What Are Kidney Function Tests?

"Kidney function tests" is a collective term used to describe a variety of individual tests and procedures, to evaluate how well the kidneys are functioning. 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 build-up of toxic waste 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 largely comprise various urine and blood tests. Among these kidney function tests are the following:

• 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, glucose and pH. 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 is another of the many kidney function tests. This 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 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.

Kidney function tests are most effective in determining not only if someone has kidney failure but also measuring the severity of the condition, as determined by the level of kidney function.

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