What labs should be ordered for a patient presenting with kidney stones?

Laboratory Evaluation for Kidney Stones

For patients presenting with kidney stones, a comprehensive metabolic evaluation should include serum chemistries (electrolytes, calcium, creatinine, uric acid), urinalysis with microscopic examination, and stone analysis when available. 1

Initial Laboratory Evaluation

Basic Laboratory Tests

• Serum chemistries including electrolytes, calcium, creatinine, and uric acid to identify underlying medical conditions associated with stone disease 1
• Urinalysis with both dipstick and microscopic evaluation to assess urine pH, indicators of infection, and identify crystals that may indicate stone type 1
• Urine culture if urinalysis suggests infection or if patient has history of recurrent UTIs 1
• Stone analysis (when stone is available) to determine composition, which is crucial for guiding preventive measures 1

Additional Testing Based on Clinical Suspicion

• Serum intact parathyroid hormone level if primary hyperparathyroidism is suspected (particularly when serum calcium is high or high-normal) 1
• Complete blood count (CBC) and platelet count if there's significant risk of hemorrhage or symptoms suggesting anemia, thrombocytopenia, or infection 1

Comprehensive Metabolic Evaluation

24-Hour Urine Collection

• Recommended for high-risk or interested first-time stone formers and all recurrent stone formers 1
• Should analyze at minimum:
  • Total urine volume
  • Urine pH
  • Calcium
  • Oxalate
  • Uric acid
  • Citrate
  • Sodium
  • Potassium
  • Creatinine 1
• One or two collections may be obtained, though two collections are preferred 1
• Additional parameters to consider:
  • Urinary cystine in patients with known cystine stones, family history of cystinuria, or suspected cystinuria 1
  • Primary hyperoxaluria should be suspected when urinary oxalate exceeds 75 mg/day in adults without bowel dysfunction 1

Timing of Metabolic Evaluation

• Initial 24-hour urine collection should be obtained before starting preventive therapy 2
• Follow-up 24-hour urine should be obtained within six months of initiating treatment to assess response 1
• Annual 24-hour urine testing thereafter (or more frequently depending on stone activity) 1

Special Considerations

Imaging

• Review available imaging studies to quantify stone burden 1
• Non-contrast CT scan is recommended for comprehensive evaluation 1
• Imaging can identify multiple or bilateral calculi which may indicate higher risk of recurrence 1
• Nephrocalcinosis on imaging suggests underlying metabolic disorders (e.g., renal tubular acidosis, primary hyperparathyroidism) 1

Persistent Hematuria

• For patients with persistent microscopic hematuria, additional testing may include:
  • Cystoscopy and imaging to assess for urinary tract abnormalities or new stone formation
  • 24-hour urine stone panel to assess for nephrolithiasis/microlithiasis 1

Clinical Pearls and Pitfalls

• Metabolic abnormalities are present in approximately 90% of patients with kidney stones, with hyperoxaluria, hypercalciuria, and hypocitraturia being most common 3
• Multiple metabolic abnormalities often coexist in stone formers (78.5% of patients) 3
• A single 24-hour urine collection may be sufficient for initial metabolic evaluation, as studies show no significant differences between two collections performed within 3 days 4
• Repeat stone analysis is justified in patients not responding to treatment, as stone composition may change over time 1
• Patients with calcium oxalate stones (the most common type at 61%) should be evaluated for hypercalciuria, hyperoxaluria, and hypocitraturia 5