Kidney function tests are simple procedures that use either the blood or urine to help identify issues in the kidneys. There are a few different types of kidney function tests that investigate various aspects of kidney function.

A kidney function test may check to see if the kidneys are filtering waste products too slowly. Another type of test may see if the kidneys are leaking proteins into the urine.

A doctor who thinks a person may have a problem with their kidneys may order a kidney function test, but regular testing can be helpful for all adults.

As the National Kidney Foundation note, 1 in 3 adults in the United States is at risk for kidney disease, and many people with early forms of the disease have no symptoms. It is important to diagnose and treat the condition early before it can do any permanent damage.

In this article, learn about a variety of kidney function tests and their results.

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A doctor may recommend kidney function tests to help diagnose kidney conditions.

Kidney function tests target either the blood or urine and measure different aspects of kidney function.

Often, doctors will order more than one test at a time to get a broader picture of the overall kidney function.

The kidneys play an essential role in keeping the body healthy. Their main job is to filter waste materials from the blood and send them out of the body in the urine.

Kidney damage can prevent the kidneys from filtering the waste properly, leaving it to remain in the body and cause dangerous symptoms.

Regular testing may help identify issues such as kidney disease in very early stages, making it possible to halt the progress of the disease.

Doctors may also order other tests to get more information about the kidney, such as imaging tests or a biopsy.

Read on to find out about the various types of kidney function tests.

Urine tests may either require a small urine sample or all of the urine a person produces in a 24-hour period.

Urinalysis

Urinalysis is a broad urine test that helps doctors identify underlying issues or determine which test to use next. Urinalysis may help identify many undesirable particles in the urine, such as:

  • blood
  • pus
  • bacteria
  • sugar
  • protein

If the test comes back positive for one or more of these particles, it may indicate an underlying issue, such as:

Microalbuminuria or albumin-to-creatine ratio tests

These two tests require a small urine sample. They both help identify levels of albumin in the urine.

Albumin is an important protein in the blood. If the kidneys filter too much albumin out in the urine, it may be a sign that they are not doing their job correctly.

A urine albumin result of 30 milligrams per gram (mg/g) or less is normal. Anything higher than this may be a sign of kidney disease.

The microalbuminuria is a much more sensitive test, which can detect even tiny amounts of the protein in the urine.

People who are at higher risk for kidney disease may need to take a microalbuminuria test even if other urine protein tests come back negative.

Creatine clearance test

A creatine clearance test is both a blood and urine test. It involves collecting all the urine a person creates in 24 hours, as well as taking a small blood sample.

Creatine is a waste product in the body that occurs naturally from daily use in the muscles.

Doctors compare the amount of creatine in the urine sample to the levels of creatine in the blood. This comparison shows how much waste the kidneys are filtering out, which may be an indicator of their overall health.

For blood tests, a doctor or nurse will insert a needle into a person's arm to draw a small sample of blood. The person may need to fast beforehand or do the test first thing in the morning.

Serum creatine test

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High serum creatine levels are a potential symptom of kidney disease.

Serum creatine levels that are too high may be a sign that the kidneys are having trouble doing their job. Doctors will also order a serum creatine test as part of the creatine clearance test.

The National Kidney Foundation note that serum creatine levels above 1.2 for women or 1.4 for men may be an early sign that the kidneys are not functioning as they should. These numbers may rise further as kidney disease progresses.

Doctors may also calculate a person's glomerular filtration rate (GFR) using this test to confirm their diagnosis or order more tests to check their results.

The GFR test takes the results of a serum creatine test and adjusts them for a number of other factors, such as age, gender, and race. The typical GFR value is 60 or above. A GFR around 60 or below is a sign of kidney problems.

Blood urea nitrogen test

The blood urea nitrogen (BUN) test checks for other waste products in the blood, such as urea nitrogen.

Urea nitrogen occurs as proteins from food break down, and elevated levels may be a sign that the kidney is not filtering these waste products effectively.

A typical BUN level falls between 7 and 20 milligrams per deciliter. Higher values could be a sign of an underlying condition affecting the kidneys.

With that said, many other things can affect BUN levels, such as medications or antibiotics. A diet that is very high in protein diet may also affect levels.

Doctors will typically compare these results to the results of a creatine test to get a better understanding of how well the kidney is filtering this waste.

Imaging tests may help doctors identify any physical changes to the kidneys, such as injuries or kidney stones.

Ultrasounds

Ultrasound tests use harmless sound waves to take pictures. Doctors may order an ultrasound to look for changes in the shape or position of the kidneys. They may also request an ultrasound to check for tumors or blockages, for example, kidney stones.

CT scans

A CT scan uses a series of X-ray images to create a more in-depth, 3D picture of the kidneys. It may help identify any structural changes or deformations in a kidney.

Sometimes the scan requires injecting the person with a dye, which can be problematic for people who may have kidney disease.

Biopsies

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A lab technician can test a kidney tissue sample.

In some cases, doctors may want to perform a kidney biopsy. This is a process that involves inserting a small needle into the kidney to remove kidney tissue. The doctor will send the tissue to a lab for testing.

Doctors may order a biopsy when they need to identify a specific disease and see how well it may respond to treatment. They may also use a biopsy to determine the progression of kidney disease.

Positive results on multiple tests are signs that there is an underlying issue in the kidneys.

The results of each of these tests help doctors get a better picture of a person's overall kidney health. The tests can also identify markers of kidney disease and justify ordering more tests.

Once they have identified that there is an issue in the kidneys, doctors will work to thoroughly diagnose the problem and develop a treatment plan.

Many possible underlying conditions can lead to the loss of kidney function. A thorough diagnosis is key to finding the correct treatment in each case.

Kidney function tests are a vital part of diagnosing and treating disorders that affect the kidneys. Even if a person does not yet have symptoms, some people may need to undergo regular testing.

The National Institute of Diabetes and Digestive and Kidney Diseases recommend that people at higher risk for kidney issues receive regular kidney function tests. People at high risk include those with:

Regular kidney function tests can help identify problems in the kidneys early and when the outlook is best.

By following a treatment plan, people can help prevent progression of the condition. Working directly with a doctor is the best way to monitor and manage any signs of kidney damage or underlying conditions.