Urine Culture Health Article

Definition

A urine culture is a diagnostic laboratory test performed to detect the presence of bacteria in the urine (bacteriuria).

Purpose

Urine cultures are performed to isolate and identify the pathogenic microorganism(s) responsible for causing a urinary tract infection (UTI). Urinary tract infections are more common in females and in children than in adult males. UTI is associated with discomfort (usually burning) on urination, and may be accompanied by fever, malaise, and lower abdominal or back pain. All of the urinary structures except the urethra are normally sterile. Most organisms reach the bladder, ureters, and kidneys by ascending the urethra. The most commonly encountered urinary tract pathogen is E. coli. Enterococcus faecalis is the most common gram positive organism to cause UTI. Infections with Klebsiella, Proteus, and other Enterobacteriaceae are also common. Some organisms not as commonly encountered such as Candida albicans, Haemophilus influenzae, Mycobacterium tuberculosis, Salmonella spp., and Staphylococcus aureus usually enter the urinary system via the blood or lymphatics.

Description

There are several different methods used to collect a urine sample for culture. The most common is the midstream clean-catch technique. Hands should be washed before beginning. For females, the external genitalia are

washed two or three times with a cleansing agent and rinsed with water. In males, the external head of the penis is similarly cleansed and rinsed. The patient is then instructed to begin to urinate, and the urine is collected midstream into a sterile container. In infants, a urinary collection bag (plastic bag with an adhesive seal on one end) is attached over a girl's labia or a boy's penis to collect the specimen.

Another method is the catheterized urine specimen in which a lubricated catheter (thin rubber tube) is inserted through the into the bladder. This avoids contamination from the urethra or external genitalia. If the patient already has a urinary catheter in place, a urine specimen may be collected by clamping the tubing below the collection port and using a sterile needle and syringe to obtain the urine sample; urine cannot be taken from the drainage bag, as it is not fresh and has had an opportunity to grow bacteria at room temperature. On rare occasions, the physician may collect a urine sample by inserting a needle directly into the bladder (suprapubic aspiration). Bladder puncture is warranted when repeated efforts to culture the urine grow contaminants from the urethra. This is especially common in infants. Suprapubic tap is also indicated when anaerobic UTI is suspected.

The urine must be cultured within one hour of collection if not refrigerated. However, refrigerated samples may be stored for up to 24 hours before plating the sample. Urine culture is a quantitative procedure. A calibrated inoculating loop that holds 0.01 or 0.001 mL of urine is inserted vertically into the urine sample and used to transfer the urine to a sterile agar plate. If urine is obtained by bladder puncture, 0.1 mL is transferred to the plate using a sterile pipet. The urine is spread evenly across the plate with a glass rod as opposed to streaking the plate with the loop. This procedure is usually performed on plates of 5% sheep blood agar, which detects growth of most organisms, and on a plate of MacConkey agar or other selective and differential medium for isolation of gram-negative organisms. Additionally, some labs plate urine on colistin-nalidixic acid agar (CNA) or other selective medium for gram-positive bacteria. The plates are incubated at 36°C for 18 to 24 hours and read for growth. The number of colonies is multiplied by the appropriate factor to give the colony count per mL urine. Some organisms, such as Mycobacterium tuberculosis, may be isolated from urine and require special culture media and growth conditions.

Plates which show no growth at 24 hours are incubated another day and read again. Growth of more than three species indicates contamination, and the culture should be repeated with a new specimen. For one to three species, plates are held and a partial identification (e.g. gram-negative rod, lactose positive) is reported when there is less than 10,000 colony forming units (CFU) per mL. Usually, when less than 10,000 CFU/mL are recovered the organism is considered a contaminant from the urethra. Exceptions are the presence of Staphylococcus aureus and organisms isolated from a catheter sample or suprapubic aspiration. Common urethral contaminants include coagulase negative staphylococci, diptheroids, and lactobaccilli. Each colony type giving 10,000 or more CFU/mL is identified and antibiotic susceptibility testing is performed. UTI is diagnosed when a species produces greater than 100,000 CFU/mL. Counts between 10,000 and 100,000 may be significant depending on the organism and patient-specific conditions (e.g. urine collected from a catheter or a patient receiving antibiotic treatment.