From the Guidelines
For patients with calcium oxalate nephrolithiasis, I strongly recommend a comprehensive diagnostic evaluation including a detailed medical and dietary history, serum chemistries, urinalysis, and 24-hour urine collection to identify underlying metabolic abnormalities contributing to stone formation, as suggested by the American College of Physicians guideline 1. This approach is crucial because calcium oxalate stones form due to various metabolic disorders such as hypercalciuria, hyperoxaluria, hypocitraturia, or hyperuricosuria. Understanding the underlying metabolic abnormality allows for targeted treatment strategies to prevent recurrence.
- The diagnostic evaluation should include:
- A detailed medical and dietary history to identify any medical conditions, dietary habits, or medications that predispose to stone disease, as recommended by the American Urological Association guideline 1.
- Serum chemistries, including electrolytes, calcium, creatinine, and uric acid, to assess for underlying medical conditions associated with stone disease.
- Urinalysis, including dipstick and microscopic evaluation, to assess urine pH and identify crystals pathognomonic of stone type.
- 24-hour urine collection to measure volume, pH, calcium, oxalate, citrate, uric acid, sodium, and creatinine, which helps identify specific metabolic abnormalities contributing to stone formation.
- Additional targeted tests may include serum vitamin D levels and, in some cases, genetic testing if there's a strong family history or early onset of stones.
- Testing should be performed at least 4-6 weeks after an acute stone event to avoid skewed results from the acute phase.
- The American College of Physicians guideline 1 recommends management with increased fluid intake and pharmacologic monotherapy with a thiazide diuretic, citrate, or allopurinol to prevent recurrent nephrolithiasis in patients with active disease.
From the FDA Drug Label
2.2 Severe Hypocitraturia In patients with severe hypocitraturia (urinary citrate < 150 mg/day), therapy should be initiated at a dosage of 60 mEq/day (30 mEq two times/day or 20 mEq three times/day with meals or within 30 minutes after meals or bedtime snack) Twenty-four hour urinary citrate and/or urinary pH measurements should be used to determine the adequacy of the initial dosage and to evaluate the effectiveness of any dosage change.
The recommended diagnostic test for patients with calcium oxalate (CaOx) nephrolithiasis is 24-hour urinary citrate measurement. This test is used to determine the adequacy of the initial dosage and to evaluate the effectiveness of any dosage change of potassium citrate therapy 2. Additionally, urinary pH measurements can also be used to assess the effectiveness of treatment.
From the Research
Diagnostic Tests for Calcium Oxalate Nephrolithiasis
The following diagnostic tests are recommended for patients with calcium oxalate (CaOx) nephrolithiasis:
- Analysis of stone composition by polarization microscopy 3
- Measurement of serum Ca, phosphate, uric acid, 1,25(OH)2D3, and creatinine 3
- Twenty-four-hour urine collection for an analysis of volume, pH, and excretion of Ca, phosphorus, magnesium, uric acid, citrate, sodium, oxalate, and creatinine 3, 4, 5
- Determination of oxalate and citrate in urine and serum using ion-pairing reversed-phase (IP-RP) LC-MS/MS 5
Rationale for Diagnostic Tests
The diagnostic tests are designed to evaluate the underlying causes of CaOx nephrolithiasis, including:
- Urinary supersaturation with respect to Ca and oxalate 3, 4
- Presence of urinary inhibitors of crystal nucleation, aggregation, and growth 3
- Idiopathic hypercalciuria, characterized by hypercalciuria, normocalcemia, and intestinal Ca hyperabsorption 3
- Dietary oxalate intake and its impact on urinary oxalate excretion 6
Additional Considerations
The diagnostic tests should be conducted while the patients follow their usual diets 3. The results of the diagnostic tests can help guide therapy to prevent stone recurrence, including: