What are the indications for a renal (kidney) ultrasound in patients with symptoms such as flank pain, hematuria, or urinary tract infections?

KUB Ultrasound Indications

Renal ultrasound is indicated as the primary diagnostic tool for evaluating suspected urinary tract pathology including urolithiasis, hydronephrosis, structural abnormalities, and renal masses, particularly in children, pregnant women, and patients requiring radiation-free imaging. 1

Primary Indications for Renal Ultrasound

Suspected Urolithiasis (Kidney Stones)

• Ultrasound serves as the first-line imaging modality for acute flank pain with suspected stone disease, though it should not delay emergency care if clinical instability exists 1
• Sensitivity is 45% for ureteral stones and 88% specificity for renal stones, with better performance for larger stones (>5 mm) 1
• Ultrasound is 81% sensitive and 100% specific for detecting renal stones, and 93% sensitive and 100% specific for hydronephrosis 2
• When combined with plain KUB radiograph, sensitivity for ureteric stones increases from 46% to 77% 2
• Ultrasound demonstrates up to 100% sensitivity and 90% specificity for detecting ureteral obstruction (hydronephrosis, ureterectasis, perinephric fluid) 1

Hematuria Evaluation

• In children with isolated microscopic hematuria without proteinuria or dysmorphic red blood cells, no imaging is indicated 1
• For painful hematuria with suspected urolithiasis, ultrasound is appropriate as initial imaging, though CT has superior diagnostic accuracy 3
• Ultrasound is the best modality to assess kidney size, position, and anatomy before renal biopsy in children with glomerular disease 1

Urinary Tract Infections

• Ultrasound is strongly recommended as first-line imaging in neonates and children with UTI to detect structural abnormalities, hydronephrosis, and vesicoureteral reflux 1
• In children with first febrile UTI, ultrasound should be performed to identify anatomic abnormalities 1
• Ultrasound is rated as "usually appropriate" (rating 7-9) for initial evaluation of pediatric UTI 1

Special Populations

Pregnant Patients

• Ultrasound is strongly recommended as first-line imaging for suspected urolithiasis in pregnancy, avoiding radiation exposure 1
• MRI serves as second-line, with low-dose CT as last resort only when absolutely necessary 1

Pediatric Patients

• Ultrasound is strongly recommended as first-line imaging in children with suspected stones, followed by KUB or low-dose CT only if ultrasound is insufficient 1
• For children with renal trauma and macroscopic hematuria, ultrasound displays anatomy, kidney size, and position effectively 1

Patients with Renal Failure

• Ultrasound is particularly valuable in patients with elevated creatinine (≥1.8 mg/dl) where contrast-enhanced studies are contraindicated 2

Key Limitations and When to Escalate to CT

Critical Limitations of Ultrasound

• Ultrasound detects only 75% of all urinary tract stones and only 38% of ureteral stones 3
• Sensitivity drops significantly for stones <5 mm 1
• Within the first 2 hours of symptom onset, secondary signs of obstruction may not have developed, reducing ultrasound sensitivity 1
• Poor sensitivity (50%) for detecting hydroureter 2

When CT is Required

• Non-contrast CT should follow ultrasound when there is high clinical suspicion for urolithiasis despite negative or equivocal ultrasound findings 1, 3
• CT has sensitivity and specificity both exceeding 90% for stone detection 1, 3
• CT is the gold standard for acute flank pain evaluation, providing stone size, location, density, composition, and degree of obstruction 1
• Low-dose CT protocols maintain 93.1% sensitivity and 96.6% specificity while reducing radiation exposure 1

Additional Diagnostic Considerations

Biochemical Workup

• Every patient with suspected urolithiasis requires urine dipstick and blood tests (creatinine, uric acid, ionized calcium, sodium, potassium, blood cell count, CRP) 1
• Stone analysis should be performed for all first-time stone formers 1

Plain Radiography (KUB)

• KUB radiography has inadequate sensitivity (59-72%) for initial stone detection and provides no information about obstruction degree or soft tissue pathology 1, 3
• KUB is useful for differentiating radiopaque from radiolucent stones and aids in follow-up of known stones 1
• Detection rate is 78% for stones >5 mm but only 8% for stones ≤5 mm 1

Clinical Pearls

• The absence of hematuria should not deter evaluation when kidney stones are clinically suspected, as many patients with urolithiasis do not have hematuria 3, 4
• Ultrasound performed by trained urologists in outpatient settings achieves 99.7% sensitivity with high patient satisfaction 5
• Lack of hydronephrosis on ultrasound makes the presence of larger ureteral stones (>5 mm) less likely 1
• Color Doppler with twinkling artifact can increase sensitivity to 99% for small renal stones, but has false-positive rates up to 60% 1