What is the most appropriate imaging study to order for a patient with a urinary tract infection (UTI) due to E. coli, who has shown mild relief with Macrobid (nitrofurantoin) and is now experiencing right flank pain?

Imaging for UTI with Right Flank Pain

For a patient with UTI due to E. coli who has developed right flank pain despite mild improvement on nitrofurantoin, a non-contrast CT of the abdomen and pelvis is the most appropriate next imaging study to order.

Rationale for Non-Contrast CT

The development of flank pain in a patient with UTI raises concern for progression to pyelonephritis or a complicated infection such as obstruction from urolithiasis. This clinical scenario requires prompt imaging to guide management:

• Non-contrast CT abdomen and pelvis is the gold standard for detecting urolithiasis with sensitivity of 97-100% 1
• It can identify both urinary and non-urinary causes of flank pain
• It does not require contrast, avoiding potential nephrotoxicity in a patient who may have compromised renal function
• It can detect complications of UTI including pyelonephritis, renal/perinephric abscess, and emphysematous changes

Clinical Decision Algorithm

1. Initial presentation: UTI with E. coli, mild improvement on nitrofurantoin, now with right flank pain

2. Immediate imaging needed: Right flank pain suggests progression to pyelonephritis or complication

3. First-line imaging: Non-contrast CT abdomen and pelvis

   • Detects stones with highest sensitivity and specificity
   • Identifies secondary signs of obstruction
   • Can detect complications of infection

4. Alternative if CT contraindicated: Ultrasound with Color Doppler

   • Lower sensitivity but no radiation exposure
   • Particularly useful in pregnant patients or children 1

Interpretation of ACR Appropriateness Criteria

The American College of Radiology (ACR) Appropriateness Criteria strongly supports this approach:

• For patients with acute flank pain and suspected stone disease, non-contrast CT is the first-line imaging study 2
• For patients with complicated UTIs (including poor response to antibiotics), imaging is indicated 2
• For patients with recurrent or complicated UTIs, CT abdomen and pelvis is appropriate when abscess is suspected 2

Important Clinical Considerations

• The patient's right flank pain despite antibiotic therapy suggests progression from uncomplicated cystitis to pyelonephritis or a complicated infection
• If the non-contrast CT is negative but symptoms persist, consider:
  • CT Urography (CTU) to better evaluate the collecting system and ureter 2
  • MR Urography in patients who cannot receive iodinated contrast 1

Potential Findings and Next Steps

Based on CT findings, management may include:

• If obstruction identified: Consider urologic consultation for possible decompression
• If abscess identified: Consider drainage and broadened antibiotic coverage
• If pyelonephritis without complications: Adjust antibiotics based on culture results and clinical response

Common Pitfalls to Avoid

1. Delaying imaging in a patient with flank pain and UTI symptoms
2. Using ultrasound alone, which has lower sensitivity for detecting small stones and subtle inflammatory changes
3. Starting with contrast studies in a patient who may have impaired renal function
4. Failing to consider non-urinary causes of flank pain that CT can identify

Non-contrast CT provides the most comprehensive initial assessment for this patient with the lowest risk profile while offering the highest diagnostic yield.