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Kidney stones

Description

An in-depth report on the causes, diagnosis, treatment, and prevention of kidney stones.

Alternative Names

Calcium stones; Nephrolithiasis

Diagnosis

The doctor will perform a physical exam. This includes pressing against abdominal areas for tender locations that might indicate the presence of the stone.

Medical History

The patient's age is a significant factor. Kidney stones that occur in children and young patients are more apt to result from inherited problems that cause cystine, xanthine, or, in some cases, calcium oxalate stones. In adult patients, calcium stones are most common.

A medical history may help predict which crystal has formed the stone. The doctor will need to know the following:

• Any previous kidney stone attacks
• Histories of cancer, sarcoidosis, or small bowel disease
• Any medications being taken, including non-prescription substances, particularly high doses of vitamins D or C and calcium-containing antacids

Ruling Out Other Disorders

Many conditions can cause symptoms similar to kidney stones. Usually the diagnosis is easily made because of the specific nature of the symptoms, but it is not always clear. Urinary tract infections can cause similar, but usually less intense, pain. In fact, infection may be present with a kidney stone. Other causes of pain that may mimic kidney stones include:

• Gallstones
• Diverticulitis
• Intestinal blockage
• Blood clots
• Irritable bowel syndrome
• Appendicitis
• Stomach ulcers
• Hiatal hernia
• Pancreatitis
• Hepatitis
• Pelvic inflammatory disease
• Inflammatory bowel disease (Crohn's and colitis)
• Heart attack

Imaging Techniques

Various imaging techniques are helpful in determining the presence of kidney stones. The best approach uses spiral (or helical) computed tomography scans. It is not always available, however, in which case ultrasound or standard x-rays are usually performed. If no stones show up but the patient has severe pain indicative of kidney stones, the next step is an intravenous pyelogram.

X-Rays. A standard x-ray of the kidneys, ureters, and bladder may be adequate as a first step for identifying many stones, since most are opaque on x-rays. Calcium stones can be identified on x-rays by their white color. Cystine crystals also can show up on x-rays.

Spiral (or Helical) Computed Tomography. A computed tomography (CT) scan called a spiral or helical CT scan is currently the best method for diagnosing stones in either the kidneys or ureters. It is fast, noninvasive, and provides detailed accurate images of even very small stones. If stones are not present, it can often identify other causes of pain in the kidney area. It is superior to x-rays, ultrasound, and intravenous pyelogram--the test that was the previous standard for detecting kidney stones. Experts hope spiral CT will eventually be able to reveal the stone's composition.

Ultrasound. Ultrasound can detect translucent uric acid stones and obstruction in the urinary tract. It is not useful for finding very small stones, but some research indicates that it may be a useful first diagnostic step in the emergency room to help predict the likelihood of a stone, including suspected stones in children.

Intravenous Pyelogram. With intravenous pyelogram (IVP), the patient is injected with a dye, and x-rays are taken as the dye enters the kidneys and travels down the urinary tract. IVP is invasive but, until recently, was the most cost-effective method for detecting stones. Where it is available, spiral CT is now preferred, since it gives a faster diagnosis, is more accurate, and it is similar in cost. In any case, IVP should not be used on patients with kidney failure. There is a risk for an allergic reaction to standard dyes, although newer less allergenic ones are becoming available.

Magnetic Resonance Imaging. Magnetic resonance imaging (MRI) techniques are showing promise for diagnosing urinary tract obstruction but do not yet accurately reveal nonobstructive or small stones. Because no radiation is involved, however, it may prove to be a good option for pregnant women.

Urine Tests

Urine samples are required to evaluate features of the urine, including its acidity, the presence of red or white blood cells, whether infection is present, any crystals, and elevated or decreased components that inhibit or promote stone formation.

Clean-Catch Urine Sample for Culturing. Once it has been determined that a kidney stone is present, the patient is usually given a collection kit, including filters, to try to catch the stone or gravel as it passes out. A clean-catch urine sample is almost always required for culturing. To provide this, the following steps are taken:

• Patients must first wash their hands thoroughly, then wash the penis or vulva and surrounding area four times with downward strokes, using a new soapy sponge each time.
• The patient must then begin urinating into the toilet and stop after a few drops.
• The patient then positions the container to catch the middle portion of the stream. Ideally, this urine will contain only the bacteria and other evidence of the stone.
• The patient then urinates the remainder into the toilet.
• The cap must then be securely screwed on without the patient touching the inside of the rim.

