Urinary tract infections are caused by organisms such as bacteria overcoming the body's defenses in the urinary tract. They can affect the kidneys, bladder, and the tubes that run between them. The urinary tract can be divided into the upper urinary tract and the lower urinary tract. The upper urinary tract consists of the kidneys and the ureters, and the lower urinary tract consists of the bladder and the urethra. Most UTIs are not serious, but some can lead to serious problems, particularly with upper UTIs. Recurrent or long-lasting kidney infections can cause permanent damage, and some sudden kidney infections can be life-threatening, particularly if bacteria enter the bloodstream in a condition known as septicemia.

In general, the isolation of more than 10,000 cfu/mL of a urinary pathogen is indicative of urinary tract infection (UTI). Isolation of 2 or more organisms above 10,000 cfu/mL may suggest specimen contamination. For specimens contaminated with the usual bacterial flora, bacteria that are potentially pathogenic are identified. Although urine is normally sterile, contamination by organisms normally present in the urethra or on periurethral surfaces can allow a proliferation of these organisms yielding misleading urine culture results. Urine held at ambient temperature for more than 30 minutes supports the growth of both pathogens and contaminants, leading to potentially inaccurate colony counts.