February 1996
SAN FRANCISCO—Because urinary tract infections are the most common cause of serious bacterial infections in children, their accurate and prompt diagnosis by pediatricians is critical.
Of 4 million infants born in the United States in 1992, 50,000 had a urinary tract infection (UTI) before their first birthday, according to Ellen R. Wald, MD, Professor of Pediatrics at the University of Pittsburgh School of Medicine. Of this group, 40%, or 20,000 infants, had vesicle ureteral reflux. Of those, 20% (or 4,000) developed renal scarring. Half of the total group, or 2,000 children, will develop hypertension before age 30, and 200 children in this single year's cohort will develop end-stage renal disease, Wald noted in a presentation at the American Academy of Pediatrics annual meeting here.
By far, the most common cause of urinary tract infection in children is gram-negative bacteria entering the urinary tract by an ascending route of infection that starts when fecal flora travel from the perirectal area and gain access to the urethra and bladder urine. "Even illnesses outside the urinary tract may predispose a child to develop a UTI," Wald said. "When a child has nasal congestion, pharyngitis, anorexia or vomiting that alters fluid intake, voiding may be less frequent and not forceful enough to clear away any bacteria that has made its way to the urethra, and infection may develop."
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Symptoms of a UTI may be specific or nonspecific. Specific signs of bladder infection include frequency, urgency, and dysuria, whereas specific signs of upper urinary tract disease are flank pain and tenderness. Nonspecific symptoms of bladder infection are irritability and low-grade fever, whereas high-grade fever, rigors, and chills signal upper urinary tract disease. Failure to thrive may indicate the presence of chronic UTI. "The practitioner really has to consider the diagnosis of UTI in any child who presents with an unexplained fever," Wald said, "but a definitive diagnosis comes from a urine culture."
The exact definition of urinary tract infection varies according to the method used to collect urine for a culture, she explained. In a toilet-trained child, a mid-stream clean catch is adequate (at least 105, or 100,000 colony-forming units/mL), or urine can be collected by a urinary catheter (5 x 104, or 50,000). "The best specimen, however, is the suprapubic specimen, where any bacterium indicates UTI, because it bypasses the distal urethra, a potential area of contamination. If this cannot be done, a urethral catheterization is the next best option," she advised.
Another method of collection, bagged urine, is best used for screening evaluations rather than in cases where a diagnosis for a specific type of urinary tract infection is needed because of the potential for bacterial contamination when the urine passes through the distal urethra.
Urine cultures can easily be misinterpreted, cautioned Wald, because of several problems. One is contamination that occurs during collection. "The hard part is that the contaminating agent is usually Escherichia coli, the very same organism that is the most common cause of urinary infections," she said. A second problem is asymptomatic bacteriuria, a temporary condition where bacteria gain access to the bladder urine without causing inflammation or symptoms.
One primary measure of urinary tract infection involves the number of white blood cells in the urine, or pyuria, an issue which is variously defined. "The first question is how many white blood cells are too many, and the second is which specimen should we use for evaluation?" noted Wald. "Should we use a centrifuged urine [specimen] and evaluate the sediment, or is it better to look at unspun urine in the same way we look at cerebral spinal fluid using a counting chamber?"
Spinning urine introduces many variables, she said, and has made unspun methods more popular recently, giving rise to the belief that pyuria is present when at least 10 white blood cells/mm³ are present. "Many reports define pyuria as the number of white blood cells per high power field on a centrifuged specimen, which has led to variable reports regarding the sensitivity of pyuria as a measure of UTI, so it's not a definition I favor."
Pyuria, she said, is almost always present in children with UTI, and it is very helpful to be able to identify it in the office setting. The best way for office pediatricians to evaluate children with unexplained fever, according to Wald, is with an "enhanced urinalysis," a method developed at the University of Pittsburgh that predicts a positive urine culture more accurately than the standard urinalysis.
"The enhanced urinalysis is significantly better than the standard urinalysis both in terms of sensitivity and also positive predictive value," she explained. "The high positive predictive value of the combination of pyuria and bacteriuria gives us the freedom to initiate antimicrobial [therapy] on the basis of the urinalysis in febrile children who present in the emergency room. We can now feel confident they're going to have a positive urine culture."
An enhanced urinalysis defines pyuria as 10 white blood cells/mm³ and bacteriuria as any organism in 10 oil immersion fields. An uncentrifuged specimen should be placed on a counting chamber to enumerate the number of white blood cells/mm³, and a gram-stained smear should also be obtained in a standardized way to report the number of bacteria per oil immersion field. Neither the nitrite nor the leukocyte esterase tests are sufficiently sensitive to be used as predictors of UTI. "The information from an enhanced urinalysis will allow you to make a reasonable decision as to whether antimicrobial [therapy is] needed. Ask the laboratory your office or hospital uses to provide this service," Wald recommended.
Finally, she stressed, although the notion has crept into the literature that UTI frequently occurs without pyuria, that is simply not true if one uses an appropriate definition of pyuria. Pyuria is nearly universal in children with symptomatic UTI. If a urine culture is positive but pyuria is absent, only three possible explanations exist: it may be a very early infection where inflammation has not developed; the urine may be contaminated (the most likely explanation); or the patient may be experiencing an episode of asymptomatic bacteria with fever from another source.
A second culture should give the pediatrician the data needed to narrow these alternatives.
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