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Kidney stones are solid, stonelike lumps that can form in your kidneys. Small stones can stay in your kidney without any problems. Small stones may also pass out of your body in your urine without you noticing. But larger stones can block the flow of urine from your kidney. Or they can pass out of your kidney into the tubes that carry urine toward your bladder. This can be very painful.

Your kidneys are two organs that are each about the size of your fist. They sit near the middle of your back, just below your rib cage. Their job is to filter waste products and extra water out of your blood. They send these waste products to your bladder as urine.¹

A kidney stone is a lump of crystals made from waste chemicals in your urine. Urine contains chemicals to stop these crystals from forming, but this doesn't always work. Crystals can then clump together and form stones in your kidneys.²

Kidney stones don't just affect your kidneys. You can also get them in other parts of your body that carry urine. The parts of your body that deal with urine are called your urinary tract.

Your urinary tract includes your bladder and ureters. The bladder is the pouch where urine is stored before you urinate. Ureters are tubes that carry urine from the kidneys to the bladder.

You might hear "kidney stone" used as a general term, to mean a stone anywhere in your urinary tract. Doctors sometimes use the terms nephrolithiasis or urolithiasis to describe kidney stones.

Kidney stones come in many shapes and sizes. They can be as small as a grain of sand or as large as a golf ball. They can be smooth or jagged. They are usually yellow or brown.¹

Small stones can stay in your kidneys without causing problems. They can also travel out of your body in the flow of urine. If they are very small, they can pass out of your body without you noticing. But larger ones can block the flow of urine from your kidney. If they pass out of your kidneys, larger stones can rub against the tubes or even get stuck. This can be extremely painful.¹

About two-thirds of all kidney stones go unnoticed. The rest cause pain on their way out of the body.³

Kidney stones form for three main reasons:⁴

• You have a lot of waste chemicals in your urine
• You don't have enough citrate in your urine. This is the main chemical that prevents crystals from forming
• Your urine doesn't contain enough water to dissolve waste products.

Some things make you more likely to get stones. These are called risk factors. They include:^{2 5}

• A family history of kidney stones. If a close relative has had kidney stones, you are three times more likely to get them
• Having had a kidney stone before. If you've had a kidney stone, you have about a 1 in 2 chance of getting another one in the next few years
• High blood pressure. If you have high blood pressure, your kidneys have to work harder. This can increase your risk of kidney stones
• Gout. Gout is a medical condition where you have too much uric acid in your body. Uric acid is a chemical that is usually filtered out by the kidneys. Too much of it can make you more likely to get kidney stones
• Overactive parathyroid glands. If your body gets too much of the hormone made by the parathyroid glands, extra calcium is released into your blood. This can make you more likely to get kidney stones
• Having both ovaries removed, bringing on an early menopause
• Taking certain medicines, including some diuretics (water pills) and some decongestants.
• Problems inside your kidney, like cysts.

There are four types of kidney stones.¹

• Calcium oxalate stones (the most common type). These are usually caused by having too much calcium in your body. Calcium that is not needed elsewhere in your body goes to your kidneys as waste. Usually, unwanted calcium is flushed out in your urine. But calcium can build up in the urine. It can then join with other waste products to form a stone.
• Uric acid stones. Uric acid is a waste product produced after food is digested. If you have too much uric acid in your urine, it may not dissolve properly. It can then form stones. These kinds of stones are more common in men.
• Struvite stones. These stones develop when a urinary infection, such as a bladder infection (cystitis), affects the balance of chemicals in the urine. They are rare, but they happen more often in women, because women have more urinary infections.
• Cystine stones. These are quite rare. They are made up of cystine, one of the building blocks of protein. Cystine does not dissolve well in urine. The stones are caused by a rare inherited condition called cystinuria.

It is important to know what kind of stone you have as this will affect your treatment to prevent future stones. For example, a medicine that helps prevent calcium stones will not work if you have a struvite stone. And the changes in diet that can help prevent uric acid stones may have no effect on calcium stones.¹

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.