Kidney STONE Calculator
Predict percutaneous nephrolithotomy stone-free success using the STONE nephrolithometry scoring system.
What Is the STONE Nephrolithometry Score?
The STONE scoring system predicts the likelihood of a stone-free outcome after percutaneous nephrolithotomy (PCNL) using five CT-derived variables: stone size, tract length, hydronephrosis, calyceal involvement, and stone density. It helps urologists counsel patients before surgery and plan treatment for complex renal calculi. Related kidney assessments include the Kidney Failure Risk Calculator.
How the STONE Score Is Calculated
Stone size is measured as length multiplied by width on CT (mm²). Tract length is the skin-to-stone distance in millimeters. Obstruction reflects hydronephrosis severity. Calyceal involvement counts how many calyces contain stone material, including staghorn configurations. Stone density is measured in Hounsfield units (HU). Component points are summed and added to a base of 5, yielding a total STONE score from 5 to 13.
Interpreting STONE Results
Lower scores indicate higher stone-free probability after the first PCNL. Scores of 8 or below are low risk with expected primary stone-free rates above 90%. Scores of 9 to 10 represent medium complexity. Scores of 11 to 13 suggest high complexity where additional procedures may be needed. Always interpret STONE alongside surgeon experience, stone composition, and patient comorbidities.
Frequently Asked Questions
What CT measurements are needed for STONE scoring?
You need stone length and width (mm), tract length from skin to stone center (mm), hydronephrosis grade, number of calyces involved, and stone density in Hounsfield units from a non-contrast CT scan.
What is a staghorn kidney stone?
A staghorn stone is a large calculus that occupies multiple branches of the renal collecting system, often filling much of the renal pelvis and calyces.
What STONE score is considered low risk?
A STONE score of 8 or below is classified as low risk, with an estimated stone-free probability of about 90% or higher after the first PCNL procedure.
Why is stone density measured in Hounsfield units?
Hounsfield units quantify radiodensity on CT. Denser stones above 950 HU are harder to fragment and correlate with lower stone-free rates after PCNL.
Does STONE replace clinical judgment?
No. STONE is a prognostic aid. Final treatment decisions should combine imaging, patient factors, institutional expertise, and shared decision-making with the surgical team.
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