Laboratory Evaluation for Kidney Stones
Order serum chemistries (electrolytes, calcium, creatinine, uric acid), urinalysis with microscopy, and obtain stone analysis when available—these constitute the essential initial laboratory workup for all kidney stone patients. 1, 2
Initial Laboratory Testing (All Patients)
Serum Studies
- Electrolytes, calcium, creatinine, and uric acid are the core serum tests that identify underlying medical conditions associated with stone disease 1, 2, 3
- Serum intact parathyroid hormone (PTH) should be added if primary hyperparathyroidism is suspected, particularly when serum calcium is high or high-normal 1
Urine Studies
- Urinalysis with both dipstick and microscopic evaluation to assess urine pH, detect infection indicators, and identify pathognomonic crystals that suggest stone type 1, 2, 3
- Urine culture is indicated if urinalysis suggests infection or if the patient has recurrent UTIs 1, 2, 3
Stone Analysis
- Stone composition analysis should be obtained at least once when stone material is available, as it directly guides preventive measures 1, 2, 3
- Stone composition revealing uric acid, cystine, or struvite implicates specific metabolic or genetic abnormalities 1
Comprehensive Metabolic Testing (High-Risk Patients)
Perform 24-hour urine collection for high-risk or interested first-time stone formers and all recurrent stone formers. 1, 2, 3
Who Qualifies as High-Risk:
- Recurrent stone formers 1, 4
- Multiple or bilateral renal calculi at initial presentation 1
- Nephrocalcinosis on imaging (suggests metabolic disorders like renal tubular acidosis, primary hyperparathyroidism, or primary hyperoxaluria) 1, 2
- Single kidney 5
- Family history of stones 5
- Malabsorption or intestinal disease 5
24-Hour Urine Collection Parameters:
The collection should analyze at minimum: 1, 2, 3
- Total urine volume
- Urine pH
- Calcium
- Oxalate
- Uric acid
- Citrate
- Sodium
- Potassium
- Creatinine
Additional 24-Hour Urine Parameters:
- Urinary cystine should be measured in patients with known cystine stones, family history of cystinuria, or suspected cystinuria 1, 2
- Primary hyperoxaluria should be suspected when urinary oxalate exceeds 75 mg/day in adults without bowel dysfunction 1, 2
Collection Frequency:
- One or two 24-hour collections may be obtained, though two collections are preferred by the AUA 1, 2
- However, research demonstrates that a single 24-hour collection is sufficient, as there is no significant difference in urinary parameters between collections obtained within 3 days of each other 6
Common Pitfalls and Caveats
- Don't skip stone analysis: Stone composition provides critical information about the urinary environment during stone formation and directly guides prevention strategies 4
- Repeat stone analysis is justified in patients not responding to treatment, as stone composition may change over time (e.g., calcium oxalate formers converting to calcium phosphate) 1, 2
- Don't forget imaging review: Quantifying stone burden through available imaging studies is essential, as multiple or bilateral calculi indicate higher recurrence risk 1, 2
- The majority of stones are calcium oxalate (61%), followed by calcium phosphate (15%) and uric acid (12%), which helps guide your clinical suspicion 5, 7