How is a urinary tract infection (UTI) diagnosed and treated in pediatric patients?

Diagnosis of Urinary Tract Infection in Pediatric Patients

UTI diagnosis in febrile infants and children aged 2-24 months requires both positive urinalysis (showing pyuria and/or bacteriuria) AND urine culture demonstrating at least 50,000 CFU/mL of a single uropathogen from a properly collected specimen via catheterization or suprapubic aspiration. 1

Risk Stratification and When to Test

Before obtaining urine, assess whether the febrile infant requires testing based on clinical risk factors 1:

• Low-risk infants (who can be observed without testing): White females >12 months old with recent onset (≥2 days) of low-grade fever (<39°C) have <1% probability of UTI 1
• For boys: Circumcision status is the major risk factor; uncircumcised boys have significantly higher UTI risk 1
• Additional risk factors that increase probability: non-Black race, temperature ≥39°C, fever >24 hours, absence of another infection source 1

If the infant appears toxic or requires immediate antimicrobial therapy, proceed directly to specimen collection and treatment 2.

Specimen Collection Methods

The method of urine collection is critical and directly impacts diagnostic accuracy 1:

• Catheterization or suprapubic aspiration (SPA): Required for culture confirmation of UTI 1

  • Catheterization has nearly equal sensitivity/specificity to SPA but higher success rates and less pain 1
  • For catheterization: discard first few milliliters, use new catheter if first attempt fails 1

• Bag collection: May be used for urinalysis screening only 1

  • Positive bag culture has extremely high false-positive rate and MUST be confirmed by catheterization 1
  • Cannot be used to document UTI 1

Urinalysis Interpretation

Urinalysis serves as a screening tool but cannot substitute for culture 1:

Performance Characteristics 1:

• Leukocyte esterase: 83% sensitivity, 78% specificity
• Nitrite: 53% sensitivity, 98% specificity
• Either positive: 93% sensitivity, 72% specificity
• Microscopy (WBCs): 73% sensitivity, 81% specificity
• Microscopy (bacteria): 81% sensitivity, 83% specificity

Clinical Application:

• Negative leukocyte esterase AND negative nitrite on dipstick is sufficient to rule out UTI in moderate-risk (5% prevalence) patients who don't require immediate treatment 1
• Specimen must be fresh (≤1 hour at room temperature or ≤4 hours refrigerated) for accurate results 1
• Pyuria without bacteriuria is nonspecific and occurs in non-infectious conditions (Kawasaki disease, chemical urethritis) 1
• Bacteriuria without pyuria suggests contamination or asymptomatic bacteriuria 1

Important caveat: Urine Gram stain adds no clinical utility beyond standard urinalysis and does not influence antibiotic selection 3. The American Academy of Pediatrics does not recommend it as part of routine evaluation 3.

Culture Criteria for Diagnosis

Definitive diagnosis requires BOTH 1, 2:

1. Positive urinalysis (pyuria and/or bacteriuria)
2. ≥50,000 CFU/mL of a single uropathogen from catheterized or SPA specimen

This threshold differs from the historical 100,000 CFU/mL standard, reflecting recognition that lower counts can indicate true infection in symptomatic patients 1.

Diagnostic Algorithm Summary

For febrile infants 2-24 months with no obvious fever source 1, 2:

1. Assess illness severity: If toxic-appearing or unable to retain oral intake → immediate catheterization for culture and urinalysis, start empiric treatment
2. If not severely ill, assess UTI risk: Use demographic/clinical factors (age, sex, circumcision, race, fever characteristics)
3. Low-risk patients: Clinical observation without testing acceptable 1
4. Not low-risk: Obtain urine specimen
   • If immediate treatment not needed: bag specimen for urinalysis acceptable; if positive, confirm with catheterized culture
   • If treatment likely needed: proceed directly to catheterization for both urinalysis and culture
5. Interpret results: Diagnosis requires positive urinalysis AND ≥50,000 CFU/mL on culture

Common Pitfalls to Avoid

• Never diagnose UTI based on positive bag culture alone - confirmation with catheterized specimen is mandatory 1
• Don't rely on urinalysis alone - culture confirmation is essential even with positive urinalysis 1
• Avoid delayed specimen processing - urinalysis accuracy deteriorates if specimen sits >1 hour at room temperature 1
• Don't use molecular/PCR tests for routine diagnosis - these cannot distinguish infection from colonization or quantify bacterial load 1

Imaging After Diagnosis

Following confirmed first febrile UTI, renal and bladder ultrasonography should be performed to detect anatomic abnormalities 1, 2. Voiding cystourethrography (VCUG) is NOT routinely recommended after first UTI but is indicated if ultrasonography shows hydronephrosis, scarring, or findings suggesting high-grade vesicoureteral reflux, or if febrile UTI recurs 1, 2.