Kidney Stone Metabolic Workup
All kidney stone patients should receive a screening evaluation consisting of detailed medical and dietary history, serum chemistries (electrolytes, calcium, creatinine, uric acid), and urinalysis with microscopy, while comprehensive 24-hour urine metabolic testing should be performed in all recurrent stone formers and high-risk first-time stone formers. 1, 2
Initial Screening Evaluation (All Stone Formers)
Every patient with a newly diagnosed kidney or ureteral stone requires a basic screening evaluation that includes the following components 1:
Medical and Dietary History
- Query for stone-provoking medications and supplements that may predispose to stone disease 1
- Assess daily fluid intake including specific types and amounts of beverages consumed 1
- Evaluate protein consumption including types (animal vs. plant-based) and quantities 1
- Document calcium intake to identify consumption below or significantly above recommended dietary allowance 1
- Assess sodium intake as high sodium increases stone risk 1
- Evaluate intake of high-oxalate foods (nuts, dark leafy greens, chocolate, tea) 1
- Document fruit and vegetable consumption as limited intake increases risk 1
- Screen for underlying medical conditions including obesity, hypertension, diabetes, and metabolic syndrome which are associated with increased stone risk 1, 3
Serum Chemistry Studies
- Obtain electrolytes, calcium, creatinine, and uric acid to identify underlying metabolic conditions 1, 4
- Measure serum intact parathyroid hormone (PTH) if serum calcium is high or high-normal to evaluate for primary hyperparathyroidism 2, 4
Urinalysis
- Perform both dipstick and microscopic evaluation to assess urine pH, detect infection indicators, and identify pathognomonic crystals 1, 2
- Obtain urine culture if urinalysis suggests infection or in patients with recurrent UTIs 1, 2
Stone Analysis
- Perform stone analysis at least once when available to determine composition and guide preventive measures 2, 4
- Consider repeat stone analysis in patients not responding to treatment, as composition can change over time 4
Imaging Review
- Review imaging studies to quantify stone burden as multiple or bilateral renal calculi indicate higher recurrence risk 2, 4
- Identify nephrocalcinosis which suggests underlying metabolic disorders such as renal tubular acidosis or primary hyperparathyroidism 2, 4
Comprehensive 24-Hour Urine Metabolic Testing
Perform comprehensive metabolic testing in the following patient populations 2, 4, 5:
Indications for 24-Hour Urine Testing
- All recurrent stone formers (defined as more than one stone episode) 2, 4
- High-risk first-time stone formers with any of the following features 2, 4:
- Multiple or bilateral renal calculi
- Nephrocalcinosis on imaging
- Young age at presentation (≤25 years)
- Solitary kidney
- Strong family history of stones
- Stones composed of uric acid, cystine, or struvite
- Interested first-time stone formers motivated to prevent recurrence 1, 4
Components of 24-Hour Urine Analysis
Collect one or two 24-hour urine specimens and analyze for 2, 4:
- Total urine volume
- Urine pH
- Calcium
- Oxalate
- Uric acid
- Citrate
- Sodium
- Potassium
- Creatinine (to verify adequacy of collection)
Special Considerations
- Measure urinary cystine in patients with known or suspected cystine stones 2
- Note that patients with severe renal tubular acidosis or chronic diarrheal syndrome may have very low urinary citrate (<100 mg/day), making potassium citrate relatively ineffective and requiring higher doses 6
Follow-Up Metabolic Monitoring
After initiating dietary or pharmacologic therapy 4:
Initial Follow-Up
- Obtain a single 24-hour urine specimen within 6 months of starting treatment to assess response 4
Long-Term Monitoring
- Obtain annual 24-hour urine specimens after initial follow-up to assess adherence and metabolic response 4
- Perform periodic blood testing to monitor for adverse effects in patients on pharmacological therapy 4
Common Pitfalls to Avoid
- Failing to obtain stone analysis when available is a critical missed opportunity to guide prevention strategies 5
- Inadequate metabolic evaluation in high-risk or recurrent stone formers leads to preventable recurrences 5
- Neglecting to assess for systemic conditions (obesity, hypertension, diabetes, metabolic syndrome) misses important modifiable risk factors 5
- Delaying imaging in patients with suspected obstruction, fever, or solitary kidney can lead to serious complications 5
- Incomplete stone removal when infection stones are present results in recurrent UTIs and renal damage 5
Special Populations
Genetic Testing Considerations
Consider genetic testing for patients with 2:
- Early-onset stone disease (≤25 years)
- Suspected inherited disorders (e.g., cystinuria)
- Recurrent stones despite appropriate therapy
- Bilateral disease
- Strong family history
Stone-Specific Implications
- Cystine stones implicate genetic cystinuria requiring specific management including urine alkalinization to pH >7.0, high fluid intake, and consideration of thiol-binding agents 4
- Struvite stones indicate infection with urease-producing organisms requiring treatment of underlying infection and complete stone removal 4, 5
- Calcium phosphate stones suggest renal tubular acidosis or primary hyperparathyroidism warranting further evaluation 4