Insidious onset (>1-2 weeks) of fatigue, weight loss, night sweats, or low-grade fevers.
Persistent fever and flank pain (>4-5 days) despite appropriate IV antibiotic therapy for pyelonephritis.
Lower urinary tract symptoms (dysuria, frequency, urgency) are not always present (often absent if the abscess does not communicate with the collecting system).
Physical Examination (PE):
High-grade fever, chills, and rigors.
Unilateral costovertebral angle (CVA) tenderness.
Palpable flank or abdominal mass (in up to 30-40% of cases).
Ipsilateral scoliosis due to psoas muscle spasm.
Diagnosis
Initial/Screening:
Renal Ultrasound (US): Useful first-line in pregnant/pediatric patients or in emergency settings. Shows a hypoechoic or anechoic perirenal fluid collection with possible internal debris or septations. Less sensitive for small (<3 cm) abscesses.
Confirmatory/Gold Standard:
Contrast-Enhanced CT of Abdomen and Pelvis: Diagnostic test of choice. Shows a well-defined, hypodense perirenal fluid collection with a ring-enhancing capsule/rim after IV contrast. May reveal gas-fluid levels or underlying nephrolithiasis.
Key Labs:
CBC: Markedly elevated WBC (leukocytosis) with left shift.
BMP: Checked to monitor renal function (Creatinine/BUN).
Urinalysis (UA) & Urine Culture: Normal UA in up to 30% of cases (if no communication with collecting system). c
Blood Cultures: Positive in up to 50% of cases; mandatory to evaluate for urosepsis or hematogenous spread.
Inflammatory Markers: Elevated ESR and CRP.
Biopsy/Aspiration:
Ultrasound- or CT-guided fine-needle aspiration of pus for Gram stain and culture (essential for targeting therapy).
Differential Diagnostics
Acute Pyelonephritis:
Differentiating features: Pyelonephritis typically responds to appropriate IV Abx within 48-72 hours, whereas perinephric abscess features persistent fever despite treatment. UA is almost always positive in pyelonephritis; CT in pyelonephritis shows wedge-shaped perfusion defects rather than a localized perirenal fluid collection.
Renal Abscess (Intrarenal):
Differentiating features: Collection is completely confined to the renal parenchyma without breaching the renal capsule. Usually smaller and less associated with psoas spasm.
Renal Cell Carcinoma (RCC):
Differentiating features: Pt presents with a flank mass, hematuria, and flank pain but typically lacks acute infectious signs (fever, leukocytosis). CT shows a solid, hypervascular tissue mass rather than a rim-enhancing fluid collection.
Emphysematous Pyelonephritis:
Differentiating features: Severe, life-threatening necrotizing infection. CT shows gas within the renal parenchyma (rather than fluid/pus in the perinephric space).
Management
First-line / Immediate Stabilization:
Fluid resuscitation (IVF) and supportive care for sepsis if hemodynamically unstable.
Broad-spectrum IV Empiric Antibiotics:
Must cover Gram-negative bacilli and S. aureus. E.g., Piperacillin-Tazobactam (Zosyn), Carbapenem (e.g., Meropenem), or Cefepime + Vancomycin (if MRSA suspected).
If abscess is small (<3 cm) and patient is stable, may attempt IV Abx alone.
Second-line / Definitive Intervention:
Percutaneous Drainage (PCD):
Indicated for abscesses >3 cm or for smaller abscesses failing to respond to 48-72 hours of IV Abx.
Pus must be sent for Gram stain, culture, and sensitivity.
Refractory / Surgical Intervention:
Surgical Drainage / Open Debridement: Indicated if PCD fails, if the abscess is multiloculated, or if there is extensive contiguous spread.
Nephrectomy: Indicated if the affected kidney is non-functioning (<15% function on differential renogram), has severe parenchymal destruction, or in refractory urosepsis.
Complications
Sepsis and Septic Shock (secondary to urosepsis).
Abscess Rupture:
Into the peritoneal cavity (causing peritonitis).
Into the pleural space (causing empyema).
Renal Parenchymal Destruction leading to chronic kidney disease (CKD) or end-stage renal disease (ESRD).
Psoas Abscess: Spread of infection along the psoas muscle.