Kidney STONE Score Calculator for PCNL
The STONE nephrolithometry score predicts the likelihood of a completely stone-free result after percutaneous nephrolithotomy (PCNL). Enter five values from the patient's preoperative CT scan - stone size, tract length, degree of hydronephrosis, number of calyces involved, and stone density - to get the total STONE score and the corresponding stone-free rate drawn from Okhunov et al. (2013). Lower scores mean a higher probability of success.
What is the STONE score?
The STONE nephrolithometry score is a five-parameter radiological scoring system designed to quantify the complexity of a kidney stone before percutaneous nephrolithotomy (PCNL). It was introduced by Okhunov and colleagues in 2013 after studying 117 patients and has since been validated in multiple independent cohorts. The acronym stands for Stone size, Tract length, Obstruction (hydronephrosis), Number of calyces involved, and Essence (stone density in Hounsfield units). Each parameter is scored from one CT scan obtained before surgery, and the five component scores are simply added together. The minimum possible score is 5 (the simplest stone) and the maximum is 13 (a large, dense, fully-branching staghorn in a deeply situated kidney). A lower score predicts a higher chance of becoming completely stone-free after a single PCNL session.
How each parameter is scored
Stone size is the product of the longest diameter multiplied by the perpendicular width, both in millimetres, measured on axial CT. Areas below 400 mm² score 1 point; 400-799 mm² score 2; 800-1599 mm² score 3; 1600 mm² or above score 4. Tract length is the shortest path from the stone centre to the skin surface measured at 0, 45, and 90 degrees, selecting the minimum; up to 100 mm scores 1, longer than 100 mm scores 2. Obstruction is graded by the degree of hydronephrosis: none or mild scores 1, moderate or severe scores 2. The number of calyceal groups containing stone material is rated 1 for one or two calyces, 2 for three calyces, and 3 for a complete staghorn. Stone density is the mean CT attenuation of the stone in Hounsfield units; 950 HU or below scores 1, above 950 HU scores 2.
Interpreting the result and planning PCNL
Scores of 5 to 7 are generally regarded as low complexity, with stone-free rates of 88-94% after a single standard PCNL. Scores of 8-10 are moderate complexity, where stone-free rates range from 42 to 83%; clinicians may consider staging the procedure or planning for an auxiliary procedure such as second-look PCNL or flexible ureteroscopy. Scores of 11 to 13 are high complexity - the predicted stone-free rate falls to around 27%, and the patient should be counselled that a multi-session approach is likely necessary. The score also helps with logistical planning: higher scores predict longer operating times, greater blood loss, and more frequent need for nephrostomy drainage after the procedure. The STONE score is one of several validated systems (alongside Guy's Stone Score and the S-ReSC score) that serve as objective complements to a surgeon's clinical judgement.
Limitations and clinical context
The STONE score is a probabilistic guide, not a guarantee. The original study sample of 117 patients was relatively small, and while subsequent validations have confirmed its predictive value, confidence intervals around the quoted stone-free rates are wide, particularly at extreme scores. The score does not capture every factor that influences PCNL outcome: patient body habitus, patient position during surgery (prone versus supine), the surgeon's experience, available equipment (mini-PCNL versus standard PCNL), and the composition of the stone (uric acid versus calcium oxalate) all contribute to the final result. The score should be used as part of a broader preoperative assessment rather than as a standalone decision tool. All results from this calculator are for educational and informational purposes; operative planning should involve a qualified urologist reviewing the full clinical picture.
STONE score interpretation table
| STONE Score | Complexity | Stone-free rate after first PCNL |
|---|---|---|
| 5 | Low | 94% |
| 6 | Low | 92% |
| 7 | Low | 88% |
| 8 | Moderate | 83% |
| 9 | Moderate | 64% |
| 10 | Moderate | 42% |
| 11-13 | High | 27% |
Stone-free rates from Okhunov et al. (2013), 117 patients. Scores of 11 and above carry the same 27% estimate.
Frequently asked questions
What does the STONE score stand for?
Each letter represents one CT parameter scored before percutaneous nephrolithotomy. S is Stone size (area in mm²), T is Tract length (distance from stone to skin), O is Obstruction degree (hydronephrosis), N is Number of calyces with stone material, and E is Essence, meaning the stone density in Hounsfield units. Adding the five component scores gives the STONE nephrolithometry score.
What is a good or bad STONE score for PCNL?
Lower is better. A score of 5 to 7 indicates a low-complexity stone with a predicted stone-free rate of 88-94% after the first procedure. Scores of 8-10 are moderate complexity (42-83% stone-free). A score of 11 or above is high complexity, with a predicted stone-free rate of around 27%, and the patient should expect that additional procedures may be necessary to achieve complete stone clearance.
How do I measure tract length for the STONE score?
On the preoperative CT scan, measure the straight-line distance from the centre of the stone to the skin surface at three positions: 0 degrees (straight posterior), 45 degrees, and 90 degrees. Record the shortest of the three measurements in millimetres. A tract length of 100 mm or below scores 1 point; longer than 100 mm scores 2 points. The shorter measurement represents the most accessible surgical approach.
What stone density threshold does the STONE score use?
The STONE score uses 950 Hounsfield units (HU) as the cutoff for stone density measured on non-contrast CT. Stones at or below 950 HU are generally softer, such as uric acid or struvite, and score 1 point. Stones above 950 HU are harder, typically calcium oxalate monohydrate or brushite, and score 2 points. Harder stones are more resistant to lithotripsy and may require more energy or multiple passes to fragment.
Is the STONE score the same as Guy's Stone Score?
No. Guy's Stone Score and the STONE nephrolithometry score are two different validated systems for predicting PCNL outcomes. Guy's Stone Score uses four grades (I to IV) based on stone number, location, and the presence of a staghorn. The STONE score uses five quantitative CT parameters and produces a numeric total from 5 to 13. Both have been externally validated and can be used alongside each other to cross-check complexity estimates.
Can I use the STONE score for mini-PCNL or ureteroscopy?
The STONE score was originally derived and validated for standard-tract PCNL. Several studies have applied it to mini-PCNL and tubeless PCNL variants with broadly consistent predictive value, but the original calibration was done for conventional PCNL. It is not designed for ureteroscopy, extracorporeal shock wave lithotripsy (ESWL), or other non-PCNL approaches, where different factors predict success.