What laboratory tests are recommended for a patient suspected of having kidney stones?

Laboratory Tests for Suspected Kidney Stones

For patients suspected of having kidney stones, the recommended laboratory tests include urinalysis, serum chemistries, and imaging studies, with additional metabolic testing for high-risk patients.

Initial Laboratory Evaluation

• Urinalysis with both dipstick and microscopic evaluation should be performed to assess urine pH, indicators of infection, and identify crystals that may indicate stone type 1
• Serum chemistries including electrolytes, calcium, creatinine, and uric acid to identify underlying medical conditions associated with stone formation 1
• Complete blood count (CBC) if there is a significant risk of hemorrhage or symptoms suggesting anemia, thrombocytopenia, or infection 2
• Urine culture if urinary tract infection is suspected based on urinalysis or clinical findings 2, 1
• Non-contrast CT scan is the standard imaging modality for suspected kidney stones after ultrasound, providing information on stone location, burden, density, and anatomy 2, 1

Comprehensive Metabolic Evaluation

• 24-hour urine collection is recommended for high-risk or interested first-time stone formers and all recurrent stone formers 2, 1
• The 24-hour urine should analyze at minimum:
  • Total urine volume
  • Urine pH
  • Calcium
  • Oxalate
  • Uric acid
  • Citrate
  • Sodium
  • Potassium
  • Creatinine 2, 1
• Either one or two 24-hour urine collections may be obtained, though two collections are preferred by experts 2
• For patients with suspected cystine stones or family history of cystinuria, urinary cystine should additionally be measured 2
• Primary hyperoxaluria should be suspected when urinary oxalate excretion exceeds 75 mg/day in adults without bowel dysfunction 2

Imaging Studies

• Ultrasound (US) is the primary diagnostic tool with 45% sensitivity and 94% specificity for ureteral stones 2
• KUB radiography (kidney-ureter-bladder) helps differentiate between radiopaque and radiolucent stone types and aids in follow-up 2
• Non-contrast CT is the standard modality after ultrasound with high diagnostic accuracy (93.1% sensitivity, 96.6% specificity) 2
• Low-dose CT reduces radiation risks while maintaining high diagnostic accuracy 2

Special Considerations

• Stone analysis (when stone is available) is crucial for determining composition and guiding preventive measures 1
• All first-time stone formers should have stone analysis performed when possible 2
• For patients with persistent microscopic hematuria, additional testing may include cystoscopy and imaging to assess for urinary tract abnormalities 1
• Microscopic hematuria is strongly associated with urinary stone disease with an odds ratio of 67.7 3

Clinical Pearls and Pitfalls

• Calcium stones are the most common type (79%), followed by uric acid stones (16.5%) 4
• Nephrocalcinosis on imaging suggests underlying metabolic disorders (e.g., renal tubular acidosis, primary hyperparathyroidism) 1
• Multiple or bilateral renal calculi at initial presentation may place a stone former at greater risk of recurrence 2
• Medications such as protease inhibitors, antibiotics, and some diuretics can increase the risk of kidney stone formation 5
• CBC parameters including increased RDW, MPV, PDW, WBC, and granulocyte percentage may be associated with urinary stone disease 3

By following this comprehensive laboratory evaluation approach, clinicians can effectively diagnose kidney stones and identify underlying metabolic abnormalities that contribute to stone formation.