What is the approach to diagnose and treat a suspected kidney infection?

Assessment and Management of Kidney Infection (Pyelonephritis)

The diagnostic approach for kidney infection should include blood tests (CBC with differential, C-reactive protein), urinalysis, urine culture, and appropriate imaging in complicated cases, followed by targeted antibiotic therapy with fluoroquinolones or other appropriate antibiotics based on culture results. 1, 2

Diagnostic Approach

Clinical Presentation

• Kidney infection typically presents with fever, flank pain, and may include symptoms of lower urinary tract infection such as dysuria, frequency, and urgency 1, 3
• In elderly patients, presentations may be atypical with confusion, incontinence, or absence of typical symptoms 1

Laboratory Evaluation

• Complete blood count (CBC) with differential should be performed within 12-24 hours of symptom onset to assess for leukocytosis and left shift 1, 2
• White blood cell count >11 × 10^9/L is a diagnostic feature of kidney infection 1
• C-reactive protein ≥50 mg/L is highly suggestive of kidney infection when combined with clinical symptoms 1, 2
• Blood cultures should be obtained if upper urinary tract infection or kidney cyst infection is suspected 1

Urinalysis and Urine Culture

• Urinalysis should be performed to check for pyuria (≥10 WBCs/high-power field), leukocyte esterase, and nitrite 1
• Urine culture with antimicrobial susceptibility testing should be obtained before starting antibiotics to guide therapy 1, 4
• Negative urinalysis for WBCs and negative dipstick tests for leukocyte esterase and nitrite are useful to exclude a urinary source for suspected infection 1

Imaging Studies

• Imaging is not necessary in uncomplicated cases that respond to initial therapy 1, 4
• Consider imaging in patients with:
  • No improvement in symptoms after 48-72 hours of appropriate therapy 4
  • Recurrence of symptoms after initial improvement 1
  • Suspected complications (abscess, obstruction) 1, 5
• Contrast-enhanced CT is the preferred imaging modality for complicated cases 1, 5
• Ultrasound can be used as an initial screening tool, especially for detecting hydronephrosis or renal enlargement 1, 5

Treatment Approach

Outpatient Management

• Appropriate for patients with uncomplicated pyelonephritis who can tolerate oral therapy 4
• Oral fluoroquinolones are recommended as first-line therapy if local resistance rates are ≤10% 3, 4
  • Ciprofloxacin 500 mg twice daily for 7-14 days 6
• If local fluoroquinolone resistance exceeds 10%, consider initial dose of ceftriaxone followed by oral fluoroquinolone 4
• Alternative oral options include trimethoprim-sulfamethoxazole based on local susceptibility patterns 1

Inpatient Management

• Indicated for patients with:
  • Severe illness, sepsis, or hemodynamic instability 4
  • Inability to tolerate oral medications 4
  • Failed outpatient therapy 4
  • Suspected complications 1
• Initial parenteral therapy options:
  • Fluoroquinolones 4
  • Aminoglycosides with or without ampicillin 4
  • Third-generation cephalosporins 4
• Duration of therapy is typically 7-14 days, depending on severity and clinical response 6, 7

Special Considerations

• For kidney cyst infections, lipid-soluble antibiotics (e.g., trimethoprim-sulfamethoxazole, fluoroquinolones) may have better penetration into cysts 1
• In patients with renal impairment, dosage adjustments may be necessary:
  • For creatinine clearance 30-50 mL/min: ciprofloxacin 250-500 mg every 12 hours 6
  • For creatinine clearance 5-29 mL/min: ciprofloxacin 250-500 mg every 18 hours 6

Follow-up and Complications

• Most patients should respond to appropriate therapy within 48-72 hours 4
• Consider repeat urine culture 1-2 weeks after completing antibiotic therapy 7
• If no improvement occurs, consider:
  • Resistant organisms 4, 7
  • Underlying anatomic or functional abnormalities 7
  • Alternative diagnoses 4
  • Need for imaging studies 4, 5
• Potential complications include renal abscess, sepsis, and acute kidney injury 5, 8

Prevention

• Investigate for underlying predisposition in patients with recurrent UTIs 1
• Consider antibiotic prophylaxis for women with recurrent UTIs after discussing risks and benefits 1
• Do not treat asymptomatic bacteriuria 1