Determining Kidney Stone Composition and Guiding Treatment
Stone analysis should be performed at least once when a stone is available, as stone composition directly guides preventive measures and treatment strategies, with 24-hour urine metabolic testing recommended for high-risk or recurrent stone formers to identify modifiable risk factors. 1
Stone Analysis: The Gold Standard
When a stone is available, obtain stone analysis at least once to determine composition. 1 Stone composition reveals specific metabolic or genetic abnormalities:
- Calcium oxalate stones (most common at 61-79%) suggest hypercalciuria, hyperoxaluria, or hypocitraturia 2, 3
- Uric acid stones (12-16.5%) indicate unduly acidic urine pH and may benefit from dissolution therapy 2, 4
- Cystine stones implicate genetic cystinuria requiring specific management 1
- Struvite stones indicate infection with urease-producing organisms 1
- Calcium phosphate stones suggest renal tubular acidosis or primary hyperparathyroidism 1
Repeat stone analysis is justified in patients not responding to treatment, as stone composition can change over time (e.g., calcium oxalate formers converting to calcium phosphate). 1
Initial Laboratory Evaluation
Perform these tests for all stone formers:
Serum Studies
- Electrolytes, calcium, creatinine, and uric acid to identify underlying medical conditions 1, 5
- Serum intact parathyroid hormone if primary hyperparathyroidism is suspected (when serum calcium is high or high-normal) 1, 5
Urinalysis
- Both dipstick and microscopic evaluation to assess urine pH, detect infection indicators, and identify pathognomonic crystals 1, 5
- Urine culture if urinalysis suggests infection or in patients with recurrent UTIs 1, 5
24-Hour Urine Metabolic Testing
Perform additional metabolic testing in high-risk or interested first-time stone formers and all recurrent stone formers. 1 High-risk features include:
- Multiple or bilateral renal calculi 1
- Nephrocalcinosis on imaging 1
- Young age at presentation 1
- Family history of stones 3
- Solitary kidney 1
Components of 24-Hour Urine Collection
Obtain one or two 24-hour urine collections (two preferred) analyzed for: 1, 5
- Total urine volume
- Urine pH
- Calcium
- Oxalate
- Uric acid
- Citrate
- Sodium
- Potassium
- Creatinine
Additional measurements:
- Urinary cystine in patients with known cystine stones, family history of cystinuria, or suspected cystinuria 1, 5
- Suspect primary hyperoxaluria when urinary oxalate exceeds 75 mg/day in adults without bowel dysfunction 1
Predictive Value of Urine Studies
Urinary supersaturation correlates with stone composition, allowing 24-hour urine testing to inform and monitor treatment protocols. 1, 6 Research demonstrates that parameters including BMI, urinary sodium, calcium, oxalate, and uric acid can predict stone type with 85.5% accuracy, helping distinguish uric acid from calcium oxalate stones. 7
Imaging to Quantify Stone Burden
Review imaging studies to quantify stone burden. 1, 5
- Ultrasound as primary diagnostic tool (45% sensitivity for ureteral stones, 94% specificity) 1, 5
- Non-contrast CT as standard follow-up for detailed assessment of stone location, burden, density, and anatomy (93.1% sensitivity, 96.6% specificity) 1, 5
- Low-dose CT reduces radiation while maintaining diagnostic accuracy 1, 5
- KUB radiography differentiates radiopaque from radiolucent stones and aids follow-up 1, 5
Multiple or bilateral stones indicate higher recurrence risk; nephrocalcinosis suggests underlying metabolic disorders (renal tubular acidosis type 1, primary hyperparathyroidism, primary hyperoxaluria, medullary sponge kidney). 1, 5
Treatment Guidance Based on Composition
Uric Acid Stones
- Alkalinize urine with citrate supplementation or diet high in fruits and vegetables 4
- Consider dissolution therapy with urinary alkalinization 4, 7
- Target urine pH 6.5-7.0 4
Calcium Oxalate Stones
- Alkalinize urine to prevent formation 4
- Address hypercalciuria with thiazide diuretics if present 1
- Reduce dietary oxalate and increase dietary calcium 4
Calcium Phosphate and Struvite Stones
- Acidify urine with cranberry juice or betaine 4
- Struvite stones require treatment of underlying infection 1
Cystine Stones
- Alkalinize urine to pH >7.0 4
- High fluid intake (>3 L/day) 4
- Consider thiol-binding agents if conservative measures fail 1
Follow-Up Monitoring
Obtain a single 24-hour urine specimen within six months of initiating treatment to assess response to dietary and/or medical therapy. 1
After initial follow-up, obtain annual 24-hour urine specimens or more frequently depending on stone activity to assess adherence and metabolic response. 1
Obtain periodic blood testing to monitor for adverse effects in patients on pharmacological therapy (e.g., hypokalemia with thiazides, hyperkalemia with potassium citrate, liver enzyme elevation with allopurinol). 1