Laboratory Testing for Kidney Stones
All patients with kidney stones should undergo basic metabolic evaluation including urinalysis with microscopy, and high-risk or recurrent stone formers require comprehensive 24-hour urine collection analyzing volume, pH, calcium, oxalate, uric acid, citrate, sodium, potassium, and creatinine. 1, 2
Initial Laboratory Workup
Blood Tests
- Complete blood count (CBC) and C-reactive protein (CRP) are required for all emergency presentations to assess for infection and systemic inflammation 2
- Basic metabolic panel including serum creatinine, blood urea nitrogen, electrolytes (Na+, K+, Ca2+, Mg2+, Cl−), and bicarbonate to evaluate renal function and metabolic abnormalities 1
- Blood coagulation studies can be omitted if no intervention is planned 2
Urinalysis
- Dipstick and microscopy to detect hematuria, proteinuria, pyuria, crystals, and casts 1
- Urine culture if infection is suspected 1
- Urine pH measurement is critical as it guides stone type prediction (acidic pH suggests uric acid stones, alkaline pH suggests calcium phosphate or struvite stones) 1, 3
Metabolic Evaluation for High-Risk Patients
Who Requires Metabolic Testing
Metabolic testing with 24-hour urine collection is mandatory for: 1, 2, 4
- All recurrent stone formers (≥2 stone episodes)
- High-risk first-time stone formers with:
- Family history of stones
- Solitary kidney
- Malabsorption or intestinal disease
- Multiple or bilateral stones
- Nephrocalcinosis
- Young age at first stone (<25 years)
24-Hour Urine Collection Parameters
The comprehensive metabolic panel should measure: 1, 2, 4
- Total urine volume (goal ≥2.5 liters/day)
- Urine pH
- Calcium (hypercalciuria is the most common metabolic abnormality, present in 43% of men and 31% of women) 5, 6
- Oxalate (hyperoxaluria present in 17% of men and 7% of women) 6
- Uric acid (hyperuricosuria present in 22% of men and 10% of women) 6
- Citrate (hypocitraturia is protective when low)
- Sodium (high sodium increases calcium excretion)
- Potassium
- Creatinine (to verify adequate collection)
Two 24-hour collections are preferred over one for accuracy 1, 2
Additional Specialized Testing
- Urinary cystine should be measured when cystine stones are suspected, family history of cystinuria exists, or hexagonal crystals are seen on microscopy 1
- Primary hyperoxaluria should be suspected when urinary oxalate exceeds 75 mg/day in adults without bowel dysfunction 1, 2
- Phosphate excretion (TmPO4/GFR) may identify phosphaturia, present in 22% of men and 8% of women with normal parathyroid hormone 6
Stone Analysis
Stone material should be sent for physicochemical and crystallographic analysis whenever available to guide prevention strategies 2, 5, 4. This is particularly critical for:
- All first-time stone formers
- Patients not responding to treatment
- When stone composition is unknown
Common stone compositions: 5
- Calcium oxalate and phosphate: 79%
- Uric acid: 16.5%
- Mixed calcium salts and uric acid: 2%
- Cystine: 0.6%
Key Metabolic Diagnoses by Stone Type
Calcium Stones
Major risk factors identified through metabolic testing: 5
- Idiopathic hypercalciuria (most common)
- Unduly acidic urine pH
- Hyperuricosuria
- Hypocitraturia
Uric Acid Stones
Primary metabolic abnormalities: 5, 3
- Unduly acidic urine pH (most important factor)
- Hyperuricosuria (less common)
Important Caveats
- Timing of collection: Obtain 24-hour urine on random diet to reflect actual stone-forming risk, not during acute stone episode 1
- Hematuria workup: If microscopic hematuria is present, rule out menstruation, urinary tract infection, and stones before considering more invasive evaluation including possible renal biopsy for glomerular disease 1
- Proteinuria: If present (>2+ by dipstick), confirm with 24-hour collection and consider glomerular source 1
- Age and sex differences: Metabolic parameters vary significantly by age and sex, with calcium, oxalate, and uric acid excretion higher in men, and women having more frequent urinary infections and structural abnormalities 6