What imaging study to order for a patient with a urinary tract infection (UTI), urine culture positive for Escherichia coli (E. coli), unresponsive to Macrobid (nitrofurantoin) and currently on levofloxacin 750mg, presenting with low back pain?

Imaging for UTI with Low Back Pain After Failed Antibiotic Treatment

Non-contrast CT of the abdomen and pelvis is the most appropriate initial imaging study for a patient with UTI who has developed low back pain despite antibiotic therapy, as it can detect complications such as pyelonephritis, abscess, and urolithiasis with high sensitivity and specificity. 1

Clinical Scenario Analysis

This patient presents with several concerning features:

• UTI with E. coli (common uropathogen)
• Failed first-line therapy (nitrofurantoin/Macrobid)
• Currently on second-line therapy (levofloxacin 750mg)
• Persistent low back pain despite treatment
• Possible complicated UTI or pyelonephritis

These features suggest this is no longer an uncomplicated UTI but rather a complicated infection requiring imaging evaluation.

Recommended Imaging Approach

First-line Imaging:

• Non-contrast CT of the abdomen and pelvis
  • Gold standard for detecting urolithiasis (sensitivity 97-100%)
  • Excellent for identifying complications of infection including abscess
  • Can detect both urinary and non-urinary causes of back pain
  • Avoids potential nephrotoxicity from contrast in a patient with possible renal infection 1

Rationale:

The American College of Radiology recommends CT abdomen and pelvis for patients with recurrent or complicated UTIs, particularly when an abscess is suspected 2. This patient's presentation with persistent back pain despite appropriate antibiotic therapy raises concern for complications such as:

• Pyelonephritis with or without abscess formation
• Obstructive urolithiasis
• Perinephric abscess
• Non-urologic causes of back pain

Alternative Imaging Options

If CT is contraindicated or unavailable:

1. Ultrasound of kidneys and bladder

   • Less sensitive than CT but can detect hydronephrosis, renal abscess, and larger stones
   • No radiation exposure
   • Limited ability to visualize ureteral stones 2

2. CT Urography (CTU)

   • Consider if initial non-contrast CT is negative but symptoms persist
   • Better evaluates the collecting system and ureter
   • Requires IV contrast 2

Management Based on Imaging Findings

• If obstruction is identified: Urologic consultation for possible decompression
• If abscess is identified: Drainage and broadened antibiotic coverage
• If pyelonephritis without complications: Adjust antibiotics based on culture results

Clinical Pearls and Pitfalls

• Pitfall: Relying solely on ultrasound may miss small stones or subtle inflammatory changes
• Pitfall: Assuming back pain in a UTI patient is always due to pyelonephritis; consider alternative diagnoses
• Pearl: E. coli resistance to fluoroquinolones has been increasing, which may explain treatment failure 3
• Pearl: Patients with persistent symptoms despite appropriate antibiotic therapy should be evaluated for complications or anatomic abnormalities

Special Considerations

• For patients with recurrent UTIs, imaging is particularly important to rule out anatomical abnormalities or foreign bodies that may contribute to infection persistence 2
• If the patient has signs of sepsis with obstruction, urgent decompression via percutaneous nephrostomy or retrograde ureteral stenting is indicated 1

The persistence of back pain despite appropriate antibiotic therapy strongly suggests the need for imaging to identify potential complications requiring intervention beyond antimicrobial therapy alone.