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).
For patients with indwelling catheters, urine cultures may not reflect bladder bacteriuria because sampled organisms may have arisen from biofilms on the inner surface of the catheter.
In conclusion, when making a diagnosis of UTI, the patient's clinical condition and symptoms should be taken into account. A count of >105 cfu/ml is likely to be associated with UTI regardless of symptoms. However, lower counts of 102 cfu/ml may be potentially significant in symptomatic patients, regardless of sex. A pure isolate of between 104-105 cfu/ml needs to be evaluated on clinical information or confirmed by repeat culture.
The main classification of UTI is (1) complicated and (2) uncomplicated. Uncomplicated UTI occurs in healthy patients with a structurally and functionally normal urinary tract. UTIs can also be classified anatomically into lower (cystitis, urethritis, prostatitis, epididymitis or orchitis) or upper tract infection (pyelonephritis).
Complicated UTI occurs in patients with underlying anatomical or functional abnormality (Table 2.1). Complicated UTIs can take longer to eradicate and tend to recur.
Table 2.1Conditions predisposing to complicated UTI (Johnson and Stamm, 1987)