Minimum Size of Kidney Stones That Can Get Stuck in the Ureter
Kidney stones as small as 4 mm can get stuck in the ureter, with stones 5 mm or larger having a significantly higher risk of becoming lodged and requiring intervention. 1, 2
Stone Size and Passage Rates
- Stones smaller than 4 mm have a high likelihood of spontaneous passage, with studies showing passage rates of approximately 98% for stones 0-3 mm in diameter 3, 1
- Passage rates decrease significantly as stone size increases: 81% for 4 mm stones, 65% for 5 mm stones, 33% for 6 mm stones, and only 9% for stones ≥6.5 mm 1
- The American Urological Association (AUA) guidelines suggest that stones smaller than 10 mm can potentially pass spontaneously, but the likelihood decreases dramatically with increasing size 4
- The European Association of Urology (EAU) guidelines use 6 mm as a cutoff size for observation, while the Société Internationale d'Urologie (SIU) guidelines suggest intervention for stones greater than 7 mm 4
Anatomical Considerations
- The ureter has natural narrowing points where stones are more likely to become lodged, regardless of their size 5
- The critical size of a kidney stone that can pass through the ureter is related to the ureter's inner diameter, with research suggesting stones approximately 11-22% larger than the ureter's inner diameter will become stuck 6
- Stone location affects passage rates - proximal ureteral stones tend to be larger than distal stones when they become lodged (mean transverse diameter 8.47 mm vs. 6.74 mm) 5
- Stones in the distal ureter have higher spontaneous passage rates than those in the proximal ureter 2
Time to Stone Passage
- Smaller stones pass more quickly: stones ≤2 mm take an average of 8.2 days to pass, stones 2-4 mm take about 12.2 days, and stones 4-6 mm take approximately 22.1 days 2
- For 95% of stones to pass spontaneously, it can take up to 31 days for stones ≤2 mm, 40 days for stones 2-4 mm, and 39 days for stones 4-6 mm 2
- The AUA guidelines suggest a maximum duration of conservative treatment of 4-6 weeks from initial clinical presentation 4
Management Implications
- For ureteral stones <10 mm, both ureteroscopy (URS) and shock wave lithotripsy (SWL) are recommended treatment options 4
- For distal ureteral stones >10 mm, URS is recommended as first-line treatment by all major guidelines 4
- For proximal ureteral stones, URS is generally recommended regardless of stone size, though SWL may be an equivalent option for stones <10 mm 4
Monitoring Considerations
- Imaging follow-up is essential when managing stones conservatively 4, 7
- CT is superior to ultrasound for accurate stone size determination, as ultrasound tends to overestimate stone size 7
- Factors that should prompt more urgent urological evaluation include evidence of urinary tract infection, intractable pain, or anatomical abnormalities that may complicate stone passage 7
In conclusion, while stones as small as 4 mm can become lodged in the ureter, the risk increases substantially with stones 5 mm or larger, and intervention is frequently required for stones 6 mm and above.