Should I order a CT scan with or without IV contrast for suspected infected stone versus pyelonephritis in a patient with flank pain radiating to the groin?

CT Imaging for Flank Pain: Infected Stone vs Pyelonephritis

For a patient with flank pain radiating to the groin with suspected infected stone versus pyelonephritis, a non-contrast CT scan of the abdomen and pelvis should be ordered as the initial imaging test, with a contrast-enhanced CT only if there are specific concerns for complications or alternative diagnoses. 1, 2

Imaging Algorithm for Suspected Urolithiasis with Infection Concerns

First-Line Imaging

• Non-contrast CT abdomen and pelvis
  • Gold standard for urolithiasis detection with sensitivity up to 97% 1, 2
  • Can visualize stone size, location, and secondary signs of obstruction
  • Can detect periureteral and perinephric inflammation
  • Low-dose protocols available to reduce radiation exposure while maintaining diagnostic accuracy

When to Add IV Contrast

Consider adding IV contrast (either as a separate study or as part of a protocol with both non-contrast and contrast phases) in these specific scenarios:

1. Strong suspicion of complicated infection:

   • Signs of sepsis
   • High fever (>38.5°C)
   • Significant leukocytosis
   • Hemodynamic instability

2. Diagnostic uncertainty after non-contrast CT:

   • Persistent symptoms without visible stone
   • Need to differentiate between ureteral stone and phlebolith
   • Concern for alternative diagnoses

Rationale for Imaging Selection

Why Non-Contrast CT is Superior for Stone Detection

• Virtually all renal calculi are radiopaque on CT, allowing for accurate detection even of small stones 1
• Enhancing renal parenchyma during contrast phases may obscure stones within the collecting system 1
• Non-contrast CT provides accurate assessment of stone size, which is critical for management planning 1

Limitations of Contrast-Only CT for Stone Detection

• CT with IV contrast alone has lower sensitivity (81%) for detecting small renal stones compared to non-contrast CT 1
• However, contrast-enhanced CT has 95% sensitivity for stones ≥3mm and 98% for stones ≥6mm 1

When IV Contrast Adds Value

• Better delineation of "soft tissue rim" sign to differentiate ureteral stone from phlebolith 1
• Enhanced detection of urinary obstruction through delayed nephrogram 1
• Evaluation of complications such as abscess formation or renal parenchymal involvement 2
• Assessment of alternative diagnoses that might cause similar symptoms 1

Important Clinical Considerations

Infection Risk Assessment

• Urinalysis is essential to confirm presence of infection markers (leukocyturia)
• If sepsis is suspected with obstruction, urgent decompression via percutaneous nephrostomy or retrograde ureteral stenting is indicated 2

Pitfalls to Avoid

• Don't order CT with and without contrast routinely: There is no documented additional benefit of obtaining both non-contrast and contrast phases for simple urolithiasis evaluation 1
• Don't miss alternative diagnoses: In patients without stones, over 30% may have another identifiable cause of flank pain 3
• Don't delay imaging in septic patients: If the patient has signs of sepsis with suspected obstruction, expedited imaging and intervention are critical 2

In summary, start with non-contrast CT for suspected urolithiasis, and only add contrast if there are specific concerns for complications or if the initial non-contrast study doesn't provide a clear diagnosis.