What labs and tests are recommended for patients presenting with kidney stones?

Laboratory Evaluation for Kidney Stones

All patients newly diagnosed with kidney stones should undergo a screening evaluation consisting of serum chemistries (electrolytes, calcium, creatinine, uric acid), urinalysis with microscopic examination, and stone analysis when available. 1, 2

Initial Laboratory Testing (All Patients)

Serum Studies

• Electrolytes, calcium, creatinine, and uric acid are essential to identify underlying metabolic conditions such as hyperparathyroidism, renal tubular acidosis, or hyperuricemia that predispose to stone formation 1, 2
• Serum intact parathyroid hormone (PTH) should be obtained if primary hyperparathyroidism is suspected, particularly when serum calcium is high or high-normal 3

Urinalysis

• Both dipstick and microscopic evaluation are required to assess urine pH (which helps predict stone type), detect infection indicators, and identify pathognomonic crystals 1, 2
• Urine culture is indicated if urinalysis suggests infection or if the patient has recurrent UTIs 1, 2

Stone Analysis

• Send all available stone material for compositional analysis to determine stone type (calcium oxalate, calcium phosphate, uric acid, struvite, or cystine) and guide targeted prevention strategies 2, 3
• Stone analysis should be performed at least once for all first-time stone formers and repeated in patients not responding to treatment, as composition may change over time 2, 3

Comprehensive Metabolic Evaluation (High-Risk Patients)

A 24-hour urine collection is recommended for all recurrent stone formers and high-risk first-time stone formers (those with family history, solitary kidney, malabsorption, intestinal disease, early-onset disease ≤25 years, or bilateral stones). 2, 3, 4

24-Hour Urine Parameters

The collection should analyze at minimum:

• Total urine volume (assesses hydration status) 2, 3
• Urine pH (acidic favors uric acid stones; alkaline favors calcium phosphate) 2, 3
• Calcium, oxalate, uric acid, citrate (primary lithogenic factors) 2, 3
• Sodium, potassium, creatinine (validates collection adequacy and assesses dietary factors) 2, 3

Additional Specialized Testing

• Urinary cystine should be measured in patients with known cystine stones, family history of cystinuria, or suspected cystinuria 2, 3
• Primary hyperoxaluria should be suspected when urinary oxalate exceeds 75 mg/day in adults without bowel dysfunction 2, 5
• Two 24-hour collections are preferred over one for improved accuracy, though one collection may be adequate as studies show strong correlation between samples collected within 3 days 2, 5, 6

Imaging Studies

Initial Imaging

• Ultrasound is recommended as the primary diagnostic tool (45% sensitivity for renal stones, 88% specificity), followed by non-contrast CT for definitive assessment 5, 3
• Non-contrast CT is the gold standard for evaluating stone location, size, burden, and density (93.1% sensitivity, 96.6% specificity), with low-dose protocols preferred to minimize radiation 5, 3

Imaging Findings That Require Additional Workup

• Multiple or bilateral renal calculi indicate higher recurrence risk and warrant comprehensive metabolic evaluation 3
• Nephrocalcinosis suggests underlying metabolic disorders such as renal tubular acidosis or primary hyperparathyroidism 2, 3

Special Populations

Genetic Testing Considerations

Genetic testing should be considered in patients with early-onset stone disease (≤25 years), suspected inherited disorders, recurrent stones, bilateral disease, or strong family history 3

Persistent Microscopic Hematuria

• 24-hour urine stone panel may be performed to assess for nephrolithiasis/microlithiasis 2
• Additional testing may include cystoscopy and imaging to assess for urinary tract abnormalities or new stone formation 2

Common Pitfalls

• Do not assume normal-sized kidneys on ultrasound exclude chronic kidney disease, as stone formers as a group have decreased renal function compared to normal individuals 5
• Avoid delaying stone analysis until after discharge, as perioperative characterization provides valuable information about the urinary environment during stone formation, especially when only fragments are available 7
• Do not skip metabolic evaluation in "low-risk" first-time stone formers if they express interest in prevention, as 24-hour urine analysis guides individualized dietary and pharmacologic interventions even when stones cannot be retrieved 2, 7