Definitions

Urinary tract infection (UTI) is currently defined as the inflammatory response of the urothelium to bacterial invasion usually associated with bacteriuria and pyuria.

Bacteriuria is the presence of bacteria in the urine. This can be symptomatic or asymptomatic.

Pyuria is the presence of white blood cells (WBC) in the urine, which indicates inflammation of the urothelium. This could be due to bacterial infection or other pathology such as tumour, stones, foreign body or tuberculosis.

In 1960 Edward Kass proposed defining UTI based on the finding of at least 10⁵ bacteria colonies/ml of urine regardless of symptoms. He found that a single culture of 10⁵ cfu/ml or more had a 20% chance of representing contamination. Since then, it has been shown that about 20-40% of women with symptoms of UTI have bacterial counts of less than 10⁵ (Stamey et al., 1965). In men, counts as low as 10³ cfu/ml of a pure or predominant organism have been shown to be significant in voided urine (Lipsky et al., 1987). Where there is evidence of contamination, a carefully collected repeat specimen should be examined.

In children, confirmation of UTI is dependent on the quality of the collected specimen. Negative cultures or growth of <10⁴ cfu/ml from bag urine may be diagnostically useful. However, counts of 10⁵ cfu/ml should be confirmed by culture of a more reliable specimen. This could be either a single urethral catheter specimen or, preferably, a suprapubic aspirate (SPA). Bacteriuria usually exceeds 10⁵ cfu/ml in SPAs from children with acute UTI (Ginsburg and McCracken, 1982).