Clinical and laboratory criteria can help differentiate these conditions. Women with dysuria and pyuria who have less than 102 bacteria/ml may have urethritis caused by Chlamydia trachomatis, Neisseria gonorrhoeae, or herpes simplex virus or vulvovaginitis caused by Trichomonas vaginalis or Candida species. In women with acute dysuria, the presence of at least 102 organisms/ml of a single coliform species appears to be a more accurate criterion for infection than a threshold of 105 organisms/ml. The number of organisms per milliliter of a clean-voided urine specimen is a useful indicator.4 The growth of 105 or more colonies of a single bacterial species per milliliter of urine in two consecutive urine cultures is the diagnostic criterion for asymptomatic bacteriuria in women. On the other hand, pyuria is highly associated with inflammation of the urinary tract, which is most commonly caused by infection. Another marker of contamination is the presence of squamous epithelial cells in a urine specimen. epidermidis, diphtheroids, and lactobacilli, seldom cause UTI except in complicated settings. Organisms that are commonly found in the vaginal introitus and distal urethra, such as S. Polymicrobial infection may occur in certain settings, however, including long-term catheterization, incomplete bladder emptying because of neurologic dysfunction, and the presence of a fistula between the urinary tract and the GI or genital tract.
Thus, in most instances, a culture that grows out mixed bacterial species is contaminated and needs to be repeated. More than 95% of UTIs result from a single bacterial species. When evaluating urine cultures, therefore, the physician should consider both the species and the number of bacteria found. In addition, it is frequently difficult to obtain a urine specimen that is uncontaminated by the normal microbial flora of the distal urethra, vagina, or skin. The correlation between clinical symptoms and the presence of infection is imprecise.
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