What investigations should be done for calcium oxalate (CaOx) crystals found in urine?

From the Guidelines

The presence of calcium oxalate crystals in urine warrants further investigation to rule out underlying conditions such as nephrolithiasis or hyperparathyroidism.

Recommended Investigations

• A comprehensive metabolic panel, including serum calcium, phosphorus, and parathyroid hormone levels, should be performed 1.
• A 24-hour urine collection to assess for hypercalciuria, hyperoxaluria, and hypocitraturia is recommended 1.
• Patients should also undergo renal ultrasonography to evaluate for kidney stones 1.
• Those with recurrent stones may benefit from thiazide diuretics, such as hydrochlorothiazide 25mg daily, to reduce calcium excretion 1.

Additional Considerations

• Genetic testing may be necessary for patients with a high clinical suspicion of primary hyperoxaluria (PH) 1.
• Urine oxalate measurement should be performed in a 24-hour urine sample, collected into acid or acidified within 24 hours after collection to achieve a pH of <2 1.
• Dietary modifications, such as increasing fluid intake and reducing dietary oxalate, may also be beneficial in preventing recurrent nephrolithiasis 1.

From the Research

Investigations for Calcium Oxalate Crystals in Urine

To investigate calcium oxalate (CaOx) crystals found in urine, several tests and analyses can be conducted, as suggested by various studies 2, 3, 4, 5, 6. These include:

• Analysis of stone composition by polarization microscopy 2
• Measurement of serum calcium, phosphate, uric acid, 1,25(OH)2D3, and creatinine 2
• Twenty-four-hour urine collection for analysis of volume, pH, and excretion of calcium, phosphorus, magnesium, uric acid, citrate, sodium, oxalate, and creatinine 2, 3, 4, 5, 6
• Random urine test during nocturnal fasting to screen for hypercalciuria, hyperoxaluria, and hypocitraturia 4
• Examination of urine sediment for calcium oxalate crystals and other abnormalities 5

Metabolic Factors to Consider

Several metabolic factors can contribute to the formation of CaOx stones, including:

• Hypercalciuria (excretion of more than 200 mg of calcium per 24 hours) 2, 4, 5, 6
• Hyperoxaluria (excretion of more than 40 mg of oxalate per 24 hours) 2, 4, 5, 6
• Hypocitraturia (excretion of less than 320 mg of citrate per 24 hours) 2, 4, 6
• Low urine magnesium and high urine calcium-to-oxalate ratio 3
• High urine supersaturation with respect to calcium oxalate monohydrate 3

Clinical Correlates

Clinical factors that may be associated with CaOx stone formation include:

• Primary hyperparathyroidism 5
• Age, with younger patients more likely to form CaOx-dihydrate stones 6
• Body mass index, hypertension, diabetes mellitus, and hyperlipidemia, although these factors were not found to be significantly different between CaOx-monohydrate and CaOx-dihydrate stone formers 6