Evaluation of 1 mm Renal Stone on CT
A 1 mm renal stone requires active surveillance with follow-up imaging rather than intervention, as stones this small have an extremely high likelihood of spontaneous passage and do not meet size thresholds for surgical treatment.
Initial Management Approach
Conservative management with observation is the appropriate strategy for this stone size. 1 International guidelines (AUA/ES, EAU, and SIU/ICUD) consistently support active surveillance for asymptomatic, non-obstructing caliceal stones, with EAU guidelines specifically stating that active surveillance is acceptable for stones up to 15 mm. 1
Expected Natural History
- Spontaneous passage is highly likely for stones <4 mm, with 38-71% of symptomatic ureteral calculi in this size range passing without intervention. 2
- For stones <2 mm specifically, only 4.8% will require intervention during surveillance. 2
- At 1 mm, this stone falls well below any intervention threshold and has the highest probability of spontaneous passage or remaining asymptomatic.
Follow-Up Imaging Strategy
Annual follow-up imaging is recommended for asymptomatic renal stones under observation. 2, 3
- Non-contrast CT remains the gold standard for follow-up, with sensitivity of 97% and specificity of 95% for stone detection. 4
- Low-dose CT protocols (<3 mSv) should be utilized to minimize cumulative radiation exposure while maintaining diagnostic accuracy (pooled sensitivity 97%, specificity 95%). 4
- Ultrasound can be alternated with CT for annual surveillance to reduce radiation burden, though ultrasound has limited sensitivity (54%) for small stones and tends to overestimate stone size. 5, 2
Indications for Intervention
Surgical treatment would only be indicated if the stone: 1
- Demonstrates significant growth on follow-up imaging
- Becomes associated with infection
- Causes symptomatic obstruction
- Develops other complications
None of these scenarios are expected with a 1 mm stone, which is far below the 10-20 mm size range where surgical options (fURS, SWL, or PCNL) are typically considered for renal stones. 1
Risk Stratification
This patient has minimal risk of stone-related events. 3
- Stones >5 mm are associated with higher rates of symptomatic events and need for intervention, but a 1 mm stone falls well below this threshold. 3
- Stone location matters for larger stones (lower pole stones are less likely to pass), but at 1 mm, location is less clinically relevant. 3
Common Pitfalls to Avoid
- Do not order contrast-enhanced CT for stone surveillance, as IV contrast may obscure small stones within the collecting system and provides no additional benefit. 4
- Avoid relying solely on ultrasound for follow-up, as it has poor sensitivity (24-57%) for small stone detection and significantly overestimates stone size in the 0-10 mm range. 1, 5
- Do not pursue intervention based on stone presence alone without symptoms, growth, or complications, as this represents overtreatment. 1
Practical Follow-Up Schedule
- First follow-up at 1 year with low-dose non-contrast CT or ultrasound. 2
- Continue annual imaging for 2-3 years if stone remains stable and asymptomatic. 2
- Patient education regarding symptoms of stone passage (flank pain, hematuria) should be provided, though intervention is rarely needed for stones this small. 2