Urine Culture

Urine specimen - No growth in 24-48 hours

A positive urine culture is based on the growth of bacteria at a high number of colony forming units (CFUs).

Urine culture results should be interpreted in conjunction with clinical symptoms of urinary tract infection (UTI), such as dysuria, urinary frequency, suprapubic pain, flank pain, and fever. For clean-catch urine samples, a positive urine culture as indicated by the growth of bacteria greater than 100,000 CFUs/mL is suggestive of UTI; growth of 1000-100,000 CFUs/mL may still indicate UTI, especially for a specimen taken at cystoscopy or other invasive procedures. ^[1]

Reliability of results is determined by the quality of the specimen and specimen collection, transport, and handling to the laboratory. ^[2]

Growth of 2 or more different bacteria or polymicrobial growth is likely to the result of contamination.

A positive urine culture is further tested to identify the organism and to test its susceptibility to antibiotics, in order to guide with antimicrobial therapy, if necessary.

Specimen type:

• Random urine specimen – More commonly used

• First-void morning specimen – Used in certain situations (eg, diagnosing urethritis due to Neisseria gonorrhea or Chlamydia trachomatis)

Specimen container: Sterile plastic containers

Specimen volume: 1 mL

Collection methods ^{[3, 4]} :

• Midstream clean catch - Patient voids first portion of urine, then collects urine specimen midstream and discards the latter portion

• Catheterization - Urine collected directly from an indwelling urethral catheter or from intermittent catheterization

• Suprapubic aspiration - Urine collected from needle aspiration through suprapubic abdominal wall into the bladder

• Cystoscopy or other invasive procedures – Sample can also be obtained during this type of procedure ^[2]

Specimen handling: Urine should be processed within 2 hours of collection. If it cannot be processed in a timely manner, then either (1) refrigerate the specimen at 2-8°C (specimen will be stable for 24 hours) or (2) place the sample in preservative fluid and store at room temperature for up to 24-72 hours; boric acid is the most common preservative fluid used for culture.

Urine in the urinary bladder is normally sterile or can be transiently colonized with small numbers of organisms. The urethra is usually colonized with a larger number of organisms. Urinary tract infections (UTIs) occur most commonly thru the ascending route; they can also occur via hematogenous and lymphatic routes. Females are much more commonly predisposed to UTIs than males, mainly because of anatomic differences: females have shorter urethras and their urethras have closer proximity to the vagina and perianal areas. The most common urinary tract pathogens grow rapidly. Escherichia coli is the most common UTI pathogen. Most infections are caused by a single bacterial species. ^{[2, 5, 6]}

A study by Meckes et al of women seeking UTI symptom treatment found that in older women, patients with a history of recurrent UTIs, and individuals who presented with dysuria, the chance of having a positive urine culture was greater than it was for those with urinary frequency. The adjusted odds ratio for women with recurrent UTIs was 2.45, while that for patients with dysuria was 1.53, and that for age was 1.02. ^[7]

Indications/Applications

Urine culture is used for the diagnosis of UTI, namely, cystitis, urethritis, and pyelonephritis, and to identify the pathogen and guide in the antimicrobial therapy.

Considerations

As mentioned, a positive urine culture and the need for treatment should be interpreted in the context of clinical symptoms and signs of UTI. All symptomatic UTIs should be treated. ^[2] Because of the higher risk of developing a symptomatic UTI, especially pyelonephritis, asymptomatic bacteriuria should be treated in pregnant women and those undergoing invasive urologic instrumentation; ^[8]  most authorities also treat renal transplant recipients who have asymptomatic bacteriuria. Treatment decisions for asymptomatic bacteriuria in children are based on imaging studies. ^[2]

Recent or concurrent antibiotic therapy can lead to false-negative urine culture results.

Urine specimens that have not been processed immediately (eg, left at room temperature for >2 hours) are susceptible to the growth of bacteria, including contaminants, leading to false-positive results. These specimens should be discarded. Urine specimens obtained from catheter bags are also unacceptable.