Laboratory Tests and Ancillaries
Febrile infants of unknown origin that are unlikely to have urinary tract infection are sufficient to have clinical monitoring without testing.
Urine Specimen Collection
It is difficult to obtain an uncontaminated urine specimen. Urine samples should be collected before initiating antibiotic therapy.
Bag
Urinary Tract Infection Pediatric_Diagnostics 1
Bag is the least traumatic method. This is useful for infants and non-toilet-trained children. A plastic bag is taped at the perineal area, and urine is collected after the child voids. Because of the high risk of contamination, a bagged specimen is not useful in accurately documenting urinary tract infection. This is useful in ruling out urinary tract infection when the result is negative. This is not ideal for urine culture. This has an 88% false positive result, 63% specificity, a 95% false positive rate for febrile boys, and a 99% false positive rate for circumcised boys.
Clean-catch Midstream Urine Specimen
A clean-catch midstream urine specimen may be obtained from toilet-trained patients with no apparent infection or abnormality of the external genitalia. Cleansing before specimen collection is needed. This is likely to be contaminated by periurethral and preputial organisms, especially in young girls and uncircumcised boys. This is useful in ruling out urinary tract infection when the result is negative.
Transurethral Catheterization
Urinary Tract Infection Pediatric_Diagnostics 2
Transurethral catheterization is a traumatic and invasive procedure that may introduce periurethral organisms into an otherwise sterile urinary tract. This has 95% sensitivity and 99% specificity of urine samples when used for culture. This requires cleansing and strict aseptic technique. The initial portion of urine should be discarded because it may be contaminated by periurethral organisms. Transurethral catheterization is recommended for febrile infants of unknown origin that appear to be ill and would require antimicrobial therapy.
Suprapubic Aspiration (SPA)
Suprapubic aspiration is the gold standard for identifying bacteria within the bladder. This is the most sensitive method to collect an uncontaminated urine sample. This is traumatic and difficult to perform. Suprapubic aspiration is recommended for diapered, uncircumcised boys whose urethral openings are difficult to see; patients with urgent indications for treatment who cannot produce a clean-catch midstream urine specimen and cannot be catheterized; and febrile infants of unknown origin that appear to be ill and would require antimicrobial therapy.
Urinalysis
Urine Dipstick Test
Urinary Tract Infection Pediatric_Diagnostics 3
A urine dipstick test may reduce the need for culture, especially in patients with a low likelihood of urinary tract infection (eg vague urinary complaints with an alternative cause of fever). Urine dipstick may have lower sensitivity in infants. This is not recommended for children who frequently void.
Leukocyte Esterase
Leukocyte esterase is produced by activated white blood cells (WBC). This may be falsely negative if WBC are not present during a urinary tract infection, very early infection, and immunocompromised patients. This has a sensitivity of 48-86% and a specificity of 17-93%.
Nitrites
Gram-negative bacteria reduce dietary nitrates to nitrites. This may be falsely negative if the pathogen is Gram-positive or bacterial metabolism has not yet produced nitrites. The presence of urine in the bladder should be at least 4 hours to process nitrates to nitrites. This has a sensitivity of 45-60% and specificity of 85-98%.
Urine Microscopy
The finding that is supportive of urinary tract infection is pyuria or ≥5-10 WBC/high power field. Any bacteria seen on the Gram stain of unspun urine has a sensitivity of 93% and a specificity of 95%. Urine specimens should have been collected <1 hour after voiding or <4 hours after voiding when refrigerated. WBC casts are almost pathognomonic of pyelonephritis.
Flow Imaging Analysis Technology
Flow imaging analysis technology may be used to classify particles (eg WBCs, squamous epithelial cells, and red cells) in uncentrifuged urine.
Urine Culture
Urine culture is the gold standard for urinary tract infection diagnosis. The results of urine culture may take 24 to 48 hours. This is indicated in the following patients: Diagnosed with acute pyelonephritis or upper UTI; have a high to intermediate risk of serious illness; are <3 years of age; have a high likelihood of urinary tract infection (eg classic urinary symptoms); have cloudy urine or single positive results for leukocyte esterase or nitrite activity; and have recurrent symptoms.
The diagnostic thresholds for urine culture based on method of specimen collection are as follows:
| Collection method | Diagnostic threshold |
| Clean-catch voiding | 105 colony forming units (cfu)/mL Repeat testing if 104-105 cfu/mL |
| Urethral catheterization | 105 cfu/mL |
| Suprapubic bladder aspiration (SPA) | Any number of cfu/mL (>10 identical colonies) |
Additional Laboratory Exams
Blood Culture
Blood culture is unnecessary in most children with urinary tract infection. This must be done in children with septic syndrome or septic shock and febrile infants.
