About the Kidney STONE Calculator for Percutaneous Neprolithotomy
The S.T.O.N.E. nephrolithometry score, published by Okhunov and colleagues in 2013, is a preoperative CT-based scoring system used to predict how technically demanding a percutaneous nephrolithotomy (PCNL) will be. It converts five measurable features of a kidney stone into a single score from 5 to 13, letting surgeons compare case complexity on a consistent scale before they enter the operating room.
How the score is built
- S - Size: stone surface area, estimated as length x width x 0.785 (an ellipse-area approximation), scores 1 point below 400 mm-squared, 2 points from 400 to 800 mm-squared, and 3 points above 800 mm-squared.
- T - Tract length: the skin-to-stone distance measured on CT scores 1 point at 10 cm or less and 2 points beyond 10 cm.
- O - Obstruction: the degree of hydronephrosis scores 1 point for none or mild and 2 points for moderate or severe.
- N - Number of calices: the count of calices involved by the stone scores 1, 2, or 3 points for one, two, or three-or-more (including staghorn) calices.
- E - Essence: stone density on non-contrast CT scores 1 point below 950 Hounsfield units and 2 points at or above 950 Hounsfield units.
Adding the five component scores gives the total STONE score, which can range from 5 (smallest, simplest stones) to 13 (largest, densest, most obstructive, most anatomically complex stones).
Putting the result in context
Higher STONE scores are associated with longer operative times, more percutaneous access tracts, and a lower likelihood of clearing the stone completely in a single PCNL session. The score describes surgical complexity, not stone danger or urgency - a small, easily accessible stone with a low score can still cause severe pain, and a high-scoring stone is not automatically an emergency.
When to consult a professional
This tool performs the standard STONE nephrolithometry arithmetic for education and surgical planning. It does not diagnose, does not select a treatment approach, and does not replace the CT read or clinical judgment of the treating urologist, who weighs the score alongside symptoms, kidney function, stone composition, and patient factors.