Serwer UMW



URINARY TRACT INFCETIONS

The diagnosis of UTI is performed by urinalysis and culturing urine, with is normally sterile.

If a patient has no signs of infection on urinalysis, no symptoms of infection, but a positive urine culture, the patient by definition has asymptomatic bacteriuria, or the specimen was contaminated at the time of collection with organisms present on the skin/mucous membranes.

Asymptomatic bacteriuria denotes significant bacteriuria (>105 CFU/mL of urine) without clinical symptoms or other abnormal findings.

• Asymptomatic bacteriuria is a relatively common finding. Anatomic obstruction of any kind markedly increases the incidence of asymptomatic bacteriuria.

• Asymptomatic bacteriuria affects mainly women until men develop prostatism.

• Most patients remain asymptomatic. In patients without anatomic factors predisposing to UTI, asymptomatic bacteriuria often resolves through normal host defense mechanisms including the mechanical flushing of the urinary stream. Complications such as pyelonephritis and sepsis are likely to occur in certain patient groups: (1) pregnant women; (2) patients undergoing genitourinary surgery; and (3) patients undergoing urethral catheterization in the presence of complicating disease.

Female normal urethral flora consist of: diphtheroids, lactobacilli, coagulase-negative staphylocci, alpha streptococci and low numbers of Enterobacteriaceae.

Male normal urethral flora consists of: diphtheroids, coagulase-negative staphylococci, and Enterococcus species.

Normal urine can contain low numbers of these organisms. Lactobacilli, alpha streptococci, and diphtheroids are not urinary pathogens, but Enterobacteriaceae and enterococci are potential urinary tract pathogens.

DIAGNOSIS

Specimen collection: There are several types of urine specimens and the results of each type are determined by different guidelines.

Clean catch midstream urine, voided urine specimen or specimen collected during the middle of voiding (midstream urine specimen).

The clean-catch is the most common type of urine specimen. The technique involved in collection is based on discarding the first portion of urine. The first voided specimen should be discarded since the initial urine flushes urethral contaminants. It is the second, midstream sample that should be sent to the laboratory. First, the urethral area should be cleaned with an antiseptic cloth and then the midstream urine sample should be collected into a sterile container. It is recommended that the first voided morning specimen be collected so bacteria will have multiplied to high levels after overnight incubation in the bladder. If this is not possible, the urine should be allowed to incubate in the bladder as long as possible before collection.

Indwelling catheter or Foley catheter

Catheterized specimens are indicated in certain situations, such as patients who are unable to provide clean-catch specimens because of urologic or neurologic problems including impaired consciousness. Hospitalized patients who have indwelling or Foley catheters are especially at risk for developing UTI. To avoid contamination, urine sample should be drawn in a sterile fashion (after disinfection with alcohol) from either the catheter itself or through the port designed specifically for this purpose, NOT from the urine collection bag. Specimen collection is critical since colonization of the Foley bag or actual catheter is common.

Suprapubic Aspirate

The suprapubic aspirate technique avoids urethral contamination but is invasive and is seldom used in today's practice.  It is usually reserved for infants, from whom it is difficult to obtain urine specimens. In this procedure, the skin above the bladder is disinfected and sterile needle and syringe are plunged into the bladder. Urine is aspirated and placed in a sterile container.

Specimens transport

Because urine is an excellent culture medium for bacteria, urine specimens must be sent to the laboratory as soon as possible at a refrigerator temperature (40C). Specimens should be plated within 2 hours of collection or bacterial counts will not be valid. If this is not possible the specimen may be refrigerated for a maximum of 4 hours before plating. Specimens should not be allowed to sit out on counter tops, since aerobic bacteria double about every 20 minutes at room temperature, causing false-positive results on microscopy, dipstick urinalysis, and culture.

Screening methods

Gross inspection of urine

Urine can be cloudy because of the presence of white blood cells (>200 per mL), red blood cells (>500 per mL), bacteria (>106 per mL), fat or sediment such as crystals. Crystals are more prominent in alkaline specimens. The urine of patients with clinical UTI is typically cloudy, but cloudy urine is not synonymous with UTI.

There are several screening methods available to detect urine specimens containing high levels of bacteriuria and pyuria.

Dipstick analysis of urine

Rapid dipstick techniques as a supplement to or substitute for traditional methods of diagnosis based on microscopy and culture. Chemical reagents on separate test pads of the dipstick evaluate different properties of urine, such as pH, glucose and protein content, and the presence or absence of white blood cells (pyuria), red blood cells (hematuria), and significant bacteriuria. The procedure is to dip the stick into fresh, uncentrifuged urine, covering all of the test pads. The strip is then withdrawn immediately along the edge of the container in such a way as to remove any excess urine. It should be held horizontally before being read, in order to prevent mixing of the reagents in adjacent pads. Storage and use of dipsticks should be according to the manufacturer's recommendations.  

