Definition of Pyelonephritis
Pyelonephritis is defined as inflammation of the renal parenchyma and renal pelvis due to bacterial infection, typically presenting with fever (>38°C), flank pain, and urinary symptoms. 1
Core Diagnostic Criteria
The European Association of Urology and American College of Radiology establish pyelonephritis as a bacterial infection affecting both the kidney parenchyma and renal pelvis, distinguishing it from lower urinary tract infections that involve only the bladder. 1, 2
Clinical Presentation Components
Systemic inflammation signs are nearly universal and include fever (≥38°C), chills, malaise, vomiting, and fatigue. 2
Flank pain or costovertebral angle tenderness is present in the vast majority of cases and is considered nearly universal; its absence should prompt consideration of alternative diagnoses. 2, 3
Lower urinary tract symptoms (urgency, dysuria, frequency) occur in approximately 80% of patients but may be absent in up to 20% of cases. 2, 4
Laboratory Confirmation
Urinalysis showing pyuria and/or bacteriuria is the initial confirmatory test supporting the diagnosis. 1, 4
Urine culture yielding >10,000 colony-forming units (CFU) per milliliter of a uropathogen is the fundamental confirmatory diagnostic test, with higher thresholds used in children. 2
Urine culture with antimicrobial susceptibility testing should be performed in all cases before initiating antibiotics to guide therapy. 1, 5
Pathophysiology
The infection occurs primarily through ascending urinary contamination from intestinal and genital skin flora, explaining why Gram-negative bacilli, particularly Escherichia coli, are the predominant pathogens. 6
Microabscesses form during the inflammatory process and may coalesce into larger renal abscesses; if these rupture into the perinephric space, perirenal abscesses develop. 2, 4
E. coli accounts for >90% of cases in young healthy women and 70-90% of all pyelonephritis infections overall. 4, 7
Special Diagnostic Considerations
Diabetic patients present a diagnostic challenge, with up to 50% lacking typical flank tenderness, making atypical presentations more common. 1, 2
Elderly patients may present with atypical symptoms and carry higher risk for complications. 2
Pregnant patients are at significantly elevated risk of severe complications and warrant special consideration. 5