Diagnosis
During a physical examination, a doctor attempts to feel the kidneys. Except for newborn infants, normal kidneys cannot usually be felt in children or adults. Enlarged kidneys or a kidney tumor may be detectable. Often, a distended bladder also can be felt. The doctor performs a rectal examination in a man to determine whether the prostate gland is enlarged, although the size of the prostate as ascertained by rectal examination does not always correlate with the degree of urethral obstruction. A vaginal examination in a woman may provide information about the bladder and urethra.
Additional procedures may need to be performed to diagnose a kidney or urinary tract disorder.
Urinalysis
Urinalysis can be used to detect and measure the level of a variety of substances in the urine, including protein, glucose (sugar), ketones, blood, and other substances. These tests use a thin strip of plastic (dipstick) impregnated with chemicals that react with substances in the urine and change color. Sometimes, the test results are confirmed with more sophisticated and accurate laboratory analysis of the urine. The urine is examined under a microscope to check for the presence of red and white blood cells, crystals, and casts.
Protein: Protein in the urine (proteinuria) can usually be detected quickly by dipstick. Protein may appear constantly or only intermittently in the urine, depending on the cause. Proteinuria is usually a sign of kidney disorders, but it may occur normally after strenuous exercise such as marathon running.
Glucose: Glucose in the urine (glucosuria) can be accurately detected by dipstick. The most common cause of glucose in the urine is diabetes mellitus. If glucose continues to appear in the urine while glucose levels in the blood are normal, impaired reabsorption of glucose by the kidney tubules (renal glucosuria) is the cause of the glucosuria.
Ketones: Ketones in the urine (ketonuria) can be detected by dipstick. Ketones are formed when the body breaks down fat. Starvation, uncontrolled diabetes mellitus, and occasionally alcohol intoxication can produce ketones in the urine.
Blood: Blood in the urine (hematuria) is detectable by dipstick and confirmed by viewing the urine with a microscope and other tests. Sometimes the urine contains enough blood to be visible, making the urine appear red or brown.
Nitrites: Nitrites in the urine (nitrituria) are also detectable by dipstick. High nitrite levels indicate an infection.
Leukocyte Esterase: Leukocyte esterase (an enzyme found in certain white blood cells) in the urine can be detected by dipstick. Leukocyte esterase is a sign of inflammation, which is most commonly caused by a urinary tract infection.
Acidity: The acidity of urine is measured by dipstick. Certain foods and metabolic disorders may change the acidity of urine.
Concentration: The concentration of urine (also called the osmolality or specific gravity) may be important in diagnosing abnormal kidney function. The kidneys lose their capacity to concentrate urine at an early stage of a disorder that leads to kidney failure. In one special test, a person drinks no water or other fluids for 12 to 14 hours; in another, a person receives an injection of antidiuretic hormone. Afterward, urine concentration is measured. Normally, either test should make the urine highly concentrated. However, in certain kidney disorders (such as nephrogenic diabetes insipidus), the urine cannot be concentrated even though other kidney functions are normal.
Sediment: Sediment in urine can be examined under a microscope to provide information about a possible kidney or urinary tract disorder. Normally, urine contains a small number of cells and other debris shed from the inside of the urinary tract. A person who has a kidney or urinary tract disorder usually sheds more cells, which form a sediment if urine is centrifuged or allowed to settle.
Urine Cultures: Urine cultures, in which bacteria are grown in a urine sample in a laboratory, are performed to diagnose a urinary tract infection. The sample of urine must be obtained by the clean-catch method. Other methods to obtain an uncontaminated urine sample include passing a catheter through the urethra into the bladder or inserting a needle through the abdominal wall into the bladder (suprapubic needle aspiration).
 See the sidebar Obtaining a Clean-Catch Urine Sample.
Kidney Function Tests
Doctors can assess kidney function by performing tests on blood and urine samples. The kidney filtration rate can be estimated by measuring creatinine, a waste product, in the serum. Creatinine clearance--a more accurate test--can be approximated from a blood sample using a formula that relates the serum creatinine level to a person's age, weight, and sex. Determining creatinine clearance more precisely requires an accurately timed urine collection in conjunction with the serum creatinine determination. The level of blood urea nitrogen (BUN) can also indicate how well the kidneys are functioning, although many other factors can alter the BUN level.
Imaging Tests
X-rays: An x-ray of the abdomen can confirm that a person has two kidneys, helps assess kidney size and position, and shows stones that contain calcium in the urinary tract. However, x-rays are rarely used as the initial imaging technique in evaluating urinary tract disorders.
Ultrasonography: Ultrasonography uses reflected sound waves to produce an image of the kidney, ureters, and bladder. It is often the initial imaging technique because it can be performed safely even when kidney function is impaired. It is noninvasive, painless, and requires no radiopaque dye. Ultrasound scans provide some indirect information about kidney function, are an excellent way to estimate kidney size and position, readily detect obstruction, and help diagnose structural abnormalities. Although less accurate than computed tomography (CT) in initially detecting kidney tumors, ultrasonography is particularly useful in distinguishing a simple benign (noncancerous) cyst from a more complex cyst, or a solid mass that may be cancerous. Doctors also use ultrasonography to locate the best place for a kidney biopsy.
