What can a KUB (Kidneys, Ureters, Bladder) determine?

What a KUB Can Determine

A KUB (Kidneys, Ureters, Bladder) radiograph is a limited diagnostic tool that can detect large, radio-opaque kidney stones (>5mm) and suggest calcifications in the urinary tract, but it has poor sensitivity for most stones and cannot reliably identify obstruction, stone location, or the cause of urinary symptoms. 1

Primary Diagnostic Capabilities

Stone Detection

• KUB detects only 78% of stones >5mm and a dismal 8% of stones <5mm when compared to non-contrast CT as the reference standard 1
• Overall sensitivity for detecting stones of any size in any location is only 29%, though this improves to 72% for large stones (>5mm) in the proximal ureter 1
• For ureteral calculi specifically, KUB demonstrates limited sensitivity of 53-62% and specificity of 67-69% 1, 2
• KUB is particularly insensitive for stones <4mm and those located in the mid and distal ureters 1, 2

What KUB Can Show

• Visible calcifications in the expected location of the ureter on the side of the patient's pain, which may suggest urolithiasis 1
• Approximately 90% of stones are radio-opaque (calcium oxalate, calcium phosphate, and struvite), making them theoretically visible 1, 2

Critical Limitations

Cannot Distinguish Pathology

• KUB cannot differentiate between ureteral stones and phleboliths or other vascular calcifications on a single 2-D view 1
• Multiple calcifications may be visible but their clinical significance cannot be determined 1

Cannot Detect Obstruction

• KUB has 0% sensitivity for identifying the cause of obstruction and minimal ability (60% specificity) to localize the obstruction site 2
• Cannot visualize hydronephrosis, perinephric fluid, or other secondary signs of obstruction 1

Factors Reducing Sensitivity

• Stone composition (radiolucent stones like uric acid are invisible) 1
• Stone location (mid and distal ureter stones poorly visualized) 1
• Stone size (<5mm stones rarely detected) 1
• Patient body habitus (obesity reduces visibility) 1
• Overlying bowel contents (gas and stool obscure stones) 1

Clinical Context and Alternatives

When KUB May Have Limited Utility

• The American College of Radiology states that KUB alone is insufficient for diagnosing suspected obstruction and should not be ordered as the sole imaging modality 2
• CT scan dramatically outperforms KUB with 93-96% sensitivity and 93-100% specificity for confirming obstruction, plus 66-87% sensitivity for identifying the cause 2

Potential Adjunctive Role

• Combining KUB with ultrasound may improve stone detection rates compared to either modality alone, with one study showing the combination identified 50% of stones detected by CT and 68% of stones requiring urological intervention 1
• When ultrasound shows hydronephrosis, adding KUB can help identify radio-opaque stones that ultrasound may miss 1, 3
• A baseline KUB at the time of initial CT may aid interpretation of future follow-up KUB radiographs for tracking stone passage, though the CT scout film often provides similar information 4

Superior Alternatives

• Non-contrast CT is the gold standard for urolithiasis evaluation, with 91-96% sensitivity compared to KUB's 29-78% 1, 2
• Ultrasound has 81% sensitivity and 100% specificity for renal stones, and 93% sensitivity and 100% specificity for hydronephrosis 3
• If CT is unavailable or contraindicated, ultrasound should be the first-line alternative—not KUB 2

Common Pitfalls to Avoid

• Do not assume that because 90% of stones are radio-opaque, KUB is adequate—the sensitivity remains poor even for radio-opaque stones due to technical limitations 2
• Do not order KUB as a standalone test for suspected obstruction—it will miss the majority of clinically significant pathology 2
• Do not rely on KUB to determine if a stone is causing obstruction or to guide urgent intervention decisions 1, 2
• Be aware that KUB has no role in evaluating acute pyelonephritis or other renal parenchymal disease 2