What Are Kidney Function Tests?

Kidney function tests are physical examinations and procedures, used by a medical doctor, to test kidney function and evaluate how well the kidneys are working.

The kidneys are the primary organs of the body's natural filtration system.

Their functions include: removing waste products from the bloodstream, regulating the body's water balance, and maintaining the pH (acidity/ alkalinity) of the body's fluids.

Many conditions can affect the ability of the kidneys to carry out their vital functions. Some lead to a rapid (acute) decline in kidney function, while others lead to a gradual (chronic) decline in kidney function. Both result in the accumulation of toxic wastes and excess fluids in the blood.

It is possible to determine the extent of kidney dysfunction by performing a variety of kidney function tests. These tests primarily involve analyses of the urine and blood. The chemical composition of these two substances (blood and urine) can reveal a great deal of information about the efficiency of the kidneys and how well they are doing their jobs.

The main job of the kidneys is to remove waste products and toxic chemicals from the blood and eliminate them from the body, via the urine. In order to test kidney function, doctors consider the rate at which the kidneys remove waste and toxic products from the blood and the quantity and composition of the chemicals excreted by the kidneys.

The following are some typical kidney tests, which are used to assess kidney function.

  • Urinalysis is often the first test administered, if kidney problems are suspected. A small urine sample is examined physically to analyze its color, odor, appearance and concentration (specific gravity). It is also examined chemically for substances such as protein and glucose, and its pH balance is measured. There is also a microscopic examination for the presence of cellular elements (red and white blood cells and epithelial cells, bacteria, crystals, and casts (structures formed by deposits of protein, cells, and other substances in the kidneys' tubules.

  • Creatinine clearance test evaluates how efficiently the kidneys clear creatinine from the blood. Creatinine is a waste product of muscle energy metabolism. This test attempts to determine kidney function by measuring the rate at which the kidneys' creatinine is removed from the body.

  • Urea clearance test measures the amount of urea that is filtered or cleared by the kidneys into the urine. Urea is a waste product that is created by protein metabolism and excreted in the urine. Like many of the other kidney function tests, the urea clearance test requires urine sample as well as a blood sample. The levels of urea in the blood sample and two urine specimens (collected one hour apart) are measured, to determine the amount of urea cleared by the kidneys.

  • Urine osmolality test is a measurement of the number of dissolved particles in the urine. It is used to evaluate the ability of the kidneys to concentrate or dilute the urine. Kidneys that function normally will excrete more water into the urine, as fluid intake is increased and less water when fluid intake is decreased. This involves a series of urine tests over a period of time (24 hours or so), after changing the patient's diet some days before.

    The inability of the kidneys to concentrate the urine, in response to restricted fluid intake or to dilute the urine, in response to increased fluid intake during osmolality testing, may indicate decreased kidney function.

  • Most of the kidney function tests require urine evaluation. Such is the case with urine protein test. If the kidneys are functioning properly they allow all or most of the proteins to be reabsorbed into the blood. Therefore, the persistent presence of significant amounts of protein in the urine is an indication of decreased kidney function.

In addition to the many kidney tests that require urine evaluation, the blood can also be evaluated to determine kidney function. For example, blood tests which measure the levels of sodium, potassium, chloride, bicarbonate, calcium, magnesium, phosphorus, protein, uric acid, and glucose, can help to determine how well the kidneys are working.

After completing a series of kidney function tests, if it is determined that kidney function is impaired, it may be necessary to perform a full assessment of the kidneys in order to determine the underlying cause of kidney failure and whether the kidney failure is chronic or acute. Kidney tests such as magnetic resonance imaging (MRI), computed tomography (CT) scan, ultrasound, X rays, renal biopsy, and arteriogram (the recording of an arterial pulse) of the kidneys can help to determine the cause of kidney failure and the level of remaining kidney function.

Kidney function tests are most effective in determining whether someone has kidney failure and also measuring the severity of the condition, which is determined by the level of kidney function.