Indications for Ultrasound KUB (Kidneys, Ureters, and Bladder)
Ultrasound KUB is primarily indicated for the evaluation of obstructive uropathy and acute urinary retention, serving as a first-line imaging modality due to its non-invasive nature, rapid deployment, and absence of radiation exposure. 1
Primary Indications
- Suspected obstructive uropathy - Ultrasound is usually appropriate as the initial imaging modality for patients with painful, nontraumatic hematuria and suspected urolithiasis 1
- Acute urinary retention - To evaluate bladder distension and potential causes 1
- Evaluation of hydronephrosis - Particularly when the cause is unknown, with high sensitivity (95%) and specificity (100%) for detecting and grading hydronephrosis 1
- Microscopic hematuria with proteinuria - Ultrasound of the kidneys and bladder is usually appropriate in children initially presenting with this combination 1
- Macroscopic (gross) hematuria - Ultrasound is usually appropriate in the initial evaluation of nonpainful, nontraumatic isolated macroscopic hematuria 1
Advantages Over Other Imaging Modalities
- No radiation exposure - Unlike CT or KUB radiography 1
- No contrast agents required - Avoiding potential nephrotoxicity in patients with renal impairment 1
- Point-of-care availability - Can be performed at bedside without removing patients from the resuscitation area 1
- Cost-effective - Lower cost compared to CT or MRI 1
Limitations and Caveats
- Limited sensitivity for small stones - Ultrasound alone has reduced sensitivity for ureteric stones (45%) compared to CT 2
- Technical limitations - Examination may be limited by patient habitus (obesity), bowel gas, abdominal tenderness, or an empty bladder 1
- Operator dependence - Quality and accuracy depend on the skill of the sonographer 1
- False negatives - Absence of hydronephrosis does not rule out a ureteral stone, especially small ones 1
- Potential mimics - Hydronephrosis may be mimicked by dilated renal vasculature, renal sinus cysts, and medullary pyramids (especially in young patients) 1
Combined Approach for Suspected Urolithiasis
- Ultrasound + KUB radiography - This combination improves diagnostic accuracy compared to either modality alone 3, 4
- Ultrasound detects hydronephrosis while KUB can identify radio-opaque stones (90% of stones are radio-opaque, consisting of calcium oxalate, calcium phosphate, and struvite) 3
- For ureteric stones, combining ultrasound with KUB increases sensitivity from 45% to 77% 2
When to Consider Alternative Imaging
- CT KUB - Consider when ultrasound is inconclusive in patients with renal colic 5
- MR Urography - Consider in patients with renal impairment where contrast CT is contraindicated 1
- CT with IV contrast - Usually appropriate in children presenting with macroscopic hematuria in the setting of trauma 1
Special Populations
- Pediatric patients - Ultrasound is preferred as the initial imaging modality to avoid radiation exposure 1, 6
- Pregnant patients - Ultrasound is the imaging modality of choice due to absence of radiation 1
- Patients with renal impairment - Ultrasound avoids contrast agents that could worsen kidney function 1
Ultrasound KUB remains a valuable first-line imaging tool for urinary tract evaluation, particularly for detecting hydronephrosis and renal stones, though its limitations in detecting ureteric stones should be recognized, often necessitating complementary imaging approaches in equivocal cases.