Kidney Function Tests

What are kidney function tests?

Kidney function tests are urine (pee) and/or blood tests that evaluate how well your kidneys work. Your kidneys support your overall health by getting rid of waste and balancing body fluids and electrolytes. Most kidney function tests measure how well your glomeruli (glo-MARE-yoo-lye) work. Your glomeruli are tiny filters in your kidney that help clean your blood. The tests measure how efficiently glomeruli clear wastes from your blood.

Kidney function tests can make you feel a little anxious. It’s hard for some people to relax for a blood draw, and it can feel weird peeing into a cup and handing it over to a healthcare provider. But they’re an important tool in monitoring your kidney health. Providers understand these feelings and will do their best to make you feel comfortable.

Another name for kidney function tests is renal function tests.

What do your kidneys do?

Your kidneys are part of your urinary system. Most people have two kidneys. They typically sit in the back of your abdomen, below your ribcage on either side of your spine. They help your body filter waste products and remove them as pee.

Your kidneys are also important for producing:

• Hormones that regulate blood pressure
• Erythropoietin, which is important for red blood cell production; red blood cells carry oxygen throughout your body
• Vitamin D, which maintains bone and muscle health

Why might I need a kidney function test?

Some conditions, like diabetes or high blood pressure (hypertension), affect how well your kidneys work. If you have a condition that affects your kidneys, a healthcare provider may use kidney function tests to help monitor the condition and your overall kidney health.

You may also need a kidney function test if you have symptoms that may indicate kidney problems. These symptoms may include:

• Blood in your pee (hematuria)
• Foamy urine (may indicate proteinuria)
• Pain or burning when you pee (dysuria)
• Frequent urges to pee
• Peeing less than expected (oliguria)
• Problems starting to pee
• Body swelling (edema)
• Feeling very tired (fatigue)
• Nausea and vomiting
• Loss of appetite

What are the types of kidney function tests?

A healthcare provider may recommend one or more different types of kidney function tests. You may have blood tests for kidney function, such as:

• Blood urea nitrogen (BUN). This test measures nitrogen in your blood. Protein makes nitrogen when it breaks down.
• Estimated GFR (eGFR). This test measures your filtration rates according to your protein levels, age, sex, height and weight.
• Serum creatinine. This test looks for creatinine buildup. When your muscle tissue breaks down, it creates the waste product creatinine.

A provider may also recommend 24-hour urine tests, including:

• Microalbuminuria. This test specifically looks for the protein albumin.
• Urinalysis. This test evaluates your pee for blood, proteins, urine acidity, specific gravity and the presence of tiny, tubelike particles (casts) and crystals.

How do I prepare for kidney function tests?

It depends on exactly what kind of kidney function test a healthcare provider recommends.

For some blood tests, you may need to:

• Avoid eating or drinking (fast) for several hours before the test. A provider will let you know if you need to fast.
• Moisturize your arms. Moisturizing can make it easier for a provider to insert the needle and acquire your blood sample.

Let the provider know if you’re afraid of needles (trypanophobia). They can recommend coping strategies to make yourself as comfortable as possible during the blood test.

You usually don’t have to do anything to prepare for a urine test. Make sure you drink an extra glass of water before the test to ensure you can provide a sample that’s large enough for testing. It’s also a good idea to tell a provider if you’re currently getting your period (menstruating). The presence of menstrual blood and other vaginal discharge may affect your results.

What happens during a kidney urine test?

You complete 24-hour urine tests at home. A healthcare provider will give you a container to collect pee. On the day of the test:

• Pee into the toilet as usual after you first wake up.
• For the rest of the day, pee into the container.
• On day two, pee into the container when you first wake up.
• Complete the test by dropping your sample off at the provider’s office or lab.

What happens during a kidney blood test?

For a kidney blood test, you visit a healthcare provider’s office or a lab to give a blood sample. A provider who has blood-draw training (phlebotomist) uses a small needle and test tube to collect your blood sample — usually from a vein in your arm.

What can I expect after a kidney function test?

Kidney function tests are simple and rarely pose any long-term risks. But after blood tests, some people experience:

• Bruising
• Dizziness
• Feeling very tired

Call a provider if you have pain, fatigue or other symptoms that don’t go away.

What do the results of my kidney function test mean?

Kidney function test results can tell you whether your kidneys are functioning as expected or not. Most kidney function tests look for two measurements: estimated glomerular filtration rate and urine albumin-creatinine ratio (uACR).

Estimated glomerular filtration rate

Healthcare providers measure your eGFR in milliliters of cleansed blood per minute per body surface. For adults, a normal eGFR is about 100 or higher. An eGFR between 60 and 100 means you have mild kidney damage, but your kidneys continue to function well. An eGFR of less than 60 may indicate that you have chronic kidney disease (CKD).

What should my eGFR be for my age?

Your eGFR gets lower as you get older. This happens naturally, even if you don’t have chronic kidney disease. The following chart indicates the average eGFR for certain age ranges:

Urine albumin-creatinine ratio

The uACR measures how much albumin and creatinine are in your pee. Providers measure the amounts of albumin and creatinine in your pee to each other in milligrams to grams (mg/g). A typical uACR is less than 30 mg/g.

What level of urine albumin-creatinine indicates kidney failure?

A uACR of 300 mg/g or higher means you’re at a very high risk of kidney failure. You’re also at a greater risk of heart attack (myocardial infarction) or stroke.

When should I know the results of the kidney function tests?

The time it takes you to receive your test results can vary. Sometimes, a provider discusses the results of your test on the same day. Or they may send your test to a lab, and you’ll receive the results in a few days.

What if my kidney function test results show early kidney disease?

If your kidney function test results show early kidney disease, a healthcare provider will follow up with you about treatment options. They may prescribe antihypertensives to manage high blood pressure. They may also refer you to a healthcare provider who specializes in diagnosing and treating conditions that affect your kidneys (nephrologist).

If your test results are unexpected or atypical, a provider will likely order more frequent kidney function tests in the future. Regular testing helps them track your health and any underlying conditions.

When should I call a healthcare provider?

If you have chronic kidney disease or are at risk of CKD, call a healthcare provider if you have any of the following symptoms:

• Peeing more than usual
• Bloody pee
• Foamy pee
• Extreme fatigue
• Muscle spasms or cramps
• Nausea or vomiting
• Shortness of breath (dyspnea)
• Swelling, especially in your hands, feet or ankles

Can drinking water lower creatinine?

Drinking a lot of water right before you take a kidney blood test can lower your creatinine levels for a short period. But it’s very important to know that this only temporarily affects your kidney blood test results. Drinking a lot of water can’t reverse kidney damage or have positive long-term effects on your creatinine levels.

If you’re in the early stages of CKD, you should work with a nephrologist and kidney (renal) dietitian to develop kidney-friendly eating patterns.