Gram-negative enteric bacteria are the most commonly isolated organisms in UTI. While P-fimbriated Escherichia coli accounts for the vast majority of infections, Klebsiella, Proteus, and Enterobacter species are also important pathogens. Enterococcus species, Staphylococcus aureus, and group B streptococci are the most frequently isolated gram-positive organisms and are more likely to be causative organisms in the neonatal period. Coagulase-negative staphylococcal UTI occurs in teens and young adults. Adenovirus may cause acute cystitis, occurs more commonly in young boys, and is clinically indistinguishable in many cases from bacterial disease.
While urine culture is the gold standard in the diagnosis of UTIs in children, culture results are rarely, if ever, available to physicians in the emergency department. Therefore, emergency physicians must rely on clinical data and other laboratory tests while obtaining a urine culture to subsequently confirm the diagnosis.
With an incontinent child, a specimen for urinalysis and culture should be obtained through direct bladder sampling (catheterization or suprapubic sampling) or through a cleanly voided specimen in boys and older girls. Bagged urine specimens are not acceptable because of the high degree of contamination. Urine specimens from older children with voiding control should be obtained from a cleanly voided midstream clean catch. 5 Parents should be instructed on the proper technique for avoiding contamination of specimens. Periurethral contamination of specimens can be avoided by having girls sit backward (facing the rear of the toilet). This position favors labial retraction and better exposure of the urethral meatus. Because bacterial contaminants grow rapidly at room temperature, a urine specimen that cannot be cultured immediately should be kept on ice or at a temperature of 4°C (39.2°F) until culturing can be accomplished.
While urinalysis is the most common adjunctive diagnostic test for a possible UTI, urine chemical test strips are also a quick means of initial urine screening for the detection of leukocyte esterase and urinary nitrates. Esterases are released into the urine after the breakdown of white blood cells, providing presumptive evidence of infection. Occasionally, false-negative urine test strip results are obtained for very young infants because their leukocyte response may be limited. Nitrates are converted to nitrites by gram-negative urinary pathogens, and urine test strips may also provide indirect evidence of bacteriuria. Hematuria, proteinuria, and pyuria are commonly associated with UTIs but are nonspecific and can occur in the absence of infection. The presence of bacteria in catheterized urinary sediment also lends support to the presence of a UTI. In most studies, the sensitivities of a positive test result for urinary leukocyte esterase or nitrite or a positive urine culture result are less than 50 percent. The combined presence of pyuria (more than five white blood cells per high-power field) and bacteriuria on urine microanalysis improves the sensitivity to approximately 65 percent. The positive predictive value of a urinalysis is 81 percent.
Peripheral white blood cell counts and erythrocyte sedimentation rates are also nonspecific indicators that should only be interpreted in conjunction with the results of the urinalysis and urine culture. A guide to interpreting urine culture results based on the various methods of obtaining urine samples is contained in Iableii136:l.
TABLE 136-1 Interpretation of Positive Urine Culture Results
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