Pediatric Urinary Tract Infection Workup

Good clinical and experimental data support the opinion that the best way to prevent kidney damage due to a urinary tract infection (UTI) is through prompt diagnosis and effective treatment of a febrile UTI.

The diagnosis of UTI is based on the quantitative culture of a properly collected urine specimen. A midstream, clean-catch specimen may be obtained from children who have urinary control. In the infant or child unable to void on request, the specimen for culture should be obtained by means of suprapubic aspiration or urethral catheterization.

Suprapubic aspiration is also the method of choice for obtaining urine from uncircumcised boys with a redundant or tight foreskin and from children of either sex with clinically significant periurethral irritation.

A culture of a urinary specimen from a sterile bag attached to the perineal area that shows no or scant growth (< 10,000 colony-forming units [CFUs]/mL) is strong evidence that UTI is absent. However, the false-positive rate is so high that this method of urine collection is not suitable for diagnosing UTI.^[5]

As a means of documenting the presence of UTI, urinalysis is not a substitute for urine culture. However, it can help in identifying those febrile children who should receive antibacterial treatment while culture results from a properly collected urine specimen are pending.

Numerous studies have compared dipstick tests for leukocyte esterase and nitrite with microscopic examination of the urinary sediment for bacteria with urine culture. These studies have been performed to determine whether the dipstick test can eliminate the need for urine culture. The invasiveness, time involved, and cost of the culture could be eliminated.

Studies report that the specific components of the dipstick test and complete urinalysis cannot detect positive culture findings in all children who have positive urine culture findings and unexplained fever or voiding symptoms. The conclusion is that urine should be obtained for culture, and initial treatment should be given to those children with fever and a presumptive diagnosis of a UTI, regardless of the result of urinary dipstick testing or urinalysis.^{[6, 7, 8, 9, 10, 11, 12]}

A study by Lunn et al supports the use of automated microscopy for screening urine samples for culture in children.^[13] In 280 urine samples collected from 263 pediatric patients, automated microscopy performed comparably to urine dipstick in the diagnosis of UTI; automated microscopy had better specificity and likelihood ratios than dipstick testing, but it had slightly lower sensitivity.

Hematologic studies do not tend to help in the diagnosis of UTIs, although they should be obtained in patients who appear ill. In patients with suspected bacteremia or urosepsis, blood cultures should be obtained.

Renal function can be measured by serum creatinine and BUN (blood urea nitrogen) levels and may be elevated in severe disease. Moreover, electrolyte abnormalities may be present in UTI.

Procalcitonin, a propeptide of calcitonin that has been found to be elevated in response to bacterial endotoxins, has shown promise in helping to diagnosis pyelonephritis and early renal damage.^{[14, 15, 16, 17]}

Obtain a complete blood count (CBC) and basic metabolic panel for children with a presumptive diagnosis of pyelonephritis. Perform blood cultures in febrile infants and older patients who are clinically ill, toxic, or severely febrile.

Imaging

The American Academy of Pediatrics recommends that all infants and young children (aged 2mo to 2y) with a first UTI undergo urinary tract ultrasonography and voiding cystourethrography (VCUG). These tests should be acquired promptly if patients fail to show expected clinical response within 2 days of treatment.^[5]

If imaging studies of the urinary tract are warranted, they should not be obtained until the diagnosis is confirmed with a quantitative urine culture. The following types of pediatric patients do not need imaging studies after a first UTI:

• Infants and children with a first febrile UTI who are assured follow-up, who respond promptly to treatment (afebrile within 72 h), and who have a normal voiding pattern (no dribbling) and no abdominal mass
• Infants and children with cystitis

With regard to the second above item, children with an abnormal voiding pattern after receiving effective treatment of the UTI may need to undergo an evaluation for voiding dysfunction; this may include standard VCUG.

The criterion standard for diagnosing urinary tract infections (UTIs) is the isolation of a pathogen from a urine culture obtained via suprapubic aspiration.^[18] However, catheterization, rather than aspiration, is the most commonly used collection technique in infants and younger children.

Midstream clean-catch urine specimens are adequate for older children who can provide them. More than 100,000 CFU/mL in a midstream clean-catch urine specimen is defined as a UTI.

Due to high rates of false positives, bagged urine specimens are of no value in the diagnosis of UTIs, and their use should be discouraged.

Because it may take as long as 48 hours for a urine culture to be positive, urinalysis is often needed to help make the initial diagnosis of UTI. A urine specimen that is found to be positive on dipstick for nitrite, leukocyte esterase, or blood may indicate a UTI. Dipstick tests have sensitivities approaching 85-90%.

Microscopic examination of spun urine can evaluate for the presence of white blood cells (WBCs), red blood cells (RBCs), bacteria, casts, and skin contamination (eg, epithelial cells).

With regard to suprapubic aspirate, the presence of 5 or more WBCs/high-power field suggests an infection. The presence of 10 or more WBCs/μL is also consistent with infection.^{[19, 20]} Gram stain of unspun urine obtained via suprapubic aspirate may reveal organisms.

