In an adult with acute flank pain and a non‑contrast computed tomography that shows no radiopaque stone, what is the appropriate next step in evaluation and management?

Evaluation and Management of Flank Pain with Negative Non-Contrast CT

When non-contrast CT shows no stone in a patient with acute flank pain, the next step is to obtain CT urography (CTU) with IV contrast or contrast-enhanced CT abdomen/pelvis to evaluate for radiolucent stones, alternative urinary tract pathology, and non-urologic causes of flank pain. 1

Rationale for Contrast-Enhanced Imaging

The 2023 ACR Appropriateness Criteria explicitly state that when non-contrast CT is inconclusive for stones, contrast-enhanced imaging may be appropriate as the next study 1. This recommendation is based on several key diagnostic advantages:

• Contrast-enhanced CT identifies alternative diagnoses in 5-18% of cases that would be missed on non-contrast imaging, though it changes clinical management in only 2-3% of patients 1
• CTU with delayed excretory phase confirms ureteral location of calculi and distinguishes stones from mimics such as phleboliths or vascular calcifications 1
• Contrast enhancement detects radiolucent stones (though rare) by demonstrating filling defects in the opacified collecting system 1
• IV contrast better delineates the "soft tissue rim" sign that helps differentiate ureteral stones from phleboliths and enhances detection of urinary obstruction through delayed nephrogram 1

Diagnostic Performance of Contrast CT for Stones

While non-contrast CT remains the gold standard with 97% sensitivity 1, contrast-enhanced studies maintain excellent performance:

• Contrast-enhanced CT demonstrates 95% sensitivity for stones ≥3 mm and 81% overall sensitivity for all stone sizes 1
• Detectability of stones ≥6 mm on contrast CT is approximately 98%, meaning clinically significant stones requiring intervention are rarely missed 1

Alternative and Non-Urologic Diagnoses to Consider

Approximately one-third of patients with flank pain have non-stone etiologies identified on imaging 2. Contrast-enhanced CT evaluates for:

Urologic Causes

• Pyelonephritis or perinephric abscess – suggested by fever, chills, or pain worsening with external flank pressure 2, 3
• Hydronephrosis from non-stone obstruction (tumor, stricture, blood clot) 2
• Hemorrhage within a renal cyst – can cause acute pain mimicking stone disease 4
• Urothelial masses – detected on excretory phase imaging 1

Gastrointestinal Causes

• Colonic diverticulitis (especially left-sided) – frequently mimics renal colic 2
• Inflammatory bowel disease – shows bowel wall thickening and mesenteric edema 2
• Gastroenteritis or colitis – demonstrates colonic wall edema 2
• Bowel obstruction – CT identifies transition point and dilated loops 2

Gynecologic Causes (in women)

• Pelvic congestion syndrome – demonstrated by dilated pelvic veins on contrast-enhanced imaging 2
• Adnexal masses or ovarian torsion – require urgent evaluation 2
• Ectopic pregnancy – must be excluded with β-hCG in all women of reproductive age 2

Clinical Algorithm for Management

Step 1: Review Initial Non-Contrast CT Findings

• Confirm absence of hydronephrosis, perinephric stranding, or ureteral dilation – their presence suggests obstruction despite no visible stone 1
• Assess for secondary signs of obstruction that may indicate a small or radiolucent stone 5

Step 2: Obtain Contrast-Enhanced Imaging

• Order CTU (CT with and without IV contrast plus delayed excretory phase) as the preferred next study 1
• Alternative: CT abdomen/pelvis with IV contrast alone if excretory phase is not available 1
• MR urography may be appropriate in patients with contrast allergy or renal insufficiency, though MRI shows only 69% sensitivity for stone impaction site versus 100% for CT 2

Step 3: Reassess Clinical Context

• Pain independent of body position and radiating to groin = classic renal colic, suggesting stone despite negative imaging 2
• Pain worsening with external pressure or associated with fever = infection (pyelonephritis, abscess) 2, 3
• Pain after prolonged static positioning = more likely musculoskeletal origin 2

Step 4: Red Flags Requiring Urgent Intervention

• Fever, chills, or sepsis signs with suspected obstruction = obstructive pyelonephritis requiring immediate admission, IV antibiotics, and decompression 2
• Hemodynamic instability = immediate hospital transfer 2
• Solitary kidney with suspected obstruction = urgent imaging and urology consultation 2
• Inability to urinate or anuria = emergent evaluation 2

Common Pitfalls to Avoid

• Do not assume absence of stone on non-contrast CT excludes urolithiasis – over 20% of confirmed stone patients have negative urinalysis, and small stones may be missed 2
• Do not delay imaging in young women – gynecologic emergencies (ectopic pregnancy, ovarian torsion) require prompt diagnosis 2
• Do not miss the diagnosis window for radiolucent stones – though rare, uric acid stones and matrix calculi may not be visible on non-contrast CT 1
• Do not overlook alternative diagnoses – the broad differential for flank pain extends well beyond the urinary tract 2, 6
• Recognize that hydronephrosis may not appear within the first 2 hours of acute obstruction, reducing early ultrasound sensitivity 2

When Contrast is Contraindicated

If IV contrast cannot be administered due to severe allergy or renal insufficiency:

• Renal ultrasound with Doppler can assess for hydronephrosis (up to 100% sensitivity), perinephric fluid, and vascular complications 2
• MR urography without contrast using heavily T2-weighted sequences shows 77% sensitivity for perirenal fluid in acute obstruction 1
• Clinical observation with repeat imaging if symptoms persist or worsen 1

Follow-Up Based on Imaging Results

• If contrast CT identifies a stone: Manage based on size and location – stones <5 mm typically pass spontaneously; larger stones or complete obstruction may require endoscopic intervention 2
• If alternative diagnosis found: Treat accordingly (antibiotics for infection, surgical consultation for diverticulitis, etc.) 2
• If imaging remains negative: Consider non-urologic causes, musculoskeletal evaluation, or empiric trial of NSAIDs with close outpatient follow-up 2