A woman with recurrent calcium phosphate kidney stones.

Kidney stones composed predominantly (50% or more) of calcium phosphate constitute up to 10% of all stones and 15%-20% of calcium stones, 80% of which are composed of calcium oxalate. Calcium phosphate is a minor component of up to 30% of calcium oxalate stones as well. The cause of calcium phosphate stones is often obscure but most often related to a high urine pH. Some patients with calcium phosphate stones may have incomplete renal tubular acidosis. Others have distal renal tubular acidosis characterized by hyperchloremic acidosis, hypocitraturia, and high urine pH. The use of carbonic anhydrase inhibitors such as acetazolamide, topiramate, and zonisamide leads to a similar picture. Treatment options to specifically prevent calcium phosphate stone recurrence have not been tested in clinical trials. Increases in urine volume and restriction of sodium intake to limit calcium excretion are important. Citrate supplementation is probably effective, although the concomitant increase in urine pH may increase calcium phosphate supersaturation and partially offset the inhibition of crystallization resulting from the increased urine citrate excretion and the alkali-associated reduction in urine calcium excretion. Thiazides lower urine calcium excretion and may help ensure the safety of citrate supplementation.