Best Initial Test for Right Flank Pain
Non-contrast CT of the abdomen and pelvis is the definitive first-line imaging test for right flank pain, with 97-100% sensitivity and 95-100% specificity for detecting kidney stones and identifying alternative diagnoses. 1, 2, 3, 4
Why CT is Superior
Non-contrast helical CT has replaced older imaging modalities as the gold standard because it:
- Detects virtually all stones regardless of size, location, or composition with 98-100% sensitivity and 92-100% specificity, far exceeding other modalities 1, 3, 4, 5
- Identifies non-stone causes of flank pain in approximately one-third of patients, including right colonic diverticulitis, appendicitis, ovarian pathology, and inflammatory bowel disease 2, 4, 6
- Provides critical management information including exact stone size, precise location, and degree of obstruction—all essential for determining whether conservative management or intervention is needed 2
- Requires no IV contrast, eliminating risks of nephrotoxicity and allergic reactions while being faster to perform (completed within 5 minutes) 5
Radiation Concerns Addressed
Use low-dose CT protocols (<3 mSv) which maintain 97% sensitivity and 95% specificity while minimizing radiation exposure, particularly important for recurrent stone formers 1, 7, 2
Alternative Imaging: When to Consider Ultrasound
Ultrasound combined with plain radiography (KUB) is a reasonable alternative in specific populations:
- Pregnant patients where radiation must be avoided 1
- Children as a first-line approach 1
- Combined ultrasound + KUB achieves 79-90% sensitivity for clinically significant stones, though it misses smaller stones more frequently than CT 1, 7, 2
Ultrasound Limitations to Recognize
- Sensitivity drops to only 24-57% for direct stone visualization, making it unreliable as a standalone test 1, 7
- Secondary signs (hydronephrosis) may not develop within the first 2 hours of symptom onset, leading to false-negative results if performed too early 2
- Absence of hydronephrosis makes stones >5 mm less likely, which can be clinically useful 2
- Ultrasound is 100% sensitive and 90% specific for detecting obstruction (hydronephrosis, ureterectasis, perinephric fluid), even when stones aren't directly visualized 1, 2
What NOT to Order
Avoid plain radiography (KUB) alone, which has only 29% overall sensitivity and 72% sensitivity even for large stones in the proximal ureter—inadequate for diagnosis or exclusion 1, 7, 2
Intravenous urography (IVU) is obsolete with lower sensitivity (87%) and specificity (94%) compared to CT, plus risks of contrast nephrotoxicity and allergic reactions 1, 5
Critical Red Flags Requiring Urgent Imaging
Order CT emergently if the patient has:
- Fever, chills, or signs of systemic infection suggesting obstructive pyelonephritis requiring emergent decompression 2
- Inability to urinate or decreased urine output indicating possible complete obstruction 2
- Visible hematuria with systemic symptoms warranting immediate assessment 2
Common Pitfalls to Avoid
- Don't use contrast-enhanced CT as first-line imaging—contrast may obscure small stones 1
- Don't rely solely on ultrasound for small stones (<5 mm) where sensitivity decreases significantly 1
- Don't dismiss the possibility of non-stone pathology—CT identifies alternative diagnoses in about one-third of flank pain cases 2, 4
- Don't order ultrasound within the first 2 hours of symptom onset expecting to see hydronephrosis, as secondary signs take time to develop 2