What is the appropriate workup and treatment for a patient presenting with symptoms suggestive of acute pyelonephritis, including flank pain, fever, and dysuria?

Workup for Acute Pyelonephritis

The workup for acute pyelonephritis should include urinalysis and urine culture with antimicrobial susceptibility testing in all cases, while imaging is reserved only for patients who remain febrile after 72 hours of appropriate antibiotics or have clinical deterioration. 1, 2

Clinical Diagnosis

The diagnosis is primarily clinical, based on characteristic symptoms combined with laboratory confirmation:

• Fever ≥38°C (100.4°F) is the hallmark systemic symptom, though it may be absent early in illness 2, 3
• Flank pain or costovertebral angle tenderness is nearly universal and distinguishes pyelonephritis from lower urinary tract infections 2, 3
• Lower urinary tract symptoms (dysuria, urgency, frequency) may be present but are absent in up to 20% of cases 2, 3
• Systemic symptoms including nausea, vomiting, chills, and malaise are characteristic 2, 3

Essential Laboratory Testing

Urinalysis

• Perform urinalysis in all suspected cases to evaluate for pyuria, bacteriuria, white blood cells, red blood cells, and nitrites 1, 2
• The combination of leukocyte esterase and nitrite testing (with either positive) has 75-84% sensitivity and 82-98% specificity for urinary tract infection 4
• Pyuria and/or bacteriuria on urinalysis confirms the diagnosis when combined with compatible clinical presentation 2, 3

Urine Culture

• Urine culture with antimicrobial susceptibility testing is mandatory in all cases of pyelonephritis before initiating antibiotics 1, 2, 5
• Cultures are positive in 90% of patients with acute pyelonephritis 4
• A threshold of >10,000 CFU/mL of a uropathogen is the confirmatory diagnostic standard 2

Blood Cultures

• Blood cultures should be obtained only if the patient appears systemically ill with high fever or has signs of sepsis 1
• Blood cultures are also indicated for immunocompromised patients, those with uncertain diagnosis, or suspected hematogenous infection 4
• Routine blood cultures are not necessary in uncomplicated cases 5

Imaging: When and What

Initial Imaging (NOT Recommended)

• Imaging is NOT indicated for initial evaluation of uncomplicated acute pyelonephritis 1, 2
• 95% of patients with uncomplicated pyelonephritis become afebrile within 48 hours of appropriate antibiotics, and nearly 100% within 72 hours 1, 2

Indications for Delayed Imaging

Perform imaging if any of the following occur:

• Persistent fever after 72 hours of appropriate antibiotic therapy 1, 2
• Clinical deterioration at any point during treatment 1, 2
• High-risk populations including:
  • Diabetic patients (up to 50% lack typical flank tenderness and are at higher risk for complications including emphysematous pyelonephritis and renal abscess) 2
  • Immunocompromised or transplant recipients 2
  • Pregnant patients 2
  • History of urolithiasis or anatomic abnormalities 1, 2
  • Suspected urinary obstruction 1

Imaging Modality Selection

• Ultrasound is the preferred initial imaging to evaluate for obstruction, stones, or abscess formation 1
• Contrast-enhanced CT scan is the study of choice when complications are suspected or ultrasound is inconclusive 1, 2
• Upper urinary tract ultrasound should be performed in patients with history of urolithiasis, renal function alterations, or elevated urine pH 1

Special Population Considerations

Diabetic Patients

• Consider early imaging in diabetic patients due to atypical presentations and higher risk of complications 2
• Up to 50% lack typical flank tenderness, making clinical diagnosis more challenging 2
• Higher risk for emphysematous pyelonephritis and renal abscesses 2

Pregnant Patients

• Pregnant patients with pyelonephritis are at significantly elevated risk of severe complications and warrant hospital admission 5

Elderly Patients

• May present with atypical symptoms and are at higher risk for complications 2

Common Pitfalls to Avoid

• Do not obtain routine imaging in uncomplicated cases responding to therapy - this leads to unnecessary testing and does not change management 1, 2
• Do not delay imaging beyond 72 hours in patients with persistent fever - this can miss complications like obstruction, abscess, or emphysematous pyelonephritis 1
• Do not rely solely on urinalysis without clinical correlation - pyuria has low positive predictive value as it can indicate non-infectious genitourinary inflammation 6
• Do not skip urine culture - it is essential for guiding targeted therapy, especially given increasing antibiotic resistance 1, 5
• Do not routinely obtain blood cultures in uncomplicated cases - reserve for systemically ill or high-risk patients 1, 5