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 laboratory evaluation should include serum chemistries (electrolytes, calcium, creatinine, and uric acid), urinalysis with microscopic examination, and stone analysis when available. 1

Initial Laboratory Workup

• Serum chemistries including electrolytes, calcium, creatinine, and BUN should be ordered to identify underlying medical conditions and assess renal function 1, 2
• Urinalysis with both dipstick and microscopic evaluation is essential to assess urine pH, indicators of infection, and identify crystals that may indicate stone type 1, 2
• Urine culture should be collected if urinalysis suggests infection or if the patient has a history of recurrent urinary tract infections 1, 2
• Stone analysis is crucial when stone material is available, as it determines composition and guides preventive measures 1, 2

Comprehensive Metabolic Evaluation

• A 24-hour urine collection is recommended for:

  • High-risk first-time stone formers
  • Interested first-time stone formers
  • All recurrent stone formers 1, 2

• The 24-hour urine collection should analyze at minimum:

  • Total urine volume
  • Urine pH
  • Calcium
  • Oxalate
  • Uric acid
  • Citrate
  • Sodium
  • Potassium
  • Creatinine 1, 2

• Additional parameters to consider in 24-hour urine collection:

  • 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

Imaging Studies

• Non-contrast CT scan is recommended for comprehensive evaluation to quantify stone burden and identify multiple or bilateral calculi 1
• In patients with complex stones or anatomy, additional contrast imaging may be needed for better definition of the collecting system and ureteral anatomy 2

Special Considerations

• One 24-hour urine sample is typically sufficient for metabolic evaluation of recurrent stone disease, as research shows no significant difference in urinary parameters between samples collected within 3 days of each other 3
• For patients with persistent microscopic hematuria, additional testing may include cystoscopy and imaging to assess for urinary tract abnormalities or new stone formation 1
• Nephrocalcinosis on imaging suggests underlying metabolic disorders (e.g., renal tubular acidosis, primary hyperparathyroidism) 1

Clinical Pearls and Pitfalls

• Stone analysis should ideally be performed during surgery, especially when only a few fragments are available 4
• Repeat stone analysis is justified in patients not responding to treatment, as stone composition may change over time 1
• Understanding stone composition is critical as different stone types require different prevention strategies:
  • Calcium stones (oxalate and phosphate) account for approximately 79% of all stones 5
  • Uric acid stones account for about 16.5% of stones 5
  • For calcium oxalate and uric acid stones, urine should be alkalinized 6
  • For calcium phosphate and struvite stones, urine should be acidified 6
• The extensiveness and frequency of metabolic work-up should be tailored to the type of stone, severity of disease, and patient's comorbidities 4