Doctors sometimes need to do tests or procedures to diagnose a kidney or urinary tract disorder.
Urinalysis is testing of the urine. A urine sample is usually collected using the clean-catch method or another sterile method. For example, a method to obtain an uncontaminated urine sample involves passing a catheter through the urethra into the bladder.
Urinalysis can be used to detect and measure the level of various 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 quickly change color. Sometimes, the test results are confirmed with more sophisticated and accurate laboratory analysis of the urine. The urine may be examined under a microscope to check for the presence of red and white blood cells, crystals, and casts (impressions of the kidney tubules created when urinary cells, protein, or both precipitate out in the tubules and are passed in the urine).
Protein in the urine (proteinuria) can usually be detected 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 in the urine (glucosuria) can be accurately detected by dipstick. The most common cause of glucose in the urine is diabetes mellitus, but absence of glucose does not mean a person does not have diabetes or that the diabetes is well controlled.
Ketones in the urine (ketonuria) can often be detected by dipstick. Ketones are formed when the body breaks down fat. Ketones can appear in the urine from starvation, uncontrolled diabetes mellitus, and occasionally after drinking significant amounts of alcohol.
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 in the urine (nitrituria) are also detectable by dipstick. High nitrite levels indicate a urinary tract infection.
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.
The acidity of urine is measured by dipstick. Certain foods, chemical imbalances, and metabolic disorders may change the acidity of urine.
The concentration of urine (also called the osmolality or specific gravity) can vary widely depending on whether a person is dehydrated, how much fluid a person has drunk, and other factors. Urine concentration is also sometimes 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 test, a person receives an injection of antidiuretic hormone (also called vasopressin). 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 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, in which bacteria from a urine sample are grown in a laboratory, are performed to diagnose a urinary tract infection. Cultures are not part of routine urinalysis. The sample of urine must be obtained by the clean-catch method or by briefly inserting a sterile catheter through the urethra into the bladder.
Kidney Function Tests
Doctors can assess kidney function by performing tests on blood and urine samples. Creatinine, a waste product, is increased in the blood when the kidney filtration rate is decreased by a large amount. Creatinine clearance—a more accurate test—can be approximated from a blood sample using a formula that relates the creatinine level in the blood to a person's age, weight, and sex. Determining creatinine clearance more precisely requires an accurately timed urine collection in conjunction with the blood 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.
X-rays are usually not helpful in evaluating urinary tract disorders.
Ultrasonography is often the initial imaging technique because it can be done safely even when kidney function is impaired. It is noninvasive and painless and requires no radiopaque contrast agent (see Ultrasonography). Ultrasonography provides some indirect information about kidney function, is an excellent way to estimate kidney size and position, readily detects obstruction, and helps diagnose structural abnormalities. Ultrasonography is not as accurate as computed tomography (CT) in the diagnosis of kidney tumors. Doctors also use ultrasonography to locate the best place for a kidney biopsy.
Urinary tract stones may be detected by ultrasonography, although stones smaller than about ¼ inch (5 millimeters) 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. Ultrasonography is not as accurate as CT in the diagnosis of bladder tumors.
Computed Tomography (see ):
CT is used to evaluate kidney masses. Helical CT (sometimes called spiral CT), performed by continuously moving the person through the CT scanner, permits special images of certain structures and more rapid completion of the scanning process. Helical CT without the use of a radiopaque contrast agent is useful for people who may have kidney stones or for people who have suffered trauma in whom bleeding into the kidney or surrounding tissues must be identified rapidly. A radiopaque contrast agent is often used in CT examinations. The intravenous contrast agent provides extra detail about the kidney arteries and veins, about certain kidney tumors (such as renal cell cancer), and about polycystic kidney disease. Use of contrast agents may result in allergic-type reactions or, rarely, kidney damage (see see Side Effects).
Magnetic Resonance Imaging (see ):
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. People who require evaluation of the kidney blood vessels and who are at risk of reactions to radiopaque contrast agents can undergo MRI rather than CT.
Intravenous urography (IVU, also called intravenous pyelography or IVP) uses a radiopaque contrast agent given through a vein to provide an x-ray image of the kidneys, ureters, and bladder. Usually, an ultrasound, CT scan, or MRI scan is done instead. However, IVU can better detect small abnormalities of the ureters and some abnormalities of the kidneys. IVU is often done for people with blood in the urine, even if the blood is not visible to the naked eye. It is also often done for people who doctors suspect may have cancer involving the ureters or other urinary passages. Use of contrast agents may result in allergic-type reactions or, rarely, kidney damage.
In cystourethrography, a radiopaque contrast agent similar to that used in IVU is injected directly through a scope or catheter passed through the urethra and into the bladder. When x-ray 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. Cystourethrography may result in infection. Use of contrast agents may result in allergic-type reactions or, rarely, kidney damage.
Radionuclide Scanning (see ):
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 study assesses kidney blood flow (renogram). Radionuclide scans are useful in evaluating other kidney problems.
Angiography (see ):
Angiography involves injecting a radiopaque contrast agent into an artery. Because it involves inserting a catheter into an artery and injecting the contrast agent under high pressure, angiography has higher risks than all other kidney imaging procedures. Thus, angiography is reserved for special situations (such as prior to balloon angioplasty and following angioplasty for the placement of a stent) to hold one of the kidney arteries open or to provide detailed information about the kidney arteries before kidney surgery. Complications of angiography may include injury to the injected arteries and neighboring organs, bleeding, and reactions to radiopaque contrast agents.
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 (30 to 150 centimeters) in length, but only 6 to 12 inches (about 15 to 30 centimeters) of the scope are inserted into the urethra and bladder. 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 sample (biopsy) of the bladder lining. Cystoscopy can be done while a person is awake and causes only minor discomfort. The doctor usually inserts an anesthetic gel into the urethra before the procedure. Possible complications include bleeding in the urine and, rarely, perforation of the bladder.
Tissue and Cell Sampling
A kidney biopsy (in which a sample of kidney tissue is removed and examined under a microscope) is primarily used to help the doctor diagnose disorders that affect the specialized blood vessels of the kidney (glomeruli) and tubules and 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 into the urine around the kidney and formation of small arteriovenous fistulas (abnormal connections between very small arteries and veins) within the kidney.
Urine cytology (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. 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 slow growing.
Last full review/revision March 2007 by Ralph E. Cutler, MD