Kidney Stone Size and Spontaneous Passage
Stones ≤5 mm have approximately a 68% chance of passing spontaneously and can be managed with observation, while stones >10 mm are unlikely to pass and require surgical intervention. 1, 2
Stone Size-Based Passage Rates
The likelihood of spontaneous stone passage is directly related to stone size:
- Stones ≤5 mm: Approximately 68% will pass spontaneously (95% CI: 46% to 85%) 1, 2
- Stones >5 mm but ≤10 mm: Approximately 47% will pass spontaneously (95% CI: 36% to 59%) 1, 2
- Stones >10 mm: Unlikely to pass spontaneously and typically require surgical intervention 2, 3
Initial Management Algorithm
For Stones ≤5 mm
Offer observation with or without medical expulsive therapy (MET) using alpha-blockers as first-line management. 1, 2 These stones rarely require surgical intervention and most will pass within 6 weeks. 3, 4
For Stones 5-10 mm
Observation with MET is appropriate as initial management, though up to 50% may ultimately require intervention. 1, 3 Alpha-blockers increase stone passage rates by an absolute 29% (95% CI: 20% to 37%) compared to control. 1, 2
For Stones >10 mm
Discuss with urology immediately as these stones are unlikely to pass spontaneously and will require surgical intervention (ureteroscopy or shock wave lithotripsy). 2, 3
Critical Time Limit
Limit conservative therapy to a maximum of 6 weeks from initial presentation to avoid irreversible kidney injury. 1, 2 This applies to all stone sizes being managed conservatively.
Location Matters
Stone location significantly affects passage rates beyond size alone:
- Distal ureteral stones have the highest spontaneous passage rates 1, 2
- Proximal and mid-ureteral stones have lower passage rates even when size-appropriate for observation 1
In pediatric patients, distal ureteral stones <5 mm have a 62% passage rate, while stones >5 mm have only a 35% passage rate. 1
Common Pitfalls
Do not rely on ultrasound alone for stone sizing decisions. Ultrasound has only 54% sensitivity for detecting renal stones and significantly overestimates stone size in the 0-10 mm range, leading to inappropriate management recommendations in 22% of cases. 5 Non-contrast CT is the gold standard with >99% sensitivity and should be used for accurate stone size determination. 3, 5
Beware that even "insignificant" stones ≤5 mm require surgical treatment in approximately 20% of cases within 5 years, though this is typically due to stone relocation into the ureter rather than failure to pass. 6
High-Risk Patients Requiring Closer Follow-Up
Patients at higher risk for stone-related events during observation include those with:
- Stone size >5 mm (higher risk of symptoms and need for intervention) 7
- Diabetes mellitus or hyperuricemia (higher risk of stone growth) 7
- Non-lower pole stones (higher risk of growth and symptoms) 7
- Age >60 years (higher risk of requiring surgical intervention) 7
These patients warrant more frequent monitoring even during conservative management. 7