What is the STONE Score?
The STONE Nephrolithometry Scoring System is a validated clinical tool developed by Okhunov et al. in 2013 to predict stone-free rates after percutaneous nephrolithotomy (PCNL). The acronym STONE stands for the five CT-based variables that comprise the scoring system: Stone size (S), Tract length (T), Obstruction degree (O), Number of involved calyces (N), and Essence or stone density (E).
The STONE score ranges from 5 (minimum, lowest complexity) to 13 (maximum, highest complexity). It was designed to provide a standardized, reproducible method for urologists to assess kidney stone complexity before surgery, predict outcomes, and communicate effectively about case difficulty. Prior to the development of such scoring systems, stone complexity assessments were largely subjective and inconsistent between practitioners.
The scoring system was validated in a multicenter study involving over 400 patients undergoing PCNL at multiple institutions. The study demonstrated a strong correlation between the STONE score and stone-free rates, complication rates, and operative times. Higher STONE scores were associated with lower stone-free rates and higher complication rates, confirming the system's predictive value.
PCNL Procedure Explained
Percutaneous nephrolithotomy (PCNL) is the preferred surgical treatment for large kidney stones (typically >2 cm), staghorn calculi, and stones that have failed other treatment modalities such as shock wave lithotripsy (SWL) or ureteroscopy. The procedure involves creating a small tract through the skin and kidney tissue to directly access and remove the stone.
During PCNL, the patient is placed under general anesthesia. A needle is inserted through the back into the kidney under fluoroscopic or ultrasound guidance. The tract is then dilated to allow passage of a nephroscope — a specialized instrument with a camera and working channel. The stone is visualized and then fragmented using ultrasonic, pneumatic, or laser lithotripsy before the fragments are extracted.
Standard PCNL uses a 24–30 French tract (approximately 8–10 mm in diameter), while mini-PCNL uses a smaller 14–20 French tract. The choice of tract size depends on stone burden, surgeon preference, and patient anatomy. Stone-free rates for PCNL range from 64% to 95% depending on stone complexity, making the STONE score an important tool for preoperative counseling and planning.
Scoring Components in Detail
| Component | Criteria | Score |
|---|---|---|
| S – Stone Size | < 400 mm² | 1 |
| 400 – 799 mm² | 2 | |
| 800 – 1599 mm² | 3 | |
| ≥ 1600 mm² | 4 | |
| T – Tract Length | ≤ 100 mm | 1 |
| > 100 mm | 2 | |
| O – Obstruction | No hydronephrosis | 1 |
| Mild hydronephrosis | 2 | |
| Moderate to severe hydronephrosis | 3 | |
| N – Number of Calyces | 1 calyx involved | 1 |
| 2–3 calyces involved | 2 | |
| Staghorn calculus | 3 | |
| E – Essence (Density) | > 950 HU | 1 |
| ≤ 950 HU | 2 |
Stone Size is measured as the total surface area on CT scan. Larger stones require more fragmentation and extraction time, reducing stone-free rates. Tract Length is the distance from the skin surface to the stone, measured on CT. Longer tracts (common in obese patients) increase procedural difficulty and complication risk.
Obstruction is assessed by the degree of hydronephrosis (kidney swelling). Paradoxically, mild hydronephrosis can actually facilitate PCNL by dilating the collecting system, while severe obstruction indicates more advanced disease. Number of Calyces with stones reflects stone distribution — stones spread across multiple calyces may require multiple access tracts or flexible nephroscopy. Essence refers to stone density on CT in Hounsfield Units (HU); lower density stones (≤950 HU) may be harder to fragment with certain lithotripsy modalities.
Score Interpretation
| STONE Score | Complexity | Stone-Free Rate | Clinical Implication |
|---|---|---|---|
| 5 – 6 | Low | ~90% | Standard single-tract PCNL likely sufficient |
| 7 – 8 | Moderate | ~83% | May need flexible nephroscopy; consider staged procedure |
| 9 – 13 | High | ~64% | Multi-tract access may be needed; higher complication risk |
It is important to note that stone-free rates represent the likelihood of complete stone clearance after a single PCNL session. Residual fragments may require secondary procedures, including second-look nephroscopy, SWL, or flexible ureteroscopy. The STONE score helps set realistic expectations for both clinicians and patients regarding surgical outcomes.
Types of Kidney Stones
Kidney stones vary in composition, and understanding stone type is important for both treatment and prevention:
- Calcium Oxalate (70–80%): The most common type. Can be monohydrate (whewellite, very hard, >1200 HU on CT) or dihydrate (weddellite, moderately hard). Associated with hyperoxaluria, hypercalciuria, and low urine volume.
