Renal Ultrasound Detection of 7x4mm Distal Ureteral Stone
A renal ultrasound will likely miss a 7x4mm stone in the distal ureter, as ultrasound has poor sensitivity for direct visualization of ureteral stones, detecting only up to 61% of ureteral calculi even when associated signs of obstruction are present. 1
Direct Stone Visualization
Ultrasound performs poorly for detecting stones within the ureter itself:
- Gray-scale ultrasound demonstrates sensitivity of only up to 61% for ureteral stone detection (with 100% specificity when stones are seen), and this sensitivity is improved only if there are associated signs of obstruction present 1
- The overall sensitivity of ultrasound for stone detection ranges from only 24% to 57% compared to noncontrast CT, with decreased sensitivity for smaller stones 1
- Ultrasound has nearly 100% sensitivity for large stones (>5 mm) in the kidney, but accuracy for small stones (<3 mm) is poor, and this does not apply to ureteral stones 1
Indirect Signs May Be Present
However, ultrasound can detect secondary signs of obstruction that suggest a stone is present:
- Ultrasound is up to 100% sensitive and 90% specific for diagnosing ureteral obstruction (hydronephrosis, ureterectasis, and perinephric fluid) in patients with acute flank pain 1
- Critical caveat: Within the first 2 hours of presentation, these secondary signs are less sensitive because obstruction findings may not have had time to develop 1
Clinical Implications
For a 7x4mm distal ureteral stone, noncontrast CT remains the reference standard with 97% sensitivity, far superior to ultrasound 1
- Your 7mm stone falls into a size category where intervention is often recommended, as stones >6-7mm have lower spontaneous passage rates 2
- Spontaneous passage rate for stones 5-7mm is approximately 60%, and for stones 7-9mm drops to 48% 3
- Distal ureteral location is favorable, with 75% spontaneous passage rate for distal stones overall 3
Recommendation
If clinical suspicion for a distal ureteral stone remains high despite negative or equivocal ultrasound findings, proceed directly to noncontrast CT for definitive diagnosis, as ultrasound's poor sensitivity for direct ureteral stone visualization makes it an unreliable modality for excluding this diagnosis 1