A hemacytometer measures cells per volume and has been found to be more sensitive and specific than standard microscopic examination.^{[19, 20]} A combination of hemacytometer cell count and Gram stain has been shown in studies to have a sensitivity approaching 95%.

In pediatric patients, urine cultures should be sent to the laboratory even if urinalysis results are inconclusive. Approximately 10-20% of pediatric patients with UTI have normal urinalysis results. Multiple organisms may be present if structural abnormalities exist.

Methods of urine collection and examination, as well as salient findings, are shown in tables 1 and 2.

Table 1. Urinalysis for Presumptive Diagnosis of Urinary Tract Infection* (Open Table in a new window)

Table 2. Quantitative Urine Culture for the Diagnosis of Urinary Tract Infection* (Open Table in a new window)

Ultrasonography of the urinary tract has replaced the use of intravenous pyelography (IVP). However, studies have shown that ultrasonograms of the urinary tract obtained after a first febrile urinary tract infection (UTI) seldom provide information that changes management. Nonetheless, the American Academy of Pediatrics Clinical Practice Guidelines recommend routine ultrasonography after a first febrile UTI in children aged 2-24 months.^[5]

However, a current recommendation is that urinary ultrasonography be omitted after a first febrile UTI in infants and children if they respond to treatment (afebrile within 72h), good follow-up is assured, and no voiding abnormality (no dribbling of urine) or abdominal mass is present.

The clinician's judgment should guide the decision regarding imaging studies, as opposed to a rigid rule. Urinary ultrasonography is a safe, noninvasive study that is easy to perform. It is useful in excluding obstructive uropathy and in identifying children with a solitary or ectopic kidney and some patients with moderate renal damage caused by pyelonephritis.^[21]

Recommendations

Febrile infants aged 2-24 months with UTI should undergo renal and bladder ultrasonography.^[5] In addition, the following pediatric patients should also undergo ultrasonography of the urinary tract after a febrile UTI:

• Children with a delayed or unsatisfactory response to treatment of the first febrile UTI
• Children with an abdominal mass or abnormal voiding (dribbling of urine)
• Children with a first febrile UTI caused by an organism other than E coli
• Children with recurrence of a febrile UTI after they have had a satisfactory response to treatment of the initial febrile UTI

Renal ultrasonography should also be considered for any child with a first febrile UTI in whom good follow-up cannot be ensured.

Traditionally, VCUG has been recommended for infants and children after a first febrile UTI. This is based on the assumptions that most upper UTIs occur because of urinary bladder infection and that vesicoureteral reflux (VUR) transfers bacteria in the bladder to the kidney. However, using cortical imaging, current data show that upper tract infection occurs equally in children with or without VUR.

VCUG is indicated if renal and bladder ultrasonography reveals hydronephrosis, scarring, or other findings that suggest either high-grade VUR or obstructive uropathy. VCUG should be performed if there is a recurrence of a febrile UTI, even if previous ultrasonographic examination findings were unremarkable.^[5]

The assumption that antibacterial prophylaxis prevents a recurrence of UTI seems reasonable, although this has not been proven. However, a review of literature published since 2005 suggested that the risk of developing long-term complications after a UTI is extremely low, that imaging should be used only in a select group of children at risk, and that the use of antibiotic prophylaxis remains questionable.^[22] Because data do not support the use of antimicrobial prophylaxis to prevent febrile recurrent UTI in infants with no VUR or with grade I-IV VUR, VCUG is not recommended routinely after the first UTI.^[5]

Of note, a first febrile UTI is as frequent in infants with VUR as in those without radiographically demonstrated reflux. The recommendation for VCUG after a first febrile UTI is based on expert opinion and judgment, not on evidence-based guidelines.

Some experts suggest that cystography that requires catheterization of the urinary bladder be avoided. Their recommendation, which is not evidence based, is that renal cortical scanning (renal scintigraphy) should be performed. This study helps in identifying kidney injury and/or pyelonephritis. If the scan findings are normal, cystography is not needed. However, if the results are abnormal, VCUG should be obtained.

If a VCUG is to be obtained, it should be obtained after the voiding pattern returns to its pre-UTI state. If the organism that caused the UTI was susceptible to the antibacterial used to treat the febrile UTI and if the response to therapy was satisfactory, follow-up urinalysis or cultures are not needed. The child should receive antibacterial therapy at least until the cystogram is obtained.

Some clinicians recommend waiting 4-6 weeks after febrile UTI is treated to perform VCUG. If the child is given suppressive antibacterial treatment during this period, this recommendation is acceptable. However, studies have shown that the VCUG may be obtained within the first few days of treating febrile UTI if the voiding pattern has returned to its pre-UTI state.

Recommendations

The following types of pediatric patients should undergo VCUG after a first febrile UTI:

• Those in whom treatment fails after 48-72 hours
• Patients with an abnormal voiding pattern (dribbling of urine)
• Infants and children in whom good follow-up is not assured
• Those with an abdominal mass
• Infants and children with recurrence of a febrile UTI