What laboratory tests are recommended for a patient presenting with symptoms suggestive of kidney stones, particularly those with a history of kidney stones or at high risk?

Laboratory Evaluation for Kidney Stones

Initial Screening Laboratory Tests

All patients presenting with suspected kidney stones should undergo a basic laboratory evaluation consisting of serum chemistries (electrolytes, calcium, creatinine, and uric acid) and complete urinalysis with microscopy. 1, 2, 3

Serum Studies

• Electrolytes, calcium, creatinine, and uric acid are essential to identify underlying metabolic conditions associated with stone disease 1, 2, 3
• Serum intact parathyroid hormone (PTH) should be obtained when serum calcium is high or high-normal, as this suggests primary hyperparathyroidism 2, 3
• Elevated serum calcium with elevated PTH confirms primary hyperparathyroidism, a surgically correctable cause of recurrent stones 2

Urine Studies

• Urinalysis with both dipstick and microscopic examination to assess urine pH, detect infection indicators, and identify pathognomonic crystals (e.g., cystine hexagons, struvite coffin-lids) 1, 2, 3
• Urine culture is mandatory if urinalysis suggests infection or if the patient has recurrent UTIs 1, 2, 3
• Urine pH provides critical information: persistently acidic pH (<5.5) suggests uric acid stones, while alkaline pH (>7.0) suggests struvite/infection stones 1

---

Stone Analysis

Stone composition analysis should be performed at least once when stone material is available, as this directly guides preventive therapy. 2, 3, 4

• Stone analysis determines whether stones are calcium oxalate (61%), calcium phosphate (15%), uric acid (12%), struvite, or cystine 5
• Repeat stone analysis is justified in patients not responding to treatment, as composition may change over time 3
• Intraoperative stone characterization should be performed during surgical procedures, even when only fragments are available 4

---

Comprehensive Metabolic Evaluation (24-Hour Urine Collection)

A 24-hour urine collection should be obtained for all recurrent stone formers and high-risk first-time stone formers (those with family history, young age, single kidney, or multiple/bilateral stones). 1, 2, 3

Who Needs 24-Hour Urine Testing

• All recurrent stone formers (≥2 stone episodes) 1, 2, 3
• High-risk first-time stone formers, including those with:
  • Family history of stones 1, 2
  • Young age (≤25 years) 2
  • Single/solitary kidney 5
  • Multiple or bilateral stones 2, 3
  • Nephrocalcinosis on imaging 2, 3
  • Intestinal disease or malabsorption 5

Parameters to Measure in 24-Hour Urine

• Total urine volume (goal >2.5 L/day) 1, 2, 3
• Urine pH 1, 2, 3
• Calcium (hypercalciuria defined as >250 mg/day in women, >300 mg/day in men) 1, 2, 3
• Oxalate (hyperoxaluria >40-45 mg/day; >75 mg/day suggests primary hyperoxaluria) 2, 3, 6
• Uric acid (hyperuricosuria >800 mg/day in men, >750 mg/day in women) 1, 2, 3
• Citrate (hypocitraturia <320 mg/day) 1, 2, 3
• Sodium (high sodium intake increases calcium excretion) 1, 2, 3
• Potassium 1, 2, 3
• Creatinine (to verify adequacy of collection) 1, 2, 3

Special Considerations

• Urinary cystine should be measured in patients with known or suspected cystine stones, family history of cystinuria, or hexagonal crystals on urinalysis 1, 2, 3
• Two 24-hour urine collections are preferred over one to account for day-to-day variability, though one collection is acceptable 2, 3
• Primary hyperoxaluria should be suspected when urinary oxalate exceeds 75 mg/day in adults without bowel dysfunction, prompting genetic testing 3, 1

---

Imaging Studies as Part of Laboratory Evaluation

Non-contrast CT scan should be reviewed to quantify stone burden, as multiple or bilateral stones indicate higher recurrence risk and warrant more aggressive metabolic evaluation. 2, 3

• Nephrocalcinosis on imaging suggests underlying metabolic disorders such as renal tubular acidosis, primary hyperparathyroidism, or primary hyperoxaluria 2, 3
• Stone density on CT (measured in Hounsfield units) helps predict composition: uric acid stones are radiolucent (<500 HU), while calcium stones are radiopaque (>500 HU) 2

---

Common Pitfalls and Caveats

• Do not skip 24-hour urine testing in recurrent stone formers: metabolic abnormalities are found in >90% of patients with recurrent stones, and identifying these abnormalities is essential for targeted prevention 6
• Hyperoxaluria is the most common metabolic abnormality (found in 64.5% of stone formers), followed by hypercalciuria and hypocitraturia 6
• Obtain PTH only when serum calcium is elevated or high-normal: routine PTH screening in normocalcemic patients is not indicated 2, 3
• Ensure adequate urine collection: 24-hour urine creatinine should be 15-20 mg/kg in women and 20-25 mg/kg in men to verify completeness 1
• Timing matters: perform metabolic evaluation at least 4-6 weeks after acute stone episode or intervention, as acute illness and dietary changes during stone passage can alter results 4