Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

Kidney Stones

What Is It?

Kidney stones are abnormal, hard, chemical deposits that form inside the kidneys. This condition also is called nephrolithiasis or urolithiasis. Kidney stones are often as small as grains of sand and pass out of the body in urine without causing discomfort. The deposits can be the size of a pea, a marble or even larger. Some of these larger stones are too big to be flushed from the kidney. Others manage to travel into the narrow tube between the kidney and bladder (ureter), where they become trapped. Trapped kidney stones can cause many different symptoms, including extreme pain, blocked urine flow and bleeding from the walls of the urinary tract.

Kidney stones are a very common problem, affecting 10% of people in the United States. There are several different types of stones, and a variety of reasons why stones form. Doctors have grouped kidney stones into four different families, based on their chemical composition:

• Calcium oxalate stones — These stones account for more than 75% of all kidney stones. They are more common in men than in women, and they are more common in adults than in children. In 50% of people with calcium stones, the stones form because of an increased level of calcium in the urine, which probably is inherited (genetic). In other people, calcium oxalate stones are caused by another condition such as extra parathyroid hormone, high uric acid levels (as in people who have gout), bowel disease, surgery for obesity, or kidney problems.

• Struvite stones — These stones are made of magnesium and ammonia (a waste product), and are related to urinary tract infections caused by certain bacteria. Struvite stones used to account for 10% to 15% of all kidney stones, but they are less common now that urinary tract infections are better recognized and treated. Struvite stones are more common in women than in men, and they develop frequently in people who have long-term bladder catheters.

• Uric acid stones — These stones account for 5% to 8% of all kidney stones, and are more common in men than in women. Uric acid stones form because of an abnormally high concentration of uric acid in the urine. They often occur in people who have gout, a disorder in which uric acid builds up in the blood and joints.

• Cystine stones — These rare stones account for less than 1% of kidney stones. They are composed of the amino acid cystine, which is a building block of proteins, and they are caused by an inherited (genetic) defect.