1) Leukocyte esterase (LE) indicates white blood cells in the urine. The leukocyte esterase test detects pyuria, which correlates with bacteriuria. The test identifies the presence of the enzyme leukocyte esterase which is produced by leukocytes. This test has a reported 75% to 96% sensitivity and a 94% to 98% specificity for detecting pyuria. False-positive tests are usually caused by contamination, often by vaginal secretions. False-negative specimens can be caused by hypertonic urine (as determined by high specific gravity), glycosuria, and urobilinogen, in case of ketonuria, proteinuria, during treatment with tetracycline, gentamycine, and after high doses of vitamin C administration. The sensitivity of the LE dipstick test can be increased by combining it with the nitrate test.

2) The nitrite test is used to screen for significant bacteriuria. It is based on two observations: (a) normal urine contains nitrates but not nitrites; and (b) about 90% of bacterial species causing UTI can convert urinary nitrates to nitrites. The nitrite test has a 92% to 100% sensitivity for UTI but only a 35% to 85% specificity. It is most useful for detecting >105 CFU/mL of aerobic gram-negative rods. The nitrite test is especially useful in patients with indwelling urinary catheters to determine whether or not they are infected. In children, the sensitivity of the nitrite test is high (up to 98%) but specificity is lower (29% to 44%) than in adults. False-positive tests can result from substances that cause red urine such as the ingestion of beets in susceptible subjects or the bladder analgesic phenazopyridine. False-negative nitrite tests can occur in “low-count” UTI (>105 CFU of bacteria per mL of urine), infections caused by bacteria that do not produce nitrites (such as enterococci, Staphylococcus saprophiticus, and Pseudomonas aeruginosa), short bladder dwell time, dilute urine specimens, or acid urine. The nitrate test is not sensitive enough to be used alone.

3) Pyuria (more sensitive than leukocyte esterase): >5-10 WBC/hpf per ml (high power field in microscope – for urine objective 40x) or >27 WBC/ml.

Sterile pyuria (positive urinalysis, but negative culture results) should be considered in cases of interstitial nephritis or cystitis, infection with fastidious organisms such as Mycobacterium tuberculosis, Chlamydia spp., fungi, viruses.

4) Gram stain of uncentrifuged/unspun urine. The gram stain is the easiest and most sensitive screening method. One drop of uncentrifuged urine is placed on a slide and allowed to dry without spreading. The slide is than gram stained and examined under oil immersion. The presence of one or more bacterial per oil immersion field correlates with a bacterial count of greater than 105 CFU/ ml. The presence of one or more PMNs per oil immersion field indicates pyuria. The gram stain cannot detect bacterial counts of less than 105 CFU/ ml.

Specimen processing

Urinalysis and urine cultures must be interpreted together in context of symptoms.

Quantitative Cultures

Quantitative cultures should be performed on all urine specimens so that the number of bacteria per milliliter of urine can be determined and expressed as CFU/ml. The most common method uses a calibrated platinum loop that delivers 0.001 ml or 0.01 ml of urine. Each plate is inoculated with one loopful of urine, and the colony count is determined by multiplying the number of colonies by the dilution factor.

For example, if a 0.001 ml loop is used and 10 colonies are observed, the colony count would be 10 x 10000 or 10,000 CFU/ml (104 CFU/ml). If a 0.01 ml loop is used and 10 colonies are observed, the colony count would be 10 x 100 or 1000 CFU/ml (103 CFU/ml). The following procedure is used to perform a quantitative culture of urine using a calibrated loop

1. Mix the urine well

2. Vertically insert a flamed and cooled calibrated loop into the specimen and immerse it just below the surface of the specimen. Only move the loop straight up and down

3. Remove a loopful of urine and inoculate each plate by making a straight line down the center and then a series of close perpendicular streaks through the first line. Inoculate each plate with one loopful of urine

4. Incubate the media for 24 hrs

Interpretation of Culture Results

To determine whether bacteria isolated from urine cultures are contaminants or clinically significant agents of UTI, some criteria must be considered, including the identity of the isolate, the number of bacterial types, the type of specimen, and the colony count.

Type of organism

Lactobacillus species, alpha-stretococci, and diphtheroids are not associated with UTI and should not be identified or have susceptibility tests performed. They are regarded as urethral or skin contaminants, no matter what is their colony count.

Number of bacterial type

95% of all UTI infection are caused by one type of organism, and it is unlikely that 2 organisms are involved. It is almost impossible that 3 organisms are involved although exceptions do occur.

Cultures containing 3 or more than organisms are generally considered to be contaminated.

Urine cultures:

If urinalysis is negative for pyuria, positive cultures are likely contamination.

Positive cultures with pyuria are defined as 100,000 (105) colonies. This cutoff is the most sensitive for a true UTI. Situations in which lower colony counts ................
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