Kidney function tests can help find signs of kidney disease early, when it’s easier to treat. If you’re at higher risk for kidney disease, your doctor may recommend these tests to make sure your kidneys are healthy and working correctly. Normal kidney function numbers can help rule out a kidney problem.
If you already have chronic kidney disease (CKD) or another health condition that can damage your kidneys, your doctor may use these tests to keep an eye on how well your kidneys are doing.
So what are normal kidney numbers? Read on to learn about the different types of kidney function tests, what’s normal for each test and what your numbers mean.
How Do Kidney Function Tests Work?
Your kidneys filter waste and extra water out of your bloodstream. Some of that waste then travels to your bladder, so you can pee it out as urine. When your kidneys aren’t working properly, it affects both your blood and your urine — so your doctor can look for signs of kidney problems with blood tests and urine tests.
Four of the common kidney function tests are:
- Estimated glomerular filtration rate (GFR) test
- Blood urea nitrogen (BUN) test
- Serum creatinine test
- Urine tests, including urine albumin
To help you understand your kidney test results, we’ll explain how each test works and the normal levels for each test.
What’s a Normal Glomerular Filtration Rate (GFR)?
The estimated glomerular filtration rate (GFR) test is a blood test that measures how well your kidneys are filtering. It does this by estimating how much blood passes through tiny filters in your kidneys (called glomeruli) in one minute.
First, your doctor measures the level of creatinine (a waste product your kidneys normally filter out) in your blood. Next, they use that number in a calculation that also factors in your age, race, gender, height and weight. The result is a number that estimates how well your kidneys are able to filter waste out of your bloodstream.
Here’s what GFR numbers mean:
- 90 or above is normal — that means you probably don’t have kidney disease.
- Below 60 means you probably have kidney disease.
- Below 15 is a sign of severe kidney problems.
What Are Normal Blood Urea Nitrogen (BUN) Levels?
A blood urea nitrogen (BUN) test measures the amount of urea nitrogen in your blood. Your liver makes urea nitrogen when it breaks down protein from the foods you eat. Then the urea nitrogen floats in your bloodstream until your kidneys filter it out. So high BUN levels may mean your kidneys aren’t working correctly.
BUN is measured in mg/dL (milligrams of urea nitrogen per deciliter of blood).
- Normal BUN levels can range from six to 24 mg/dL. However, each lab that does the testing may use a slightly different reference range for what’s considered normal. And your doctor may consider other factors like your age and general health to decide what BUN level is normal for you.
- If your BUN levels are at the higher end of this range (around 20 mg/dL or more), this may be a sign of kidney problems — but there are also other things that can make your BUN level rise, like eating a lot of protein.
- If your BUN is much higher than the normal range (like around 60 mg/dL), that’s probably a sign of serious kidney problems.
Sometimes doctors combine the BUN and blood creatinine levels to develop a “BUN creatinine ratio.” This ratio compares the amount of urea nitrogen in your blood to the amount of creatinine. Normal BUN creatinine ratios are between 10:1 and 20:1. If yours is outside of this range, that’s probably a sign of a kidney disorder.
What Are Normal Creatinine Levels?
A serum (blood) creatinine test measures the level of creatinine in your blood. Your muscles make creatinine and it stays in your blood until your kidneys filter it out. High creatinine levels may be a sign that your kidneys aren’t filtering as well as they should be.
So what are normal serum creatinine levels? They’re slightly different for men and women.
- For men, normal serum creatinine is between 0.7 and 1.3 mg/dL. A level of 1.4 or higher may be a sign of kidney problems.
- For women, normal serum creatinine is between 0.6 to 1.1 mg/dL. A level of 1.2 or higher may be a sign of kidney problems.
Your doctor may also check the amount of creatinine in your urine and compare it to the amount in your blood. This is called a creatinine clearance test. To do this test, you’ll usually have to collect your urine at home over 24 hours and return it to the doctor’s office or a lab for testing.
What Are Urine Tests for Kidney Function?
Since your kidneys make urine, urine tests can be a good way to check for kidney problems. Your doctor may have you give a urine sample at their office, or they may ask you to collect all your urine for 24 hours and return it for testing.
Two types of kidney urine tests check for albumin — a protein that’s normally in your blood. Healthy kidneys don’t filter albumin out of your blood, so it shouldn’t show up in your urine. If you have albumin in your urine, it may be a sign of kidney problems.
- An albumin dipstick test checks for very small amounts of albumin in your urine. Your doctor will dip a strip of chemically treated paper into your urine sample. If there’s albumin in the sample, the paper changes colors.
- An albumin-to-creatinine ratio (ACR) test compares the amount of albumin in your urine to the amount of creatinine. It’s measured in mg/g (milligrams of albumin per gram of creatinine). A normal ACR is 30 mg/g or lower. An ACR higher than 30 may be a sign of kidney problems.
Remember, if you have one test that doesn’t show normal kidney function numbers, don’t panic. Lots of things can affect kidney function, and it doesn’t necessarily mean that you have kidney disease. Your doctor can use your kidney function numbers along with other tests and exams to find out if you have kidney problems. And together, you can make a treatment plan to protect your kidneys.
Resource Links:
- “Kidney Tests” via MedlinePlus.gov
- “Glomerular Filtration Rate (GFR) Test” via MedlinePlus.gov
- “Blood Urea Nitrogen” via Mayo Clinic
- “Creatinine Tests” via Mayo Clinic
- “Microalbumin Test” via Mayo Clinic
- “Blood Urea Nitrogen” via University of Rochester Medical Centre
- “Blood Urea Nitrogen (BUN) Test” via University of Michigan Health
- “Tests to Measure Kidney Function, Damage and Detect Abnormalities” via National Kidney Foundation
- “Albuminuria: Albumin in the Urine” via NIDDK (National Institute of Diabetes and Digestive and Kidney Diseases)