All types of urinary tract stones, including those that do not contain calcium, may be detected by ultrasonography, although stones smaller than 5 mm (about ¼ inch) may be missed. When doctors suspect that the flow of urine from the bladder is obstructed, they sometimes use ultrasonography to measure the amount of urine that remains in the bladder after a person makes every effort possible to urinate. Although doctors may be able to identify a bladder tumor using ultrasonography, CT is more reliable.
Computed Tomography: Computed tomography (CT) is used to evaluate kidney masses. Spiral CT, performed by continuous movement of a person through the CT scanner, permits special images of certain structures and more rapid completion of the scanning process. Spiral CT is often used in the initial evaluation of people suspected of having kidney stones because all stones can be seen with this technique. Spiral CT without the use of a radiopaque dye is useful for the investigation of stones or bleeding into the kidney or surrounding tissues. A radiopaque dye is often used to enhance images with spiral CT to evaluate obstruction of the kidney arteries.
Magnetic Resonance Imaging: Magnetic resonance imaging (MRI) can provide three-dimensional images of the kidneys, blood vessels, and structures surrounding the kidneys. MRI helps distinguish tumors from cysts. When used with a paramagnetic contrast agent to enhance images, MRI can identify disorders of kidney blood vessels.
Intravenous Urography: Intravenous urography uses a radiopaque dye to provide an image of the kidneys, ureters, and bladder. Intravenous urography is sometimes used to locate the site and identify the underlying problem when the flow of urine is obstructed. Intravenous urography can also show abnormal connections (fistulas) between the urinary tract and the skin or other organs. Intravenous urography does not work well in people with poorly functioning kidneys because the kidneys cannot excrete and concentrate the radiopaque dye normally.
A cystogram, an x-ray image of the bladder, is obtained as part of intravenous urography. When films of the bladder and urethra are taken during and immediately after urination, the study is called a voiding cystourethrogram, which is especially useful in evaluating recurring urinary tract infections.
Retrograde Urography: In retrograde urography, a radiopaque dye similar to that used in intravenous urography is injected directly through a scope or catheter passed through the bladder and into the ureter. This technique provides good images of the bladder, ureters, and renal pelvis when intravenous urography has been unsuccessful. Retrograde urography is also useful in investigating an obstruction of a ureter. Disadvantages include the risk of infections and the need for anesthesia.
Radionuclide Scanning: A radionuclide scan of the kidneys is an imaging technique that relies on the detection of small amounts of radiation by a special gamma camera after the injection of a radioactive chemical. One type of radionuclide scan assesses kidney blood flow (renogram). Other types of scans are useful in evaluating other kidney problems.
Angiography: Angiography involves injecting a radiopaque dye into an artery. Because it has higher risks than all other kidney imaging procedures, angiography is reserved for special situations, such as the assessment of the blood supply to the kidneys. Complications of angiography may include injury to the injected arteries and neighboring organs, bleeding, an allergic-like reaction to the radiopaque dye, and acute kidney failure triggered by the radiopaque dye.
See the sidebar Problems With Using Radiopaque Dye.
Cystoscopy
A doctor can diagnose some disorders of the bladder and urethra by looking through a flexible viewing tube (cystoscope, a type of endoscope). A cystoscope, which has a diameter about the size of a pencil, may be between 1 and 5 feet in length. Most contain a light source and a small camera, which allows the doctor to view the inside of the bladder and urethra. Many cystoscopes also contain a small clipping device on the tip, allowing the doctor to obtain a biopsy of the bladder lining.
Tissue and Cell Sampling
Kidney Biopsy: A kidney biopsy (in which a sample of kidney tissue is removed and examined under a microscope) is primarily used to help a doctor diagnose disorders that affect the specialized blood vessels of the kidney (glomeruli) or unusual causes of acute kidney failure. A biopsy is often performed on a transplanted kidney to look for signs of rejection.
When undergoing a kidney biopsy, the person lies face down, and a local anesthetic is injected into the skin and muscles of the back over the kidney. Ultrasonography or CT is used to locate the part of the kidney where the glomeruli are located and to avoid large blood vessels. The biopsy needle is inserted through the skin and into the kidney.
This procedure is not recommended for anyone with uncontrolled high blood pressure, bleeding disorders, active urinary tract infection, or only one kidney (except for a transplanted kidney). Complications include bleeding around the kidney and formation of small arteriovenous fistulas (abnormal connections between very small arteries and veins within the kidney).
Urine Cytology: Urine cytology, which is a microscopic examination of the urine to look for cancer cells, is sometimes useful in diagnosing cancers of the kidneys and urinary tract. For people at high risk--for example, smokers, petrochemical workers, and people with painless bleeding--urine cytology may be used to screen for cancer of the bladder and kidneys. For people who have had a bladder or kidney tumor removed, the technique may be used for follow-up evaluation. However, the results can sometimes indicate cancer when none is present, or they can fail to indicate cancer when it is present, especially if the cancer is very new or very slow growing.
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