Twenty-Four Hour Urine Collection. A 24-hour urine collection may be needed to measure urine volume and levels of acidity, calcium, sodium, uric acid, oxalate, citrate, and creatinine.

• The patient should not change any of his or her usual eating or drinking patterns when performing this test.
• The patient discards the first urination on the day of the test.
• Afterward all urine passed over the next 24 hours is collected, including the first urination on the morning of day two.
• A second 24-hour urine collection may be needed to determine if treatment is working or if the first analysis was not conclusive and the doctor suspects a less common stone, such as a cystine or xanthine stone.

Urine tests that are used to determine the specific chemical and biologic factors causing the stone should be performed about 6 six weeks after the attack, since the attack itself may change the levels of such substances, including calcium, phosphate, and citrate. It should be noted that calcium levels in the urine may be abnormal even in many people without stones. In addition, high urinary concentrations of calcium may pose a greater or lesser risk depending on age. (In one 2001 study, middle-aged adults with high urinary calcium concentrations had a much greater risk than older adults with high levels.)

Microscopic Examination

The kidney stones obtained from the sample are examined under a microscope. The crystal formations are often specific enough so that the doctor is able to identify the substance causing the stone.

• Calcium oxalate crystals are eight-sided (octahedral), while calcium phosphate crystals tend to have irregular shapes.
• Uric acid stones are sometimes described as pear- or diamond-shaped.
• Some struvite stones have very specific shapes commonly described as "coffin lids." Struvite crystals may also occur in a formation known as a staghorn, which can be large and damaging to the kidney.

Testing the Acidity of Urine

Testing whether urine is acid or alkaline helps to identify the specific type of stone. The levels of acid or alkaline in any solution, including urine, are indicated by the pH scale :

• A pH value of 7.0 is neutral.
• A solution with a low pH (below 7.0) is acidic. (A low pH favors uric acid and cystine stones.)
• A solution with a high pH is alkaline. (A high pH favors calcium phosphate and struvite stones.)

Testing for Blood in the Urine

A dipstick for blood in the urine (called hematuria) is typically performed when patients appear in the emergency room with flank pain (the primary symptom of kidney stones). About a third of kidney stone patients, however, do not show blood in the urine, so other tests are needed.

Blood Tests

Blood Tests for Stone Factors. Blood and urine tests help determine the substance forming the crystal so that appropriate treatment and preventive measures can be taken. Blood tests may help determine levels of blood urea nitrogen, creatinine, calcium, phosphate, and uric acid for patients with known or suspected calcium oxalate stones. These tests are often scheduled about six weeks after the attack, particularly with recurrent stones, in order to measure these substances when the stone has been passed and the patient has been stabilized.

Parathyroid Tests. Tests to detect parathyroid hormone levels are given if the doctor suspects hyperparathyroidism based on other signs and symptoms.

Tests for Infection. A test result that shows a high white blood cell count might indicate infection, but such results could be misleading, since white cells could also increase in response to the extreme physical stress of a kidney stone attack.

Tests for Metabolic Problems. About half of children with stones have an identifiable metabolic disorder, which increases their risk of stone recurrence five-fold. Experts argue over whether tests for metabolic abnormalities are routinely needed once the stone composition has been determined. Studies suggest the following:

• People with recurrent calcium stones have a wide range of abnormal blood or urine chemistries indicating a variety of metabolic disorders. (For example, calcium stones in women may be due to parathyroid abnormalities.)
• Calcium phosphate stones raise the likelihood of renal tubular acidosis.
• People with non-calcium stones generally have identifiable metabolic disorders.
• Determining the stone composition may be sufficient for treatment and may help avoid unnecessary metabolic tests.

• Review Date: 5/22/2006
• Reviewed By: Harvey Simon, M.D., Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital

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