Urine Analysis (2/3): Urine Culture
Urine Collection for Culture
First choice for the urine collection is during sterile catheterizations and interventions. Otherwise, the urine collection is made in men by midstream specimen, in women by catheterization or midstream clean-catch specimen (which is frequently contaminated). A midstream specimen in women has only a clinical significance, if it is not pathological. The urine collection in infants is made with an adhesive bag.
Interpretation of Urine Culture
105 colony forming units per ml urine (cfu/ml) obtained in a clean midstream specimen is a sign of a clinically significant urinary tract infection (Kass criteria). With high urine output or with a sterile urine collection, lower colony counts also indicate a significant UTI.
The colony count can be determined by using a quantitative urine culture. After dilution of the urine (1:1000), 1 ml is used to inoculate an agar plate. Semiquantitatively, the bacterial count is determined with the help of commercially available systems (dip slides). A diagnostic plate is dipped into urine and incubated for 24 hours. The colony count is estimated by comparing to a visual standard.
Identification of Bacteria
After counting of the bacteria colonies, bacteria are identified by taking advantage of the different biochemical properties of bacteria. Bacteria are incubated in different medias with color reactions indicating e.g. enzyme activity (lactase, fermentation, urease), motility or use of citrate as an energy source. The results of the biochemical properties are assigned to bacteria species with the help of a table. Other identification techniques use molecularbiological assays and look for differences at DNA-level or surface antigens.
The determination of antibiotic resistance is more important than the species assignment of the bacteria. The effect of an antibiotic drug against bacteria growth can be measured with broth dilution tests or agar diffusion tests; the output of the test is the minimum inhibitory concentration (MIC) of commonly used antibiotics.
Lower Urinary Tract Culture Techniques
To distinguish urethral, bladder or prostate infections, Meares and Stamey (1968) developed the four-glass urine test. It is the gold standard for the evaluation of patients with symptoms of chronic prostatitis. Four different collections of urine and expressed prostatic secretions are sent for bacterial culture (identification and antibiotic):
- Voided Bladder 1 (VB1): first 10 ml of urine (urethral specimen)
- Voided Bladder 2 (VB2): midstream sample (bladder specimen)
- Expressed prostatic secretions (EPS): microscopic examination shows typically less than 10 leukocytes per high power field. EPS is also send for bacterial culture (prostate specimen)
- Voided Bladder 3 (VB3): first 10 ml of urine after prostatic massage (prostate specimen)
A simple, reliable and cost-effective alternative is the two-glass test (Weidner and Ebner 1985, Nickel 1995):
- Pre-massage midstream urine sample (bladder specimen)
- Post-massage urine sample: first 10 ml of urine after prostatic massage (prostate specimen)
Index: 1–9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
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