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What is a Kidney Function Test?

By Dulce Corazon
Updated Mar 03, 2024
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A kidney function test is a group of laboratory tests that helps detect problems or disorders in the kidneys. One of these tests is the analysis of urine, commonly known as urinalysis. Blood tests that are included in the kidney function test include measuring for creatinine and blood urea nitrogen (BUN). There is also the creatinine clearance test, which measures both blood and urine over a given time.

The kidneys are important body organs that eliminate waste from the blood and in the maintain fluid balance inside the body. When the kidneys are not functioning well due to injury or diseases, waste products can build up in the blood. Symptoms may then appear such as abdominal pain on the side of the affected kidney, weakness, swelling of the hands or feet, and shortness of breath. Some patients may not even present with symptoms when kidney disease affects only a small part of one or both kidneys. A nephrologist, doctor who specializes in kidney disorders, usually orders a kidney function test to evaluate and diagnose patients suspected of having kidney problems.

Urinalysis is usually the first kidney function test done to detect kidney disorders. The test is performed by collecting an adequate volume of urine from the patient. In the laboratory, the urine sample is then subjected to several processes such as observation of color and clarity, and measurement of protein and sugar. It is also examined under the microscope for presence of red blood cells, bacteria, crystals, and white blood cells. Protein is not usually present in normal urine, and its presence can frequently indicate a kidney problem.

Measurement of creatinine is also an important kidney function test. Creatinine is the end product of muscle metabolism, and is usually excreted in urine. When the kidneys are functioning normally, the amount of creatinine often remains constant and within the normal range. It is usually higher in men, which is about 0.8 to 1.2 mg/dL, than in women, which is between 0.6 to 0.9 mg/dL. This kidney function test is often considered a sensitive and reliable test in the evaluation of kidney disorder.

BUN determination is another kidney function test that can evaluate the filtering capacity of the kidneys. It is a waste product of protein breakdown and is usually excreted in urine. Its value however, can be affected by liver function and protein diet, thus a test for BUN is often done together with the blood creatinine test.

In the creatinine clearance test, the amount of creatinine excreted by the kidneys is measured usually over a 24-hour period. Blood is also collected from the arm vein for creatinine measurement. The results from both tests are then used in the calculation of the creatinine clearance of the patient. This kidney function test is also considered a specific test for kidney evaluation.

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