- Calcium Phosphate (10–15%): Often mixed with calcium oxalate. Includes hydroxyapatite and brushite (the latter being extremely hard and resistant to fragmentation). Associated with renal tubular acidosis and hyperparathyroidism.
- Uric Acid (5–10%): Radiolucent on plain X-ray but visible on CT. Form in acidic urine (pH <5.5). Can often be dissolved with urine alkalinization (potassium citrate), making medical dissolution possible.
- Struvite (5–10%): Also called infection stones or triple phosphate. Form in the presence of urease-producing bacteria (Proteus, Klebsiella). Often become large staghorn calculi. Require complete surgical removal plus antibiotic treatment.
- Cystine (1–2%): Result from a genetic disorder (cystinuria) causing excessive cystine excretion. Tend to recur frequently and may require lifelong preventive therapy.
Stone Prevention Strategies
Since kidney stones have a 50% recurrence rate within 5–10 years, prevention is crucial:
- Hydration: Drink enough fluid to produce at least 2.5 liters of urine per day. Water is the best choice. This is the single most important preventive measure for all stone types.
- Dietary Calcium: Maintain normal calcium intake (1,000–1,200 mg/day from food). Contrary to popular belief, adequate dietary calcium actually reduces stone risk by binding oxalate in the gut.
- Limit Sodium: High sodium intake increases urinary calcium excretion. Aim for less than 2,300 mg of sodium per day.
- Moderate Protein: Excessive animal protein increases uric acid production and decreases urinary citrate. Limit to 0.8–1.0 g/kg/day.
- Citrate Supplementation: Potassium citrate inhibits calcium stone formation and alkalinizes urine for uric acid stone prevention.
- Limit Oxalate: Reduce intake of high-oxalate foods (spinach, rhubarb, beets, nuts, chocolate) for calcium oxalate stone formers.
- 24-Hour Urine Analysis: This metabolic evaluation identifies specific risk factors (hypercalciuria, hyperoxaluria, hypocitraturia, etc.) to guide targeted therapy.
When is Surgery Needed?
Not all kidney stones require surgical intervention. Treatment decisions depend on stone size, location, composition, symptoms, and the presence of complications:
- Observation: Small, asymptomatic stones (<5 mm) may pass spontaneously. Medical expulsive therapy (alpha-blockers like tamsulosin) can facilitate passage of stones 5–10 mm.
- SWL (Shock Wave Lithotripsy): Recommended for renal stones <2 cm that are not in the lower pole. Non-invasive but stone-free rates are lower than PCNL for larger stones.
- Ureteroscopy (URS): Flexible ureteroscopy with laser lithotripsy is effective for stones up to 1.5–2 cm and all ureteral stones. No skin incision required.
- PCNL: Preferred for stones >2 cm, staghorn calculi, and complex lower pole stones. Highest stone-free rates for large stone burdens.
- Emergency Surgery: Required for obstructing stones with infection (obstructive pyelonephritis), complete bilateral obstruction, or obstruction of a solitary kidney.
Frequently Asked Questions
What is a good STONE score?
A STONE score of 5–6 indicates low complexity with an expected stone-free rate of approximately 90% after a single PCNL session. This is the most favorable outcome category. Scores of 7–8 (moderate) and 9–13 (high) indicate progressively more complex cases with lower expected stone-free rates.
Does a high STONE score mean surgery should not be done?
No. A high STONE score does not contraindicate surgery. Rather, it helps set realistic expectations about outcomes and guides surgical planning. High-complexity cases may benefit from staged procedures, multiple access tracts, or combined approaches (PCNL plus flexible ureteroscopy) to maximize stone clearance.
How is stone size measured for the STONE score?
Stone size is measured as the total surface area on non-contrast CT scan. This is calculated by summing the cross-sectional areas of all stone fragments on the slice where each appears largest. Some centers use 3D reconstruction for more accurate volumetric measurements. The surface area cutoffs are <400, 400–799, 800–1599, and ≥1600 mm².
Can kidney stones dissolve on their own?
Only uric acid stones can potentially be dissolved through medical therapy (urine alkalinization with potassium citrate to achieve urine pH of 6.5–7.0). Calcium-based stones, struvite stones, and cystine stones cannot be dissolved and require either passage or surgical removal. This is why stone composition analysis after treatment is important for guiding long-term prevention strategies.
What is the recovery time after PCNL?
Most patients are hospitalized for 1–3 days after standard PCNL. A nephrostomy tube is typically left in place for 1–2 days for drainage. Most patients return to sedentary work within 1–2 weeks and full physical activity within 4–6 weeks. Complications occur in approximately 20% of cases, most commonly fever, bleeding, and residual stone fragments requiring secondary procedures.