Background

Kidney stones are common throughout the world but are more common in industrialized nations. About 5 in 1000 persons are affected with stones of the upper renal urinary tract (kidney) more common than stones in the bladder. The average age of onset is during the thirties. Men are affected with calcium stones more often than women. Recurrences are common and identification of the chemical composition of stones can help in identifying risk factors.

Chemical Composition of Stones	Percentage %
Calcium oxalate/Calcium phosphate	75
Magnesium ammonium phosphate	10-15
Uric acid	6
Mixed magnesium, calcium, uric acid	3-10%
Cystine	1-2%
Xanthine	Rare
Adenine	Rare

There is overlap between the magnesium and uric acid stones since they are frequently mixed with calcium phosphate. It is important to know the pH that each stone precipitates. Calcium oxalate precipitates at acid or neutral pH. Calcium phosphate precipitates at normal urine pH of 6.0-6.5. Uric acid crystallizes at pH 5.3. Magnesium ammonium phosphate forms precipitates at an alkaline pH (pH>7.0). These latter stones are almost always associated with bacterial infections from organisms that can convert urea to ammonia and thus alkalinize the urine. Proteus and some staphylococci are the main offenders. Finally there are rare stones formed by precipitation of various amino acids such as cystine and xanthine, present in patients with inherited defects in amino acid metabolism.

Routine laboratory evaluation includes routine urinalysis with consideration for a 24 hour urine study to urine pH and to quantitate sodium, calcium, phosphorus, uric acid, oxalate, and creatinine clearance. Serum chemistries for the same electrolytes and minerals, chemical analysis of the stone if available, and a radiologic examination are also frequently performed. All stones are radiopaque except pure uric acid and xanthine stones. The pathologist can play an important role in determining the cause of the stones, thus directing preventive therapy.

Henry JB. Clinical Diagnosis and Management by Laboratory Methods. Twentieth Edition. WB Saunders. 2001.

Staghorn calculi -Large bulky stones formed in the renal pelvis, almost always a result of infection by certain bacteria such as Proteus and some staphylococci.

U/A-Urinalysis.

• UCLA Medical Center

Last Updated 4/1/2001