Is a KUB X-ray a reasonable next step to investigate persistent right flank pain in a patient with a history of diverticulosis and a previous negative CT scan?

KUB X-ray is Not Appropriate for Persistent Right Flank Pain

A KUB X-ray should not be ordered for your patient with persistent right flank pain and a history of diverticulosis, as it has poor diagnostic accuracy and will likely miss clinically significant pathology. The American College of Radiology explicitly rates KUB as "usually not appropriate" (rating 3 out of 9) for evaluating flank pain with suspected stone disease 1.

Why KUB is Inadequate

KUB has unacceptably low sensitivity and specificity for the conditions you need to rule out:

• For ureteral stones: Only 53-62% sensitivity and 67-69% specificity 1
• For small stones (<5mm): Detects only 8% compared to CT 2
• For mid and distal ureteral stones: Particularly insensitive 2, 3
• For obstruction: Cannot identify the cause (0% sensitivity) and has minimal ability to localize the site (60% specificity) 3

Even though 90% of stones are radio-opaque, KUB's sensitivity remains poor even for these radio-opaque stones 3, 4.

What You Should Order Instead

First-Line Imaging: Ultrasound

Ultrasound is the appropriate initial imaging modality for your patient 1:

• 93-100% sensitivity and specificity for hydronephrosis 2, 4
• 100% sensitive and 90% specific for ureteral obstruction 2
• 81% sensitivity and 100% specificity for renal stones 4
• No radiation exposure 1

The ultrasound examination should include 2:

• Both kidneys in longitudinal and transverse planes
• Assessment for hydronephrosis, stones, masses, or parenchymal abnormalities
• Bladder evaluation when distended, then post-void to measure residual volume
• Assessment for hydroureter and perinephric fluid

Second-Line Imaging: CT Without Contrast

If ultrasound is negative or equivocal but clinical suspicion remains high, order non-contrast CT 1:

• 96-100% sensitivity for stones 2
• 93-96% sensitivity and 93-100% specificity for obstruction 3
• Can detect alternative diagnoses 1

Low-dose CT protocols should be used to minimize radiation exposure 1.

Addressing the Diverticulosis History

Your patient's history of diverticulosis does not change the imaging algorithm for flank pain:

• Diverticulosis typically affects the sigmoid colon and presents with left lower quadrant pain, not right flank pain 5, 6
• Right-sided diverticulitis is rare in Western populations 6
• If you suspect diverticulitis recurrence, CT with IV contrast is the gold standard (98-99% sensitivity and 99-100% specificity) 5
• Non-contrast CT can detect diverticulitis findings (pericolonic fat stranding, bowel wall thickening, diverticula) but is less accurate than contrast-enhanced CT 7

Clinical Algorithm for Your Patient

Follow this stepwise approach:

1. Order renal ultrasound first to evaluate for hydronephrosis, stones, and structural abnormalities 1, 2

2. If ultrasound shows hydronephrosis or stones: Proceed to non-contrast CT to clarify the cause and guide management 2

3. If ultrasound is negative but pain persists: Order non-contrast CT to exclude small stones, subtle obstruction, or alternative diagnoses 1

4. If considering diverticulitis: Order CT abdomen/pelvis with IV contrast (not KUB), as this is the appropriate test for suspected diverticulitis 7, 5

Critical Pitfalls to Avoid

• Do not order KUB as a standalone test—it will miss the majority of clinically significant pathology 1, 3
• Do not assume previous negative CT rules out new pathology—stones can form or migrate, and new obstruction can develop 1
• Do not delay appropriate imaging—persistent flank pain warrants investigation to exclude obstruction, which can lead to renal damage if untreated 2
• Do not confuse diverticulosis location with flank pain—right flank pain is unlikely to be diverticulitis unless the patient has right-sided diverticulosis (uncommon in Western populations) 6