Complete Septic Workup
A complete septic work-up must be done in neonates to avoid missing a diagnosis of meningitis and in children with septic syndrome or septic shock.
C-reactive Protein (CRP)
C-reactive protein may help differentiate upper UTI from lower UTI and other causes of bacteriuria. A concentration of >20 g/mL signifies serious bacterial infection. This may be useful in ruling out acute pyelonephritis or upper UTI in patients with pyuria and fever who may have viral infection.
Procalcitonin
Procalcitonin may help differentiate upper UTI from lower UTI and other causes of bacteriuria. A cut-off value of >1.0 ng/mL is predictive of acute pyelonephritis. This may be seen in the early phase of acute pyelonephritis.
Imaging
In the acute setting, diagnostic urinary tract imaging is generally not necessary unless the diagnosis of urinary tract infection is equivocal. Imaging studies can most often be done after the resolution of the acute infection because management during this time is based on the patient’s clinical profile. Routine imaging for patients with a first UTI is not recommended because imaging has not been shown to alter outcomes and is also not cost-effective.
Indications for Early Imaging During Urinary Tract Infection
Indications for early imaging during urinary tract infection include: Persistence of signs and symptoms of UTI after 48 hours of appropriate antibiotic therapy; to identify conditions that require invasive therapy (eg renal abscess, anatomic abnormalities) that may be corrected surgically; possible urinary tract obstruction (eg abdominal mass, elevated creatinine, poor urine flow, sepsis); in rare cases when localization is clinically important; atypical UTI in all patients; and recurrent UTI in patients <6 months.
Indications for Delayed Imaging
Indications for delayed imaging are atypical UTI in patients <3 years old and recurrent UTI in all patients.
Types of Imaging Studies
Ultrasound
Urinary Tract Infection Pediatric_Diagnostics 4
The ultrasound identifies abnormalities in renal size and shape, scars, duplication anomalies, and ureteric dilatation. This is indicated in children <2 years old with urinary tract infection and additional risk factors. This may reveal bladder diverticula or ureteroceles and assess bladder emptying. Doppler ultrasonography can detect small areas of inflammation in the kidneys. This is recommended for early imaging tests in infants and children with atypical UTI to identify structural urinary tract abnormalities. Renal and bladder ultrasound (RBUS) is recommended for febrile infants if evaluation of the renal parenchyma and size are needed. The advantages are that it is noninvasive and radiation-free. The disadvantage is that the results are operator-dependent.
Voiding Cystourethrogram (VCUG)
Voiding cystourethrogram is an invasive procedure requiring urethral catheterization. This should only be done when hydroureter, hydronephrosis, scarring, or findings suggestive of high-grade VUR/obstructive uropathy, or atypical/complicated disease, are seen with RBUS. This detects and grades VUR accurately and can show bladder and urethral anatomy, periureteral diverticula, and spinal abnormalities. Voiding cystourethrogram is recommended for first febrile UTI in infants <12 months regardless of ultrasound results; recurrent febrile UTI; abnormal ultrasound or dimercaptosuccinic acid renal scintigraphy (DMSA) results; and atypical UTI at any age regardless of ultrasound results. Disadvantages include: Radiation exposure, the possibility of introducing infection into the urinary tract, and retrograde filling of the bladder, which may be necessary. In a patient undergoing VCUG, prophylactic antibiotics should be given for 3 days, with the procedure taking place on the second day.
Radionuclide Cystography
Radionuclide cystography may be considered for patients with VUR. The advantage is less radiation exposure. Disadvantages are poor image resolution and low sensitivity for lower urinary tract infection and other abnormalities.
Tc 99m Dimercaptosuccinic Acid Renal Scintigraphy (DMSA)
Tc 99m dimercaptosuccinic acid renal scintigraphy (DMSA) is the gold standard for localizing infection to the renal parenchyma. This is more sensitive in detecting cortical scarring than ultrasound and IV pyelogram.
Radiolabeled DMSA is injected intravenously and binds to renal proximal tubular cells, after which renal cortical images are taken. An area of decreased uptake delineates an area of focal defect in the renal parenchyma. A star-shaped defect in the renal parenchyma may indicate acute pyelonephritis. A focal defect in the renal cortex may signify a chronic lesion or renal scar. Renal scarring may indicate that vesicoureteral reflux (VUR) is likely to persist in patients with reflux.
Magnetic Resonance Imaging (MRI)
Magnetic resonance imaging may be an alternative to DMSA. This may detect late renal scars and acute pyelonephritis. The advantage is that there is less radiation exposure.