Kidney Stone Spontaneous Passage by Size
Stones ≤5 mm will pass spontaneously in approximately 68% of cases, while stones between 5-10 mm pass in only 47% of cases, making 5 mm the practical cutoff where most stones will still pass without intervention. 1
Size-Based Passage Rates
- Stones ≤5 mm: 68% spontaneous passage rate (95% CI: 46-85%) 1
- Stones 5-10 mm: 47% spontaneous passage rate (95% CI: 36-59%) 1
- Stones >10 mm: Surgical intervention typically required as spontaneous passage is unlikely 2
More Granular Size Breakdown
Recent evidence provides even more precise predictions based on exact stone width 3:
- 0-2 mm: 98% passage rate at 20 weeks 3
- 3 mm: 98% passage rate at 20 weeks 3
- 4 mm: 81% passage rate at 20 weeks 3
- 5 mm: 65% passage rate at 20 weeks 3
- 6 mm: 33% passage rate at 20 weeks 3
- ≥6.5 mm: 9% passage rate at 20 weeks 3
Time to Passage
The timeline for spontaneous passage varies significantly by stone size 4:
- ≤2 mm stones: Average 8.2 days; 95% pass within 31 days 4
- 2-4 mm stones: Average 12.2 days; 95% pass within 40 days 4
- 4-6 mm stones: Average 22.1 days; 95% pass within 39 days 4
- Most stones that pass: Do so within approximately 17 days (range 6-29 days) 5
Conservative Management Duration
The maximum duration for conservative management should be limited to 4-6 weeks from initial presentation to avoid irreversible kidney injury. 1, 5
Medical Expulsive Therapy (MET)
Alpha-blockers significantly improve passage rates and should be offered 1, 5:
- Absolute increase: 29% improvement in stone passage rate (95% CI: 20-37%) compared to observation alone 1, 2
- Stone-free rates: 77.3% with alpha-blockers versus 54.4% with placebo for distal ureteral stones <10 mm 2
- Recommended for: Distal ureteral stones, particularly those >5 mm 1
Location Matters
Stone location significantly affects passage probability 4, 3:
- Distal ureteral stones: Higher passage rates than proximal stones 4
- Right-sided stones: More likely to pass than left-sided stones 4
- Stone size and location together are the most important predictors of spontaneous passage 3
When to Intervene
Intervention is warranted before the 4-6 week mark if 5, 2:
- Uncontrolled pain despite adequate analgesia
- Signs of infection or sepsis
- Development of obstruction or hydronephrosis
- Patient/clinician shared decision-making based on individual circumstances
Common Pitfalls
Do not continue observation indefinitely beyond 4-6 weeks, as prolonged obstruction can lead to irreversible kidney damage 5. Even small stones (≤5 mm) require surgical intervention in approximately 20% of cases within 5 years when initially asymptomatic 6.
Intervention rates by size 4:
- ≤2 mm: 4.8% require intervention
- 2-4 mm: 17% require intervention
- 4-6 mm: 50% require intervention