Kidney stones (calculi) are made of substances normally found in the urine. Stones begin as small crystals in the kidney. They form when the urine is too concentrated in calcium or other chemicals that can group together. Over time, these crystals may grow into stones. Small stones can pass out of the body undetected along with urine. But when larger stones fall into and obstruct the ureter (the narrow duct that drains urine from the kidney to the bladder), they can cause sudden, severe pain.

Stones grow in an unpredictable fashion. They can grow in quick spurts or extremely slowly, depending on conditions in the urine.

About 90% of kidney stones contain calcium. Most commonly, the crystals are made of calcium oxalate and occasionally they are made of calcium phosphate. Less common stone types are uric acid, cystine and struvite.

Kidney stone symptoms

Most people are diagnosed with kidney stones after experiencing unforgettable pain. This severe pain occurs when the stone drops into the ureter (the narrow duct that drains urine from the kidney to the bladder) and blocks the drainage of urine from the kidney. This is also known as renal colic. The pain may begin in the lower back and may move to the side or groin. Stones in the kidney that do not obstruct urine flow may cause a chronic dull ache.

In addition to pain, other kidney stone symptoms include:

• Side pain that is not affected by activity
• Blood in the urine
• Frequent or persistent urinary tract infections
• Urinary urgency or frequency
• Nausea

For many people, kidney stones cause no symptoms and are detected when microscopic amounts of blood are found in the urine. They may also be detected during x-ray tests for other medical problems.

Kidney stone facts

• Around 10% of people will develop a kidney stone in their lifetime.
• 50% of people who experience a kidney stone will have a recurrence within 7 to 10 years.
• 50% of asymptomatic stones within the kidney will cause symptoms within the next 5 years.