• Acute pyelonephritis (APN; or upper urinary tract infection [UTI]) is a clinical diagnosis that may feature fever, positive urine culture, and urinary symptoms (e.g., dysuria, frequency, urgency, and/or flank pain). APN shows renal parenchymal (interstitial) inflammation secondary to bacterial invasion.
  • APN and febrile UTI (fUTI) are often used interchangeably. This usage assumes fUTIs always involve the upper tracts and is not accurate as lower tract UTIs may present with fever.
  • Chronic pyelonephritis is a term reserved for prolonged or incompletely treated infection of renal parenchyma.


  • UTIs are more likely to involve the upper renal tracts in children <3 years of age.
  • UTIs are more common in males ≤12 months of age; thereafter, UTI is more common in females.


  • 5–7% of febrile infants <8 weeks
  • 1% of school-aged children (0.03% school-aged boys)

Risk Factors

  • Previous history of UTI (e.g., recurrent UTI [rUTI])
  • Female sex, especially sexually active females
  • Bladder and bowel dysfunction (BBD)
  • Sibling with a history of UTI
  • Structural abnormalities or foreign bodies in urinary tract, including stones, catheters, or stents
  • Neurologic conditions affecting bladder and bowel
  • Vesicoureteral reflux (VUR)
    • Present in ~30–40% of children with fUTIs
    • The majority (>95%) of VUR associated with fUTIs is low or moderate grade (I to III). There is stronger association of fUTI with high-grade (≥IV) VUR
  • Uncircumcised boys <12 months
    • Circumcision can reduce risk of UTI by up to 87%.


  • Host-related factors:
    • Anatomic abnormalities (e.g., obstruction)
    • Functional abnormalities (e.g., BBD, VUR)
  • Pathogen-related factors:
    • Adherence factors (e.g., P and type 1 fimbriae, adhesins)
    • Virulence factors (e.g., lipopolysaccharide, capsular antigen)
    • Antibiotic- and immune clearance–resistant intracellular bacterial pods within urothelium
  • Bacterial adhesion to uroepithelium induces cytokine release and subsequent inflammatory response.
  • Patchy infiltration of the medullary parenchyma by polymorphonuclear leukocytes and lymphocytes leads to degradation of extracellular matrix, tubular disruption, and interstitial edema.
  • Parenchymal scarring may result as a consequence.


  • Enterobacteriaceae: Escherichia coli most frequent (90% of initial infections and up to 66% of rUTIs); Proteus, Klebsiella, Enterobacter spp. are less common.
  • Gram-positive organisms cause 10–15% of cases: Staphylococcus aureus, Staphylococcus epidermidis, Staphylococcus saprophyticus, Enterococci spp.
  • Other organisms: Pseudomonas, Haemophilus influenzae, Streptococcus group B

Commonly Associated Conditions

  • Struvite kidney stones: associated with urease-producing bacteria (e.g., Proteus and Klebsiella spp.)
  • Anatomic or physiologic abnormality of the urinary tract are found in up to 50% of infants with APN.

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