What are the next steps after ordering a Kidney, Ureter, Bladder (KUB) scan?

Next Steps After Ordering a KUB

A KUB radiograph has limited diagnostic utility and should be interpreted in conjunction with ultrasound or followed by CT imaging depending on your clinical suspicion—it is not a standalone diagnostic tool.

Understanding KUB Limitations

The KUB has significant diagnostic limitations that determine your next steps:

• For urolithiasis detection: KUB sensitivity is only 44-77% and is particularly poor for stones <4mm 1
• Cannot reliably rule out stones: KUB detected only 8% of stones <5mm in one study, though detection improved to 78% for stones >5mm 2
• Not recommended as initial imaging: For acute pyelonephritis in uncomplicated patients, KUB provides no benefit 1
• Poor sensitivity for ureteral stones: Only 53-62% sensitive with 67-69% specificity for ureteral calculi 2

Clinical Context-Specific Next Steps

If Suspecting Urolithiasis (Renal Colic):

Ultrasound should be your primary diagnostic tool, not KUB 1:

• Combine KUB with ultrasound if you've already ordered the KUB—this combination improves diagnostic accuracy compared to either modality alone 2
• Proceed to non-contrast CT if ultrasound findings are equivocal or if the patient has complicated features (recurrent stones, diabetes, immunocompromise, lack of response to therapy) 1
• Use low-dose CT protocols when possible (93.1% sensitivity, 96.6% specificity) to minimize radiation exposure 1

If KUB Shows a Radio-opaque Stone:

• Use KUB for follow-up imaging in these patients—all stones visible on CT scout films are also visible on KUB, making KUB appropriate for serial monitoring 3
• 90% of stones are radio-opaque (calcium oxalate, calcium phosphate, struvite), so KUB can be useful for tracking known stones 2

If KUB is Normal but Clinical Suspicion Remains High:

Do not stop at a normal KUB 1:

• Order non-contrast CT immediately for patients with:
  • Persistent symptoms after 72 hours of appropriate therapy 1
  • Complicated features (recurrent disease, diabetes, immunocompromise, solitary kidney, fever) 1
  • Suspected obstruction with sepsis or anuria—these require urgent decompression via nephrostomy or ureteral stent 1

If Ordered for Constipation:

KUB is not recommended for constipation diagnosis 2, 4:

• Rely on clinical diagnosis based on history and physical examination rather than radiographic assessment 2, 4
• The American College of Radiology does not recommend KUB as a primary diagnostic tool for constipation 2, 4

If Ordered for Trauma:

KUB has no role in acute trauma evaluation 1:

• Proceed directly to contrast-enhanced CT with delayed urographic phase for hemodynamically stable patients with suspected kidney or urinary tract injury 1
• Intravenous urography may be useful only in unstable patients during surgery when CT is unavailable 1

Critical Pitfalls to Avoid

• Never rely on KUB alone to exclude urolithiasis—its low sensitivity means significant stones can be missed 1, 2
• Don't delay CT in complicated patients—those with fever, solitary kidney, or persistent symptoms need definitive imaging 1
• Recognize that 10% of stones are radiolucent (uric acid, cystine) and will never appear on KUB 2
• In patients ≥65 years with colic and hematuria, if KUB is uncertain, proceed to CT urography to exclude upper tract urothelial cancer (though the miss rate is low at 0.2%) 5

Immediate Biochemical Work-up

While interpreting imaging, obtain these labs for any patient with suspected urolithiasis 1:

• Urine dipstick
• Creatinine, uric acid, ionized calcium
• Sodium, potassium (if intervention planned)
• Blood cell count and CRP
• Stone analysis if stone is passed or retrieved 1