Caveat: Failure to perform bacterial urine cultures or failure to correctly interpret the results of urine cultures may lead to
diagnostic errors and therapeutic failures. Although detection of bacteria in properly collected urine samples is highly indicative
of bacterial UTI, further information is required to confirm and localize the site(s) of infection.
Quantitative urine culture
How should urine samples be collected for diagnostic culture?
Table 6 Checklist of factors influencing interpretation of qualitative bacterial cultures of urine
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We prefer to collect urine samples for bacterial culture by cystocentesis to eliminate problems of differentiating contaminants
from pathogens. Detection of bacteria, even in low numbers, in urine aseptically collected by cystocentesis is indicative
of UTI. However, false-positive results may occur if the needle penetrates a loop of intestine during cystocentesis, or if
the sample is contaminated during transfer to culture media. For this reason, quantitative urine culture is routinely recommended,
even for samples collected by cystocentesis. Urine culture results should be interpreted in context of other clinical findings
(Table 6).
Caveat: Catheter-induced UTIs (nosocomial infections) are common in patients with urinary tract diseases, and could even result
in iatrogenic pyelonephritis, renal failure and septicemia. Therefore, transurethral catheterization of patients at increased
risk for UTI should be evaluated in context of risks and benefits.
Collect before antibiotic administration
How should urine samples be preserved prior to culture?
If diagnostic bacterial cultures are to be performed, urine should be collected for culture before antibacterial therapy is
initiated. If the patient is currently being treated with an antimicrobic, it should be discontinued for approximately three
to five days prior to diagnostic urine culture in order to minimize inhibition of in vivo and in vitro bacterial growth.
Because urine may be a good culture medium at room temperature (bacterial counts may double every 20 minutes to 45 minutes),
it should be cultured within 15 minutes to 30 minutes from the time of collection. Another indication for culture of fresh
urine samples is that destruction of some fastidious bacteria may be detectable within an hour of collection. If for any reason
culture of freshly collected urine samples is not possible, the samples should be kept in a sealed sterile container and immediately
refrigerated following collection. Refrigerated samples may be stored for six hours to 12 hours without significant additional
growth of bacteria. However, it is emphasized that fastidious organisms may be killed in the urine environment if refrigeration
storage time is prolonged. Freezing urine samples may also destroy bacteria.
Caveat: Transport of urine specimens to a commercial microbiology laboratory results in increased time between urine collection
and aerobic culture, and therefore adds a potential source of error, especially if the samples aren't properly preserved.
In addition, 80 percent to 85 percent of bacterial UTIs are caused by a single species of bacteria, while only 15 percent
to 20 percent are caused by more than one bacterial species. Therefore, detection of multiple species of bacteria suggests
sample contamination, especially in urine samples collected by voiding or catheterization.
Putting it in practice
Why are quantitative bacterial urine cultures a standard of practice?
Bacteriuria, the presence of bacteria in urine, is not synonymous with UTI because urine may be contaminated with bacteria
as it flows through the urethra, and after it is removed from the patient, but before it is cultured. Quantitative urine culture
includes determination of the number of bacteria (colony-forming units) per milliliter of urine in addition to isolation and
identification of bacteria. Because it facilitates differentiation of bacteria that have contaminated the urine sample from
bacteria that are likely to be causing UTI, quantitative culture is the preferred method of diagnostic culture for urine samples
obtained by any collection method.
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