Urine Culture (Routine)

Definition

Urine culture is used for the detection of UTI caused by common uropathogenic bacteria and yeast. The range of UTI syndromes is broad, including asymptomatic bacteriuria through pyelonephritis with systemic symptoms. Patients with uncomplicated UTIs often present with dysuria and frequency, whereas pyelonephritis may be associated with signs of sepsis, including fever, flank pain, and nausea. The risk of UTI, including complicated UTI, is increased in patients with urinary tract prosthetic materials, like stents, GU tract malformations, history of GU surgery, and medical conditions, such as pregnancy, neurologic disorders, and DM.

Acceptable specimens: Clean-catch midstream urine, straight catheterization ( "in and out " ¯), newly placed indwelling catheters, and suprapubic aspirates are commonly submitted and should be associated with low contamination rates.
- Urine collected from an indwelling catheter or from pediatric collection bags is frequently contaminated. Negative cultures may be helpful in ruling out UTI; positive cultures should be interpreted with caution. Urine for culture should never be taken from a collection bag attached to an indwelling catheter.
- 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 manufacturers 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.

Urine is cultured quantitatively. For most patients, 1 ˇ ¼L of urine is inoculated onto SBA and onto a selective, differential agar for isolation of gram-negative bacilli. Urine specimens with fewer than 103 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 ˇ ¼L of urine may be inoculated, resulting in a lower detection level of 102 organisms per milliliter. Uropathogens present in concentrations between 102 and 103 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.

Expected results: <103 colonies/mL for routine urine cultures; <102 colonies/mL for special cultures taken from patients at high risk for complicated UTI. A low concentration of genital flora is commonly seen.
Positive results: Isolation of a common uropathogen at concentrations >104 colonies/mL (>103 colonies/mL for high-risk patients), when present as the sole or predominant isolate, is considered positive.
Negative results: Urine culture is sensitive for ruling out UTI, but prior antimicrobial therapy can inhibit the growth of uropathogens, resulting in false-negative cultures.

Common pitfalls:
- Contamination, due to poorly collected or transported urine samples, limits the value of a significant proportion of specimens submitted to the laboratory.
- Clinically significant polymicrobial UTI is uncommon (<5%). Interpret mixed cultures with caution " ”they most likely indicate contaminated specimens.
- Urine is frequently transported in collection cups on which the caps are not firmly tightened, resulting in leakage and possible contamination.
- Urethritis and vaginitis may be associated with pyuria and clinically mimic cystitis.