What causes kidney stones?

A kidney stone is a solid piece of material that forms in a kidney when substances that are normally found in the urine become highly concentrated. A stone may stay in the kidney or travel down the urinary tract. Kidney stones vary in size. A small stone may pass on its own, causing little or no pain. A larger stone may get stuck along the urinary tract and can block the flow of urine, causing severe pain or bleeding.

Kidney stones can form when substances in the urine—such as calcium, oxalate, and phosphorus—become highly concentrated. Certain foods may promote stone formation in people who are susceptible, but scientists do not believe that eating any specific food causes stones to form in people who are not susceptible. People who do not drink enough fluids may also be at higher risk, as their urine is more concentrated.

People who are at increased risk of kidney stones are those with

· hypercalciuria, a condition that runs in families in which urine contains unusually large amounts of calcium; this is the most common condition found in those who form calcium stones

· a family history of kidney stones

· cystic kidney diseases, which are disorders that cause fluid-filled sacs to form on the kidneys

· hyperparathyroidism, a condition in which the parathyroid glands, which are four pea-sized glands located in the neck, release too much hormone, causing extra calcium in the blood

· renal tubular acidosis, a disease that occurs when the kidneys fail to excrete acids into the urine, which causes a person’s blood to remain too acidic

· cystinuria, a condition in which urine contains high levels of the amino acid cystine

· hyperoxaluria, a condition in which urine contains unusually large amounts of oxalate

· hyperuricosuria, a disorder of uric acid metabolism

· gout, a disorder that causes painful swelling of the joints

· blockage of the urinary tract

· chronic inflammation of the bowel

· a history of gastrointestinal (GI) tract surgery

Others at increased risk of kidney stones are people taking certain medications including

· diuretics—medications that help the kidneys remove fluid from the body

· calcium-based antacids

· the protease inhibitor indinavir (Crixivan), a medication used to treat HIV infection

· the anti-seizure medication topiramate (Topamax)