Kidney Stone STONE Score Calculator
Predict kidney stone surgery outcomes with the STONE nephrolithometry scoring system. Enter CT stone measurements, tract length, and density for stone-free probability.
About This Calculator
The Kidney Stone STONE Score Calculator uses the validated S.T.O.N.E. nephrolithometry scoring system to predict the probability of a stone-free outcome after the first percutaneous nephrolithotomy (PCNL). This tool is designed for urologists, radiologists, and healthcare professionals managing patients with kidney stones who are candidates for surgical stone removal.
The STONE scoring system evaluates five parameters on CT imaging: Stone size (length × width in mm²), Tract length (skin-to-stone distance in mm), Obstruction (hydronephrosis grade), Number of calyces with calculi, and Essence/stone density (Hounsfield units). Each parameter is scored from 1–3 points (density 1–2), giving a total score ranging from 5 to 14. Higher scores indicate more complex stones and lower stone-free probability after a single PCNL procedure.
About the STONE Scoring System
First described by Okhunov et al. in 2013 (Urology, Vol. 81, Issue 6) and externally validated by Akhavein et al. in 2015 (Urology, Vol. 85, Issue 1), the STONE nephrolithometry score provides a standardized, reproducible way to communicate stone complexity and predict surgical outcomes. The system converts qualitative CT findings into a quantitative score that correlates with stone-free rates, operating time, and complication risk. Stones scoring 5–6 have approximately 91% probability of being stone-free after the first PCNL, while scores of 13–14 drop to approximately 20%.
Regional Notes
Global: The STONE scoring system is used in urology practices worldwide. CT scan parameters and Hounsfield unit measurements are standardized internationally. PCNL is the standard of care for large renal calculi (>1.5–2 cm) across all regions.
India: Kidney stone disease affects approximately 12% of the Indian population, with higher prevalence in the northern and western regions. PCNL is widely performed in major urology centers across India. Standard CT imaging protocols (120 kVp, non-contrast) apply.
United States: Kidney stones affect about 1 in 11 Americans. PCNL is the recommended first-line treatment for staghorn calculi and large renal stones per AUA guidelines. CT KUB (kidney, ureter, bladder) without contrast is the standard imaging modality with typical radiation exposure of 4–7 mSv.
United Kingdom: NICE guidelines recommend PCNL for renal stones >20 mm or lower pole stones >10 mm with unfavorable anatomy. Non-contrast CT is the preferred imaging modality for stone assessment. The STONE scoring system is increasingly incorporated into preoperative planning.
Frequently Asked Questions
What is the STONE nephrolithometry score?
The STONE scoring system (Stone size, Tract length, Obstruction, Number of calyces, Essence/density) predicts the probability of a stone-free outcome after the first percutaneous nephrolithotomy (PCNL). It was first proposed by Okhunov et al. in 2013 and validated by Akhavein et al. in 2015.
How is the stone-free probability calculated?
Each of the five STONE criteria is scored from 1–3 points (density 1–2 points). The total score ranges from 5–14. Higher scores indicate more complex stones with lower stone-free rates: scores 5–6 have ~91% probability, 7–8 have ~81%, 9–10 have ~72%, 11–12 have ~50%, and 13–14 have ~20% probability of being stone-free after the first PCNL.
What inputs do I need for this calculator?
You need the patient's CT scan results: stone length and width in mm (to calculate stone size in mm²), tract length from skin to stone in mm, hydronephrosis grade (none, mild, moderate, severe), number of calyces with calculi, and stone density in Hounsfield units (HU).
What is a good STONE score?
Lower STONE scores (5–6) indicate less complex stones with a high probability (~91%) of being stone-free after the first PCNL. Higher scores (13–14) indicate very complex stones with only ~20% stone-free probability and may require additional procedures.
How do I calculate kidney stone size?
Kidney stone size is calculated as the product of stone length and width in millimeters (mm): size = length × width. For example, a stone measuring 15 mm × 10 mm has a size of 150 mm². This is measured from the CT scan image.
Can this calculator be used for all kidney stone types?
The STONE scoring system is specifically designed for percutaneous nephrolithotomy (PCNL) candidates. It is most accurate for large stones (>1.5 cm) or stones where other less-invasive methods have failed. It should not replace comprehensive clinical assessment by a urologist.
How accurate is the STONE scoring system?
The STONE scoring system was validated in two cohorts with area under the receiver operating characteristic curve (AUC) values of 0.70–0.79, indicating good predictive accuracy. Components include stone size, tract length, obstruction grade, number of involved calyces, and stone density on CT.
What are the risk factors for kidney stone recurrence?
Risk factors include personal or family history of kidney stones, anatomical urinary tract abnormalities, frequent UTIs, sedentary lifestyle, obesity, diet high in animal protein or sodium, hyperparathyroidism, gout, and certain medications like diuretics. Adequate hydration and dietary modifications can reduce recurrence risk.