What are the differential diagnoses for a left lumbar (flank) swelling associated with pyuria?

Differential Diagnoses for Left Lumbar Swelling with Pyuria

The most likely diagnosis is acute pyelonephritis with possible complication (renal or perinephric abscess), and the patient requires urgent renal ultrasound to exclude obstruction, abscess formation, or stone disease. 1, 2

Primary Differential: Acute Pyelonephritis with Complications

Uncomplicated Pyelonephritis

• Flank pain/tenderness combined with pyuria and fever (≥38°C) establishes the presumptive diagnosis of acute pyelonephritis. 2
• The presence of "many WBCs" in urine strongly supports bacterial infection, particularly gram-negative organisms which produce the most pronounced pyuria. 3
• Urine culture with antimicrobial susceptibility testing must be obtained before initiating antibiotics. 1, 2

Complicated Pyelonephritis (High Priority)

Left lumbar swelling is NOT a typical feature of uncomplicated pyelonephritis and suggests:

Renal or Perinephric Abscess

• Microabscesses that form during acute pyelonephritis may coalesce to form renal abscesses; if an abscess ruptures into the perinephric space, a perirenal abscess forms, which can present as flank swelling. 2
• Abscess formation should be suspected when there is visible swelling, as this indicates mass effect or fluid collection. 1
• Renal ultrasound is the preferred initial imaging modality to evaluate for abscess formation, obstruction, or stones. 1
• If ultrasound is inconclusive, contrast-enhanced CT provides superior visualization of abscess formation and complicated stone disease. 1

Obstructive Pyelonephritis

• Urinary obstruction from stones, stricture, or other causes can lead to hydronephrosis with secondary infection (pyonephrosis), presenting with flank swelling. 4, 1
• Obstructive pyelonephritis can rapidly progress to urosepsis if not promptly identified and addressed. 1
• Ultrasound is 100% sensitive and 90% specific for detecting ureteral obstruction (hydronephrosis, ureterectasis, perinephric fluid). 4

Secondary Differentials

Nephrolithiasis with Infection

• A history of kidney stones makes nephrolithiasis the leading differential diagnosis for patients presenting with flank pain and pyuria. 1
• Stones can cause both obstruction and secondary infection, producing pyuria and flank swelling from hydronephrosis. 4, 1
• Ultrasound demonstrates variable sensitivity (24-57%) for stone detection but is highly sensitive for detecting associated hydronephrosis. 4

Retroperitoneal Hematoma

• Subcapsular or retroperitoneal hematoma can present with flank pain, swelling, and hematuria mimicking nephrolithiasis. 5
• Page kidney (compression of renal parenchyma by hematoma) causes secondary hypertension and can present with flank swelling without trauma history. 5
• This diagnosis is less likely with prominent pyuria but should be considered if imaging shows mass effect. 5

Lumbar Artery Pseudoaneurysm with Retroperitoneal Hemorrhage

• Vascular injury can cause retroperitoneal hematoma with mass effect on the ureter, producing flank pain and swelling. 6
• This is rare and typically follows trauma or iatrogenic injury, making it less likely in spontaneous presentations. 6

Immediate Diagnostic Algorithm

Step 1: Obtain urine culture before antibiotics 1, 2

Step 2: Perform renal ultrasound immediately to assess for:

• Hydronephrosis or obstruction 4, 1
• Renal or perinephric abscess 1, 2
• Nephrolithiasis 4, 1
• Subcapsular hematoma 5

Step 3: If ultrasound is inconclusive or shows concerning findings, proceed to contrast-enhanced CT 1

Step 4: Initiate empiric antibiotics after cultures obtained:

• Ceftriaxone 1-2g IV once daily for hospitalized patients 1
• Total duration 7-14 days with transition to oral therapy based on culture sensitivities once afebrile 24-48 hours 1

Critical Pitfalls to Avoid

• Do not delay imaging when flank swelling is present—this is NOT typical of uncomplicated pyelonephritis and suggests complication. 1, 2
• Do not assume pyuria alone confirms infection; pyuria has low positive predictive value (25-54% bacteriuria rate depending on WBC count) and can result from non-infectious inflammation. 7
• Do not miss obstructive pyelonephritis—persistent fever after 72 hours of appropriate antibiotics mandates imaging to rule out obstruction or abscess. 1
• In diabetic or immunocompromised patients, maintain high suspicion for emphysematous pyelonephritis or abscess formation, as up to 50% lack typical flank tenderness. 2