Urine Culture :
In the past, the diagnosis of UTI was based on a quantitative urine culture yielding greater than 100,000 colony-forming units (CFU) of bacteria per milliliter of urine,termed "significant bacteriuria." This value was chosen because of its high specificity for the diagnosis of true infection, even in asymptomatic persons. However, several studies have established that one third or more of symptomatic women have CFU counts below this level (low-coliform-count infections) and that a bacterial count of 100 CFU per mL of urine has a high positive predictive value for cystitis in symptomatic women. Unfortunately, not all clinical laboratories report counts of less than 10,000 CFU per mL of urine. As a result, lowcoliform-count infections are under diagnosed.
In the majority of women with symptoms of UTI, a culture is rarely necessary and does not affect the management. An urine culture is invaluable in the following situations - in all patients with systemic symptoms suggestive of pyelonephritis, pregnant women, men and in patients who recurrent and frequent UTI.
The microbiology of uncomplicated cystitis is limited to a few pathogens. As many as 90 percent of uncomplicated cystitis episodes are caused by Escherichia coli, 10 to 20 percent are caused by coagulase-negative Staphylococcus saprophyticus and 5 percent or less are caused by other Enterobacteriaceae organisms or enterococci. In addition, the antimicrobial susceptibilities of these organisms are highly predictable. Up to one third of uropathogens are resistant to ampicillin and sulfonamides, but the majority are susceptible to trimethoprim-sulfamethoxazole (85 to 95 percent) and fluoroquinolones (95 percent). In view of the limited spectrum of causative organisms and their predictable susceptibility, urine cultures and susceptibility testing add little to the choice of antibiotic for the treatment of acute uncomplicated cystitis in young women. Therefore, urine cultures are no longer advocated as part of the routine work-up of these patients.
Instead, these patients should undergo an abbreviated laboratory work-up in which the presence of pyuria is confirmed by traditional urinalysis (wet mount examination of spun urine), the cell-counting chamber technique or a dipstick test for leukocyte esterase.
A positive leukocyte esterase test has a reported sensitivity of 75 to 90 percent in detecting pyuria associated with a UTI. Gram staining of unspun urine can be used to detect bacteriuria. In this semiquantitative test, one organism per oil immersion field correlates with 100,000 CFU per mL by culture. Because the procedure
CFU per mL by culture. Because the procedure is timeconsuming and has low sensitivity, it is not routinely performed in most clinical laboratories unless it is specifically requested.
The dipstick test for nitrite is used as a surrogate marker for bacteriuria. The urine dipstick is an increasingly available easy to use tool that can be used in any physician’s office. While many abnormalities can be detected in the strip, the nitrite test and test for leukocyte oxidase are the most useful. Nitrite is generated from urinary nitrate reducing active of bacteria while leukocyte oxidase reflects the presence of white cells in the urine. If both of these are positive then UTI is present over 90% of the time. A positive nitrite test alone on dipstick while specific for UTI is only 30% sensitive. It should be noted that not all uropathogens reducenitrates to nitrite. For example, enterococci, S. saprophyticus and Acinetobacter species do not and therefore give false-negative results. In a patient with a high index of suspicion of UTI and a negative dipstick, direct examination of the urine and or nitrates to nitrite. For example, enterococci, S. saprophyticus and Acinetobacter species do not and therefore give false-negative results. In a patient with a high index of suspicion of UTI and a negative dipstick, direct examination of the urine and or culture must be done piror to excluding infection as the cause of the symptoms.If the urine does not contain bacteria urethritis, prostatis or vaginitis must be considered as most likely. Adenovirus and chemotherapeutic agents can cause non bacterial cystits. Other considerations, include tuberculosis, bladder stone, bladder tumors and interstitial cystitis.
Urine culture rarely necessary
Urine culture invaluable in,
1. Pyelonephritis,
2. Pregnant women,
3. Men
Recurrent
90% due to E.Coli
10% due to staph saphrophyticus
5% due to Enterococci
Majority susceptible to TMP SMX and fluoroquinolones.
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