What labs are recommended for a 5 1/2 month old with suspected pyelonephritis?

Laboratory Testing for a 5½-Month-Old with Suspected Pyelonephritis

For a 5½-month-old infant with suspected pyelonephritis, both urinalysis and urine culture are essential laboratory tests, with blood cultures recommended only if the infant appears septic or toxic. 1

Essential Laboratory Tests

Urine Testing

• Urinalysis - Should include:

  • Leukocyte esterase and nitrite dipstick tests
  • Microscopy for pyuria (WBC count)
  • Gram stain of uncentrifuged urine specimen (if available)
  • Assessment of urine clarity

• Urine Culture with Antimicrobial Susceptibility Testing

  • Required for all infants this age with suspected UTI
  • Collection method is critical:
    • Catheterized specimen preferred (significant growth: 10³-10⁵ CFU/mL)
    • Suprapubic aspiration if available (significant growth: >10² CFU/mL)
    • Avoid bag specimens due to high contamination rates

Blood Tests

• Complete Blood Count (CBC)

  • To assess for leukocytosis indicating systemic infection

• Blood Culture

  • Not routinely needed unless:
    • Infant appears toxic or septic
    • Temperature >39°C
    • Immunocompromised status
    • Failure to respond to appropriate therapy

• Inflammatory Markers

  • C-reactive protein (CRP) or erythrocyte sedimentation rate (ESR)
  • May help distinguish lower UTI from pyelonephritis
  • Not required in straightforward cases

Interpretation of Urinalysis Results

• Positive urinalysis criteria:

  • Presence of either nitrite OR leukocyte esterase (88% sensitivity)
  • Presence of BOTH nitrite AND leukocyte esterase (96% specificity)
  • ≥10 WBCs/mm³ on enhanced urinalysis or ≥5 WBCs/HPF on centrifuged specimen
  • Bacteria on Gram stain (93% sensitivity)

• Important caveat: Up to 20% of infants with culture-proven pyelonephritis may have negative initial urinalysis 1

Diagnostic Pitfalls to Avoid

1. Relying solely on urinalysis - In children under 2 years, urinalysis alone can miss 10-50% of UTIs; always obtain culture 1

2. Misinterpreting urine culture results - Consider:

   • Collection method (catheter vs. suprapubic vs. bag)
   • Growth of single vs. multiple organisms
   • Colony count thresholds based on collection method
   • Clinical presentation

3. Unnecessary blood cultures - Studies show blood cultures rarely change management in uncomplicated pyelonephritis 2

4. Missing pyelonephritis with normal urine findings - Some children with pyelonephritis may have minimal or normal urine findings; maintain high clinical suspicion in febrile infants 3

Follow-up Imaging Considerations

While not laboratory tests, imaging should be considered after diagnosis:

• Renal and Bladder Ultrasound (RBUS)

  • Recommended for all infants with first UTI 1
  • To detect hydronephrosis, scarring, or other abnormalities

• Voiding Cystourethrography (VCUG)

  • Not routinely recommended after first UTI
  • Consider if RBUS reveals abnormalities suggesting high-grade VUR or obstructive uropathy 1

By following this laboratory testing approach, you can effectively diagnose pyelonephritis in a 5½-month-old infant and guide appropriate antimicrobial therapy to prevent renal scarring and other complications.