[Microbiological diagnosis of urinary tract infections]

Urinary tract infections (UTI) are the most common infectious diseases observed in primary care; up to one-third of women will have at least one symptomatic UTI by age 24, and more than one-half of women will be affected by the end of life. In addition, UTIs represent 40% of nosocomial infections, and being usually associated with urinary catheters. Although urine cultures would not be indicated in all cases, these samples are the most abundant in the laboratories of clinical microbiology. Thus, the working protocols applied to these samples have an important impact in the performance of the laboratory. The samples are collected by mid stream urine, and 60-70% of them are negative culture. At present, several commercial systems have been introduced in order to simplify and automate this process. A urine culture with ≥ 10(5) CFU/ml has classically been considered as positive, although lower counts are valued in certain clinical settings. Factors related to this count e.g. methods to obtain urine, conservation of the sample or use of chemical preservatives as well as low counts are critical points to be discussed in detail. The development of antimicrobial resistance logically affects uropathogens, mainly Escherichia coli, which remains the most frequently isolated in urine cultures. The aim of this paper is to review the most innovating aspects influencing the microbiological diagnosis of UTI.