Kidney Stones - Introduction

Calcium Stones. About 70 - 80% of all kidney stones are composed of calcium, usually combined with oxalate, or oxalic acid. (Oxalate is found in a number of common vegetables, fruits, and grains.) About 6% of calcium stones are composed of calcium phosphate (called brushite).

Uric Acid Stones. About 7% of stones are made up of uric acid, which is formed from a breakdown in purine, a nitrogen compound found in protein. (Uric acid can constitute as many as 40% of kidney stones in some countries.) Uric acid is produced in the liver and enters the bloodstream, where most passes into the kidneys and is eliminated in urine. Often, uric acid stones occur with calcium stones.

Struvite Stones. Struvite stones, which are made of magnesium ammonium phosphate, are almost always associated with certain urinary tract infections. Worldwide, they compose 30% of all kidney stones. In the US, however, less than 15% of all stones are struvite, with most occurring in women. The incidence of these stones may be declining in America, perhaps because of better control of urinary tract infections.

Cystine Stones. About 2% of stones in adults and up to 8% of kidney stones in children are caused by a build-up of the amino acid cystine, a building block of protein. The tendency to form these stones is inherited. They are marked by rapid growth and recurrence, which, if not treated promptly, can eventually lead to kidney failure.

Xanthine Stones. Others are composed of xanthine, a nitrogen compound (caffeine is a derivative of this substance). These stones are extremely uncommon and usually occur as a result of a rare genetic disorder.

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