The precise cause of kidney stones is still not fully understood but many theories exist.  Urine contains substances known to form stones and to prevent stones.   High levels of  compounds and elements known to cause stones include calcium, uric acid, phophate and oxalate.  In fact, urine often holds larger amounts of these substances than can normally be held in the same volume of pure water.  Therefore, calcium salts and uric acid can easily precipitate into crystals which can grow into larger calcium and uric acid stones.  When urine volume is low, the relative concentrations of these substance will be even higher which significantly increase the risk for stone precipitation.  The urine also contains substances which are known to prevent stone formation, such as magnesium, citrate and proteins.

Urine studies measure levels of calcium, phosphate, uric acid, and oxalate to determine the cause of a particular person's kidney stones.  These levels should not be used alone to determine the cause.  Further specialized tests should be done by  phsyicians experienced in analysis of supersaturations of calcium oxalate, calcium phosphate, and uric acid.  Supersaturations measures the interactions of the different urinary elements with each other.   The testing becomes even more complicated when pH, ammonium, sodium, urea and sulfate measurements are performed to determine the metabolic effects of diet on changes in urine composition.  These urinary changes often contribute to an individual's propensity to form stones.

In addition to the above testing.  Physicians should measure urinary citrate and magnesium levels.  These are known to prevent kidney stone formation.  In some the cause of low citrate and magnesium can be determined from the patient's history, physical exam, and analysis of blood and urine studies.  Low citrate levels can be a sign of renal tubular acidosis (RTA), while low magnesium can can occur with malnutrition or diuretics (water pills).

In conclusion, the causes of kidney stone formation are multifactorial in most individuals.  A physician experienced in kidney stone care and prevention can integrate the information from urine and blood tests to determine the most likely causes.  The important point, is that there is often more than one cause in each person.  Proper testing and analysis is critical in order to tailor a medical and dietary regimen particular to each person.