What imaging study is recommended for a patient with left flank pain who has no signs of infection?

Diagnostic Imaging for Left Flank Pain Without Signs of Infection

For a patient with left flank pain and no signs of infection, ultrasound should be performed first, followed by non-contrast CT if ultrasound is inconclusive or does not provide sufficient diagnostic information. 1, 2

Primary Diagnostic Approach

First-Line: Ultrasound

• Ultrasound is the primary diagnostic tool for evaluating acute flank pain 1
• Sensitivity is 45% for detecting stones, but specificity is high (94% for ureteral stones, 88% for renal stones) 1
• US is 100% sensitive for detecting ureteral obstruction (hydronephrosis, ureterectasis, perinephric fluid) 2
• Key limitation: Secondary signs of obstruction may not develop within the first 2 hours of presentation 2
• Ultrasound should not delay emergency care if clinical urgency exists 1

Second-Line: Non-Contrast CT

• Non-contrast CT is the standard modality after ultrasound for acute flank pain 1, 2
• Provides comprehensive assessment of stone location, size, density, and anatomy 1
• Sensitivity of 93-97% and specificity of 92-100% for stone detection 3, 4, 5
• Low-dose CT protocols reduce radiation exposure while maintaining diagnostic accuracy (93.1% sensitivity, 96.6% specificity) 1

Clinical Algorithm

Step 1: Perform ultrasound initially

• If hydronephrosis or stone visualized → diagnosis established
• If negative but high clinical suspicion → proceed to CT

Step 2: Non-contrast CT if:

• Ultrasound is inconclusive
• No hydronephrosis on US but symptoms persist
• Need to assess stone size, location, and density for treatment planning
• Alternative diagnoses need to be excluded

Step 3: Consider KUB radiography

• Useful for differentiating radioopaque vs radiolucent stones 1
• Aids in follow-up of known stones 1
• Not recommended as primary diagnostic tool (sensitivity only 29-72% depending on stone size and location) 2

Important Caveats

When Ultrasound May Miss Pathology

• Early presentation (< 2 hours): Secondary signs of obstruction may not yet be visible 2
• Small stones (< 5 mm): Detection sensitivity decreases significantly 2
• Ureteral stones: Sensitivity up to 61% compared to CT 2
• Absence of hydronephrosis on US makes stones > 5 mm less likely but does not exclude smaller stones 2

CT Advantages Beyond Stone Detection

• Identifies non-calculus causes of flank pain in > 30% of patients 5
• Can detect pancreatitis, diverticulitis, renal masses, and other abdominal pathology 6
• Superior for treatment planning as it provides stone density and precise measurements 1

Special Populations

• Pregnant women: Use ultrasound first, then MRI if needed, with low-dose CT as last resort 1
• Children: Ultrasound strongly recommended as first-line, followed by KUB or low-dose CT if US insufficient 1

Biochemical Work-Up

Alongside imaging, obtain:

• Urinalysis (dipstick)
• Serum creatinine, uric acid, ionized calcium
• Complete blood count and C-reactive protein 1

Note: The ACR Appropriateness Criteria explicitly state this guidance assumes no clinical signs or suspicion of infection, as pyelonephritis requires different imaging considerations 2.