The Cardiovascular
ToolsCalculators

S.T.O.N.E. nephrolithometry score

CT-based scoring of renal calculus complexity before percutaneous nephrolithotomy (PCNL).

S.T.O.N.E. nephrolithometry score
Stone size (greatest cross-sectional surface area on NCCT)
Surface area measured on axial non-contrast CT (length x width, or summed for staghorn/multiple stones).
Tract length (skin-to-stone distance)
Measured perpendicular from skin to the stone on axial CT at the planned access site.
Obstruction (degree of hydronephrosis/hydrocalycosis)
Number of calices with calculi
Essence (mean stone density)
Mean Hounsfield units of the stone on non-contrast CT.
Result5 points

Low surgical complexity (range 5–13). The original derivation study reported the S.T.O.N.E. score as a continuous predictor: higher scores correlated with longer operative time, greater blood loss, longer hospital stay, and a lower chance of being stone-free after a single percutaneous procedure. The low/moderate/high complexity bands shown here follow the categorization used in subsequent validation literature (Srivastava et al. 2020), not a threshold defined in the original 2013 paper.

Total S.T.O.N.E. score
5
Complexity tier
Low surgical complexity

When to use

  • Preoperative planning for percutaneous nephrolithotomy (PCNL): standardising how renal stone complexity is reported from a preoperative non-contrast CT.
  • Counselling patients on the expected likelihood of a stone-free outcome after a single PCNL session and on operative risk before consent.

Formula

Sum of 5 CT-derived components (range 5-13): Size - stone surface area <400 mm2 = 1, 400-799 mm2 = 2, 800-1599 mm2 = 3, >=1600 mm2 = 4. Tract length (skin-to-stone) <=10 cm = 1, >10 cm = 2. Obstruction - none/mild hydronephrosis = 1, moderate/severe = 2. Number of calices with calculi: 1-2 calices = 1, 3 calices = 2, full staghorn stone = 3. Essence (stone density) <=950 HU = 1, >950 HU = 2.

Pearls and pitfalls

  • All 5 components are measured on a single preoperative non-contrast CT, making the score fully reproducible before the patient reaches the operating room.
  • The 950 HU density cutoff for 'essence' approximates harder, more fragmentation-resistant stone composition (e.g. calcium oxalate monohydrate, cystine) versus softer stones.
  • The number-of-calices component is not a simple 1/2/3-calix count: 1 or 2 involved calices both score 1 point, 3 calices scores 2 points, and only a full staghorn stone (all calices) scores 3 points.
  • The original 2013 derivation study did not define discrete low/moderate/high risk categories — it validated the score as a continuous predictor of stone-free rate and perioperative outcomes. Any categorical cutoffs (as used here) come from later validation literature, not the index publication.
  • Developed and validated for PCNL planning; external validation cohorts have shown only moderate discrimination for stone-free status, so use alongside clinical judgement and surgeon experience rather than as a stand-alone decision tool.

References

  1. Okhunov Z, Friedlander JI, George AK, et al. S.T.O.N.E. nephrolithometry: novel surgical classification system for kidney calculi. Urology. 2013;81(6):1154-1159.
  2. Noureldin YA, Elkoushy MA, Andonian S. External validation of the S.T.O.N.E. nephrolithometry scoring system. Can Urol Assoc J. 2015;9(5-6):E190-E193.
  3. Srivastava A, Yadav P, Madhavan K, et al. Inter-observer variability amongst surgeons and radiologists in assessment of Guy's Stone Score and S.T.O.N.E. nephrolithometry score: a prospective evaluation. Arab J Urol. 2019;18(2):118-123.