urinary calcium and renal stones

Calcium oxalate stones are the most common type of renal stone.(around 75%). 3 types of stones can be caused by calcium- calcium oxalate, CaCO₃, or CaHPO₄. 


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. 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.


Eventhough calcium is involved in majority of stones....its measurement in urine is of limited value in predicting/diagnosing renal stones.This is because of a lot of other factors involved in stone formation( citrate, oxalate, urine Ph etc). Risk assessment using ratios such as the calcium/magnesium ratio, the calcium/citrate ratio, and the (calcium * oxalate)/(magnesium * citrate) ratio have met with little success, and are regarded to play a supplementary role in evaluation of renal stones.( Ped Nephrol 2009)


But Urine calcium can be used in assessing patients with primary hyperparathyroidism. A study done comparing biochemical parameters and treatment outcomes in stone formers with hyperparathyroidism Vs those without systemic disease...found ..a greater amount of calcium excretion in urine of stone formers with hperparathyroidism than in stone formers without systemic disease.(Brit J of Urol 2009) .Thus high Calcium creatinine clearance rate(or calcium creatinine ratio > 0.20) can be helpful in patients with hyperparathyroidism in predicting/assessing risk for stone formation. Interestingly, parathyroidectomy did not reverse the hypercalciuria or hypophosphatemia in these patients.


Increased Urinary calcium - > 300mg/24 hours
Increased calcium creatinine ratio - > 0.2 
A quick word about diet in calcium stones---- is reducing calcium in diet doesnt help( again because of multiple factors involved in stone formation). But reducing oxalate consumption prevents crystal formation and reduces stone formation.Have a look at this article from NEJM 2002 (hope u all have access)