blood agar) or to extend the incubation times due to the improvement of the diagnostic sensitivity of urine cultures. Gram staining also can help detect the presence of fastidious microorganisms or mixed infections, indicating whether rich and/or selective media should be included to enhance the diagnostic sensitivity of cultures. If this methodology is not followed, it should be noted that besides fastidious species, fastidious strains of Escherichia coli, Proteus mirabilis, Pseudomonas aerugniosa and Stenotrophomonas maltophilia will also be missed.