Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1435 page)

   Collection of urine from ileal conduits or by invasive procedures (like percutaneous nephrostomy or by cystoscopy) is obtained by personnel specifically trained in these techniques.

   The specimen should be transported to the laboratory within 2 hours after collection. If transport is delayed, the specimen should be refrigerated.

   Alternatively, urine may be inoculated into a preservative collection system, allowing transport up to 48 hours. Preservative systems must be inoculated according to the manufacturer’s instructions. Preserved specimens are transported at room temperature.

   There are several commercially available systems that allow culture media to be directly inoculated at the site of collection. These systems may be incubated prior to transport to the laboratory.

   Use

   Urine is cultured quantitatively. For most patients, 1 μL of urine is inoculated onto SBA and onto a selective, differential agar for isolation of gram-negative bacilli. Urine specimens with fewer than 10
³
organisms per milliliter of urine yield no growth on the media.

   For patients at risk for clinically significant UTI at lower concentrations of uropathogens, 10 μL of urine may be inoculated, resulting in a lower detection level of 10
²
organisms per milliliter. Uropathogens present in concentrations between 10
²
and 10
³
organisms per milliliter may be clinically significant in symptomatic patients. Repeat culture has shown that these patients may rapidly progress to higher concentrations of bacteria.

   The extent of workup and susceptibility testing is determined by the type of specimen submitted, concentration and species isolated, and patient risk factors. Workup of mixed cultures, which usually represent specimen contamination with endogenous flora, should be limited.

   Potentially pathogenic isolates are identified and susceptibility testing performed, as appropriate.

   
Turnaround time:
Urine cultures from patients at low risk for complicated UTI should be incubated for a minimum of 16 hours. Cultures from patients at risk for complicated UTI should be incubated for a minimum of 48 hours before signing out as negative. Several additional days may be required for final identification and susceptibility testing in positive cultures.

   Interpretation

   
Expected results:
<10
³
colonies/mL for routine urine cultures; <10
²
colonies/ mL for special cultures taken from patients at high risk for complicated UTI. A low concentration of genital flora is commonly seen.