Urine Culture

Laboratory examination and culture of urine are used to evaluate urinary tract infections (UTIs), especially bladder infections. Urine in the kidneys and bladder is normally sterile, but a urine specimen may contain various organisms due to bacteria in the urethra and on external genitalia. Bacteriuria generally results from one prevalent bacteria type; the presence of more than two bacterial species in a specimen strongly suggests contamination during collection. A single negative culture does not always rule out infection; a quantitative examination of urine culture is needed.

Purpose

• To diagnose UTI
• To monitor microorganism colonization after urinary catheter insertion

Patient preparation

• Explain to the patient that this test is used to detect UTI.
• Inform the patient that the test requires a urine specimen and that no restriction of food or fluids is necessary. Tell him that the first voided specimen of the day is preferred because it will reflect a high colony count after an overnight incubation period.
• Instruct him how to collect a cleanvoided midstream specimen; emphasize that external genitalia must be cleaned thoroughly.
• If appropriate, explain catheterization or suprapubic aspiration to the patient, and inform him that he may experience some discomfort during specimen collection.
• For the patient with suspected tuberculosis, specimen collection may be required on three consecutive mornings.
• Check the patient's history for current antimicrobial therapy.

Equipment

Gloves, sterile specimen cup, towelettes or sterile water, cleansing solution (such as aqueous green soap), and cotton balls or sterile gauze sponges (Note that commercial clean-catch urine kits are available.)

Procedure and posttest care

• Collect a urine specimen.
• Record on the laboratory slip the suspected diagnosis, the collection time and method, current antimicrobial therapy, and fluid- or drug-induced diuresis.

Precautions

• Wear gloves when performing the procedure and handling specimens.
• Collect at least 3 ml of urine, but don't fill the specimen cup more than halfway.
• When obtaining a specimen from an indwelling catheter, clamp the tubing below the collection port to collect a specimen in the tubing. Then use an alcohol pad to clean the port. Next, using a sterile needle and syringe, aspirate a 4-ml specimen from the port and transfer it into a sterile specimen cup.
• Seal the cup with a sterile lid, and send it to the laboratory immediately. If transport is delayed longer than 30 minutes, store the specimen at 39.2° F (4° C) or place it on ice, unless a urine transport tube containing preservative is used.

Normal Findings

Culture results that contain 10,000 or fewer organisms per milliliter are considered negative.

Abnormal findings

Bacterial counts of 100,000/ml or more of a single microbe species indicate probable UTI. Counts under 100,000/ml may be significant, depending on the patient's age, sex, history, and other individual factors. Counts under 10,000/ml usually suggest that the organisms are contaminants, except in symptomatic patients, those with urologic disorders, and those whose urine specimens were collected by suprapubic aspiration. A special test for acid-fast bacteria isolates Myco­bacterium tuberculosis, thus indicating tuberculosis of the urinary tract.

Isolation of more than two species of organisms or of vaginal or skin organisms usually suggests contamination and requires a repeat culture. Prolonged catheterization or urinary diversion may cause polymicrobial infection.

Interfering factors

• Failure to use proper collection technique
• Failure to preserve the specimen properly or to immediately send it to the laboratory
• Fluid- or drug-induced diuresis and antimicrobial therapy (possible decrease)