Kidney Stone Work-Up Protocol
All patients with suspected or confirmed kidney stones should undergo a standardized initial evaluation consisting of serum chemistries (electrolytes, calcium, creatinine, uric acid), urinalysis with microscopy, and imaging—with ultrasound as first-line followed by low-dose non-contrast CT if needed—plus stone analysis when available. 1, 2
Initial Screening Evaluation (All Patients)
Detailed History
Obtain specific information about:
- Medical conditions: Primary hyperparathyroidism, inflammatory bowel disease, recurrent UTIs, diabetes, hypertension, obesity
- Medications and supplements: Stone-provoking drugs, calcium supplements, vitamin D, vitamin C
- Dietary intake: Daily fluid volume and types of beverages, protein sources and amounts, calcium intake, sodium consumption, high-oxalate foods (spinach, nuts, chocolate), fruits/vegetables 1
Laboratory Testing
Serum chemistries (mandatory for all):
- Electrolytes, calcium, creatinine, uric acid
- Add intact PTH if serum calcium is high or high-normal (suggests primary hyperparathyroidism) 1
Urinalysis (both dipstick and microscopic):
- Assess pH (low pH suggests uric acid stones; high pH suggests struvite/infection stones)
- Identify infection indicators
- Look for pathognomonic crystals (cystine hexagons, struvite coffin-lids) 1
Urine culture: Obtain if urinalysis suggests infection or patient has recurrent UTIs 1
Imaging Protocol
First-line: Ultrasound (45% sensitivity, 94% specificity for ureteral stones; 88% specificity for renal stones) 2, 3
Second-line: Low-dose non-contrast CT if ultrasound inadequate—this is the gold standard for acute flank pain, providing stone location, size, density, and anatomic details (93.1% sensitivity, 96.6% specificity) 4, 2, 5
KUB radiography: Helps differentiate radiopaque vs radiolucent stones and aids follow-up 2
Stone Analysis
Obtain stone analysis at least once when stone material is available—composition (uric acid, cystine, struvite, calcium oxalate, calcium phosphate) directs specific metabolic evaluation and preventive measures 1
Extended Metabolic Work-Up (High-Risk Patients)
Who Qualifies as High-Risk?
- Recurrent stone formers
- First-time formers who are interested/motivated
- Multiple or bilateral stones at presentation
- Nephrocalcinosis on imaging
- Solitary kidney
- Young age (≤46 years—consider genetic counseling)
- Family history of stones
- Underlying metabolic conditions 1
24-Hour Urine Collection
Perform one or two collections (two preferred) on random diet, analyzing:
- Total volume, pH
- Calcium, oxalate, uric acid, citrate
- Sodium, potassium, creatinine 1
Additional testing:
- Urinary cystine: If cystine stones known, family history of cystinuria, or suspected
- Suspect primary hyperoxaluria: If urinary oxalate >75 mg/day in adults without bowel dysfunction 1
Quantifying Stone Burden
Review imaging to assess:
- Number of stones (single vs multiple)
- Bilateral vs unilateral
- Presence of nephrocalcinosis (implies renal tubular acidosis type 1, primary hyperparathyroidism, primary hyperoxaluria, or medullary sponge kidney) 1
Multiple or bilateral stones indicate higher recurrence risk and warrant more aggressive metabolic evaluation 1
Special Populations
Pregnant Women
Imaging hierarchy: Ultrasound → MRI → low-dose CT (last resort) 2
Children
Ultrasound first-line, followed by KUB or low-dose CT only if ultrasound insufficient 2
CKD Patients
Consider inherited disorders (Dent disease, primary hyperoxaluria, APRT deficiency) especially if:
- CKD of unknown cause
- Recurrent stones
- Early age of presentation
- Family history 6
Common Pitfalls to Avoid
- Don't skip PTH testing when calcium is high-normal—primary hyperparathyroidism is easily missed
- Don't assume all stones are calcium oxalate—stone analysis changes management dramatically for uric acid (medical dissolution possible) and cystine stones (specific therapy needed)
- Don't order 24-hour urine on all first-time formers—reserve for high-risk or motivated patients; otherwise, focus on dietary counseling
- Don't forget to assess stone burden on imaging—nephrocalcinosis or multiple stones mandate metabolic work-up even in first-time presenters
- Don't use contrast-enhanced CT as initial imaging—non-contrast is superior for stone detection and avoids obscuring stones with enhancing parenchyma 4
Key Distinction: Initial vs Extended Work-Up
The AUA guideline framework 1 clearly separates:
- Universal screening (history, serum chemistries, urinalysis, imaging, stone analysis if available) for ALL patients
- Metabolic testing (24-hour urine collections) reserved for high-risk/recurrent formers or motivated first-time formers
This tiered approach prevents over-testing while capturing patients who will benefit most from targeted dietary and pharmacologic interventions to prevent recurrence.