How do you diagnose pyelonephritis?

Diagnosis of Pyelonephritis

The diagnosis of pyelonephritis requires urinalysis showing white blood cells and nitrite, urine culture with antimicrobial susceptibility testing, and clinical assessment of symptoms including fever, chills, flank pain, nausea, vomiting, or tenderness at the costovertebral angle, with or without symptoms of cystitis. 1

Clinical Presentation

Pyelonephritis typically presents with:

• Fever (>38°C)
• Chills
• Flank pain
• Nausea and vomiting
• Costovertebral angle tenderness
• Lower urinary tract symptoms (urgency, dysuria, frequency) may or may not be present 1

Important to note: Up to 20% of patients with pyelonephritis may lack bladder symptoms, making diagnosis more challenging 1.

Diagnostic Algorithm

Step 1: Laboratory Testing

• Urinalysis: Assessment of white and red blood cells and nitrite (positive results strongly suggest UTI) 1
• Urine culture with antimicrobial susceptibility testing: Required in all cases of suspected pyelonephritis 1
  • Positive culture defined as >10,000 colony-forming units of a uropathogen per milliliter of urine 1
• Blood cultures: Consider in patients with severe illness, immunocompromise, or suspected hematogenous infection 2

Step 2: Imaging Studies

For uncomplicated cases in otherwise healthy patients:

• No imaging is required initially 1

Imaging is indicated in the following scenarios:

• Patients who remain febrile after 72 hours of appropriate antibiotic therapy 1
• Immediate imaging if there is clinical deterioration 1
• Patients with history of urolithiasis, renal function disturbances, or high urine pH 1
• Patients at high risk for complications: diabetes, anatomic abnormalities, renal obstruction, pregnancy, immunosuppression 1

Step 3: Appropriate Imaging Selection When Indicated

• Ultrasound: First-line imaging to rule out urinary tract obstruction or renal stone disease 1, 3
• Contrast-enhanced CT: Most sensitive for confirming diagnosis when complications are suspected 1
• MRI: Preferred in pregnant women to avoid radiation exposure 1

Special Considerations

High-Risk Populations

Pay particular attention to:

• Diabetic patients: Up to 50% may not have typical flank tenderness 1
• Pregnant women: Higher risk of complications; ultrasound or MRI preferred over CT 1
• Elderly patients: May present with atypical symptoms
• Patients with urinary tract abnormalities or obstruction 1

Critical Distinction

• Uncomplicated vs. Obstructive Pyelonephritis: Prompt differentiation is crucial as obstructive pyelonephritis can rapidly progress to urosepsis 1
• Pyonephrosis: Accumulation of purulent material in an obstructed collecting system requires urgent decompression 3

Common Pitfalls to Avoid

1. Delaying imaging in high-risk patients: Patients with diabetes, immunocompromise, or suspected obstruction should receive prompt imaging 1

2. Inadequate follow-up: Patients who don't respond to therapy within 48-72 hours should undergo repeat cultures and imaging 2

3. Misdiagnosing complicated pyelonephritis as uncomplicated: Always assess for risk factors that suggest complicated disease requiring more aggressive management 4

4. Relying solely on clinical presentation: Laboratory confirmation with urinalysis and urine culture is essential, as symptoms can overlap with other conditions 5

By following this diagnostic approach, clinicians can accurately diagnose pyelonephritis and identify patients who require more intensive monitoring or intervention to prevent serious complications.