A kidney stone is a hard, crystalline mineral material formed within the kidney or urinary tract. Kidney stones are a common cause of blood in the urine and often severe pain in the abdomen, flank, or groin. Kidney stones are sometimes called renal calculi. One in every 20 people develops a kidney stone at some point in their life.
The condition of having kidney stones is termed nephrolithiasis. Having stones at any location in the urinary tract is referred to as urolithiasis.

Kidney stones form when there is a decrease in urine volume and/or an excess of stone-forming substances in the urine. The most common type of kidney stone contains calcium in combination with either oxalate or phosphate. Other chemical compounds that can form stones in the urinary tract include uric acid and the amino acid cystine.
Dehydration from reduced fluid intake or strenuous exercise without adequate fluid replacement increases the risk of kidney stones. Obstruction to the flow of urine can also lead to stone formation. Kidney stones can also result from infection in the urinary tract; these are known as struvite or infection stones.
A number of different medical conditions can lead to an increased risk for developing kidney stones:

1. Gout results in an increased amount of uric acid in the urine and can lead to the formation of uric acid stones.
2. Hypercalciuria (high calcium in the urine), another inherited condition, causes stones in more than half of cases. In this condition, too much calcium is absorbed from food and excreted into the urine, where it may form calcium phosphate or calcium oxalate stones.
3. Other conditions associated with an increased risk of kidney stones include hyperparathyroidism, kidney diseases such as renal tubular acidosis, and some inherited metabolic conditions including cystinuria and hyperoxaluria. Chronic diseases such as diabetes and high blood pressure (hypertension) are also associated with an increased risk of developing kidney stones.
4. People with inflammatory bowel disease or who have had an intestinal bypass or ostomy surgery are also more likely to develop kidney stones.
5. Some medications also raise the risk of kidney stones. These medications include some diuretics, calcium-containing antacids, and the protease inhibitor indinavir (Crixivan), a drug used to treat HIV infection.

While some kidney stones may not produce symptoms (known as "silent" stones), people who have kidney stones often report the sudden onset of excruciating, cramping pain in their low back and/or side, groin, or abdomen. Changes in body position do not relieve this pain. The pain typically waxes and wanes in severity, characteristic of colicky pain (the pain is sometimes referred to as renal colic). It may be so severe that it is often accompanied by nausea and vomiting. Kidney stones also characteristically cause blood in the urine. If infection is present in the urinary tract along with the stones, there may be fever and chills. Sometimes, symptoms such as difficulty urinating, urinary urgency, penile pain, or testicular pain may occur due to kidney stones.

The diagnosis of kidney stones is suspected by the typical pattern of symptoms when other possible causes of the abdominal or flank pain are excluded. Imaging tests are usually done to confirm the diagnosis. A helical CT scan without contrast material is the most common test to detect stones or obstruction within the urinary tract. Formerly, an intravenous pyelogram (IVP; an X-ray of the abdomen along with the administration of contrast dye into the bloodstream) was the test most commonly used to detect urinary tract stones, but this test has a greater risk of complications, takes longer, and involves higher radiation exposure than the non-contrasted helical CT scan. Helical CT scans have been shown to be a significantly more effective diagnostic tool than the IVP in the diagnosis of kidney or urinary tract stones.
In pregnant women or those who should avoid radiation exposure, an ultrasound examination may be done to help establish the diagnosis.

Most kidney stones eventually pass through the urinary tract on their own within 48 hours, with ample fluid intake. Pain medications are used for symptom relief. When over-the-counter medications are not sufficient for pain control, narcotics may be prescribed. Intravenous pain medications can be given when nausea and vomiting are present.

There are several factors which influence the ability to pass a stone. These include the size of the person, prior stone passage, prostate enlargement, pregnancy, and the size of the stone. A 4 mm stone has an 80% chance of passage while a 5 mm stone has a 20% chance. Stones larger than 9 mm-10 mm rarely pass without specific treatment.

Some medications have been used to increase the passage rates of kidney stones. These include calcium channel blockers such as nifedipine (Adalat, Procardia, Afeditab, Nifediac) and alpha blockers such as tamsulosin (Flomax). These drugs may be prescribed to some people who have stones that do not rapidly pass through the urinary tract.

For kidney stones that do not pass on their own, a procedure called lithotripsy is often used. In this procedure, shock waves are used to break up a large stone into smaller pieces that can then pass through the urinary system.

Surgical techniques have also been developed to remove kidney stones when other treatment methods are not effective. This may be done through a small incision in the skin (percutaneous nephrolithotomy) or through an instrument known as an ureteroscope passed through the urethra and bladder up into the ureter.

Rather than having to undergo treatment, it is best to avoid kidney stones in the first place when possible. It can be especially helpful to drink more water, since low fluid intake and dehydration are major risk factors for kidney stone formation.
Depending on the cause of the kidney stones and an individual's medical history, dietary changes or medications are sometimes recommended to decrease the likelihood of developing further kidney stones. If one has passed a stone, it can be particularly helpful to have it analyzed in a laboratory to determine the precise type of stone so specific prevention measures can be considered.

Prevention
If you have more than one kidney stone, especially if you have a family history of stones, you are more likely to have kidney stones again. But you can take steps to help prevent them:

• Drink more fluids. Try to drink enough water to keep your urine clear, about 8 to 10 glasses of water per day. Slowly increase how much you drink, perhaps adding one more glass of water a day until you are drinking 8 to 10 glasses a day. This slow increase will give your body time to adjust to the extra fluids. You are drinking enough water when your urine is clear or light yellow. If it is dark yellow, you are not drinking enough fluids. If you have kidney, heart, or liver disease and have fluid restrictions, talk with your doctor before increasing how much you drink.
• Change your diet. This may be helpful, but it depends on what is causing your kidney stones. Your doctor may do more tests before deciding whether changing your diet will help reduce your risk for developing another stone. The results of these tests may suggest that it could be helpful to do one or more of the following:
  • Increase how much fiber you eat. Fiber includes oat bran, beans, whole wheat breads, wheat cereals, cabbage, and carrots.
  • Eat less beef, pork, and poultry.
  • Eat a moderate or high amount of calcium-rich foods, such as dairy products. Getting your recommended amounts of calcium, combined with a diet low in sodium and protein, may decrease your risk of kidney stones.
  • Try not to drink grapefruit juice. Drinking grapefruit juice may increase your risk for developing kidney stones.
  • Avoid foods that are high in oxalate, such as dark green vegetables, nuts, and chocolate.
  • Try not to add salt when you cook or eat. Try removing the salt shaker from your table.