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What Are Kidney Stones?

Kidney stones are a common health problem that affects about 15 percent of Americans. A kidney stone is a hard mass that develops when chemicals in the urine crystallize or conglomerate into a mass within the kidney. The peak age for stones is between 20 years and 50 years. White Americans are more prone to developing kidney stones than African Americans, and men develop stones more often than women.

Many stones are as small as a grain of sand. They easily pass down the ureter into the bladder and out of the body through the urethra. Other stones may be pea- or marble-sized and more difficult to pass. Some stones are too large to pass out of the kidney—these are often called staghorn stones—and some smaller stones may get trapped in the ureter, causing extreme pain, blockage of urine, and bleeding into the urine.

People who have repeated attacks can develop kidney damage and having one attack increases the chance you may have another one.


The cause of a kidney stone is not always known. An inherited condition called hypercalciuria is the cause of stones in over half of those who get them. In this condition, a high level of calcium in the urine causes crystals of calcium oxalate or calcium phosphate to form either high in the kidneys or in the lower urinary tract.

Other causes of kidney stones are hyperuricosuria (a disorder of uric acid metabolism), gout, excess intake of vitamin D, and blockage of the urinary tract. A less common type of stone is caused by infection in the urinary tract. This type of stone is called a struvite or infection stone.

In some people, certain foods may promote stone formation, but food is not the cause in most people. Having family members who have had kidney stones may increase the risk for kidney stones. 

Rare diseases that affect metabolism also can cause stones. 

People who have chronic inflammation of the bowel, or who have had an intestinal bypass operation or ostomy surgery, are more susceptible to calcium oxalate stones.

Types of stones

These are the three most common types of kidney stones:

  • Calcium stones. This common type of stone contains calcium. They're made of either calcium oxalate or calcium phosphate. The formation is linked to an increased level of calcium in the urine, possibly because of inherited, or genetic, factors.

  • Struvite stones. These are made of magnesium and ammonia and are related to urinary-tract infections.

  • Uric acid stones. These form because of an abnormally high concentration of uric acid in the urine. They often occur in people with gout.


Kidney stones that block the flow of urine require treatment to prevent kidney damage. Call your doctor if you notice any of these symptoms:

  • Severe pain in your back or side, with or without nausea and vomiting

  • Pain that radiates down into the groin

  • Urine that is colored pink or tinged with blood

  • Fever or chills, and urine that is cloudy or foul-smelling


Traditional kidney stone surgery required incisions on the abdomen to reach the stone to remove it. Today, “minimally invasive” treatment is used for almost all kidney stones. Although the majority of smaller stones will pass without a medical procedure, large stones that are unlikely to pass through on their own must be broken up by a process known as lithotripsy. The procedure can be done in several ways:

  • Extracorporeal lithotripsy, also called ESWL, in which shock waves from an outside source are used to break the stones into smaller fragments that can be swept away in urine. This is the most common procedure.

  • Percutaneous ultrasonic lithotripsy, in which a narrow, tube-like instrument is inserted into the body through a small incision in the back. When the instrument reaches the kidney, a small ultrasound transducer is used to break up the stones.

  • Laser lithotripsy, in which a laser is used to break up the stones in the ureter.

  • Ureteroscopy, in which a small telescope is passed up the ureter from the bladder; the stone is then removed by passing a basket-like device up the scope to retrieve the stones. This is often done when stones are larger or irregular in shape.


The occurrence of stones is more common when you don't drink enough fluids. Most adults should drink at least 12 glasses of water or other fluids every day. Reducing salt and losing weight, if you are overweight, also helps. You may be able to prevent kidney stones by drinking enough liquids throughout the day to produce at least two quarts of urine in every 24-hour period. Reducing the amount of cola, coffee, and tea you drink also may help if you suffer from calcium oxylate stones. If you have uric acid stones, dietary and lifestyle changes can be effective; these include avoiding alcohol and diets high in meat.

Protein is a recommended part (50 grams per day) of a balanced diet, but high protein diets can increase the occurrence of kidney stones. Protein is found in beef, chicken, pork, fish, milk, and eggs. Dairy products help prevent stones, because calcium in milk binds with oxalate before it gets into the kidneys. People with the lowest dietary calcium intake have an increased risk of kidney stones 

If your doctor has analyzed one of your kidney stones, he or she may recommend dietary changes or medication that can reduce your risk of developing new stones.