Urine Calcium: Laboratory Measurement and Clinical Utility

Kevin F. Foley, PhD, DABCC; Lorenzo Boccuzzi, DO


Lab Med. 2010;41(11):683-686. 

In This Article

Calcium Crystals and Stones

Calcium is a common ingredient in urine stones and crystals. Calcium oxalate (Ca[COO]2) crystals are the most frequently observed crystals in urine, and 75% of renal calculi have calcium oxalate as a component. Calcium oxalate crystals can form at any pH and have various microscopic morphologies. It is estimated that about half of the oxalate in urine comes from ascorbic acid (vitamin C), which is a precursor to oxalate. Calcium oxalate crystals are also associated with ethylene glycol ingestion, another oxalate precursor. Calcium carbonate (CaCO3), the main component of marine shells and egg shells, can be found as small granular crystals in alkaline urine. Calcium carbonate crystals are not common in urine but when present can be mistaken for bacteria. To help discriminate these 2, acetic acid can be added to the sample, causing the crystals to release CO2 which appears as effervescence. Calcium phosphate (CaHPO4 or Ca[H2PO4]2) crystals can have different morphologies depending on their state of hydration and can be present in the urine sediment of neutral or slightly alkaline or acidic urine. Although not all patients with calcium crystals present in urine will suffer from kidney stones, renal calculi can be caused by calcium oxalate, CaCO3, or CaHPO4.

Acidification of urine helps prevent calcium from precipitating as salts and thus prevents falsely decreased measurements of urine calcium. Because of the possible interference of crystals, acidification of urine to pH <2 or pH 4–5 is recommended by many manufacturers of urine calcium reagents. However, adding acid to urine specimens presents some risk to technologists; it will dilute the specimen (although usually only to a minor degree), and it is time-consuming and often requires training and monitoring at collection sites. A recent study has questioned the need to acidify urine.[3]


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