What is the best initial imaging study for a patient with suspected kidney stone, considering their past medical history and current symptoms?

Kidney Stone Imaging: Initial Diagnostic Approach

Ultrasound is the recommended first-line imaging modality for suspected kidney stones, followed by non-contrast CT if ultrasound is inconclusive or emergency intervention is needed. 1, 2

Primary Imaging Strategy

Start with renal ultrasound as your initial diagnostic tool for all patients presenting with suspected nephrolithiasis, regardless of age or clinical presentation. 1, 2 This recommendation comes from the 2025 European Association of Urology guidelines, which designate ultrasound as the primary diagnostic modality with a strong recommendation. 1

Ultrasound Performance Characteristics

• Sensitivity: 45% for both renal and ureteral stones 1, 2
• Specificity: 94% for ureteral stones and 88% for renal stones 1, 2
• Key advantage: No radiation exposure, making it ideal for pregnant patients, children, and patients requiring serial imaging 1
• Limitation: Lower sensitivity for stones <3 mm and in non-dilated collecting systems 3

When to Proceed to CT Imaging

Non-contrast CT of the abdomen and pelvis is the gold standard after ultrasound for acute flank pain and should be obtained when: 1, 2

• Ultrasound findings are equivocal or negative despite high clinical suspicion 1
• You need precise stone localization, size measurement, and density assessment for treatment planning 1
• Emergency intervention may be required 1
• The patient has fever suggesting infected obstructing stone (urgent indication) 1, 2
• The patient has a solitary kidney (emergent indication) 1, 2

CT Performance and Dosing

• Sensitivity and specificity: >95% for stone detection 1, 2
• Use low-dose protocols to minimize radiation exposure while maintaining diagnostic accuracy (93.1% sensitivity, 96.6% specificity) 1
• Non-contrast CT is sufficient—adding IV contrast provides minimal additional benefit (changes management in only 2-3% of cases) and is generally unnecessary 1

Special Population Considerations

Pregnant Patients

Ultrasound is the imaging tool of choice with an appropriateness rating of 8 (usually appropriate). 1 If ultrasound is non-diagnostic:

• Second-line: MRI without contrast 1
• Last-line: Low-dose non-contrast CT only if absolutely necessary 1

Pediatric Patients

Ultrasound is strongly recommended as first-line imaging, followed by KUB radiography or low-dose non-contrast CT if ultrasound doesn't provide required information. 1

Recurrent Stone Disease

For patients with known current stones on recent imaging presenting with recurrent symptoms:

• Non-contrast CT remains appropriate (rating 7) to assess for stone migration, passage, or complications 1
• Optimize radiation dose and minimize total number of studies 1
• Plain radiography (KUB) may be appropriate (rating 5) for tracking large radiopaque stones 1

Imaging Modalities to Avoid

Do NOT order these as initial studies: 1

• CT with IV contrast alone (adds no benefit for stone detection) 1
• CT urography (CTU) as initial test 1
• MRI/MRU as first-line (expensive, stones difficult to visualize) 1
• Plain KUB radiography alone (sensitivity only 44-77%, misses radiolucent stones) 1
• Intravenous urography (IVU) (outdated, no longer recommended) 1

Clinical Pitfalls to Avoid

• Don't skip ultrasound and go straight to CT unless there's an emergency requiring immediate intervention 1
• Don't order contrast-enhanced CT for routine stone evaluation—it exposes patients to unnecessary contrast and rarely changes management 1
• Don't rely on ultrasound alone when precise stone sizing is critical for treatment decisions (sensitivity limitations) 1, 3
• Don't forget that ultrasound should not delay emergency care in unstable patients 1

Algorithmic Approach

1. All patients: Start with ultrasound 1, 2
2. If ultrasound positive and uncomplicated: Proceed with clinical management
3. If ultrasound negative/equivocal OR complicated presentation: Obtain low-dose non-contrast CT 1
4. If pregnant: Ultrasound → MRI → low-dose CT (only if absolutely necessary) 1
5. If emergency signs present (fever, sepsis, solitary kidney, anuria): May proceed directly to low-dose non-contrast CT after ultrasound without delay 1, 2