A kidney function test is a group of blood and urine tests that measure how well your kidneys filter waste from your blood. The most important number these tests produce is your estimated glomerular filtration rate (eGFR), which tells you how many milliliters of blood your kidneys clean per minute. Together with a few other markers, these tests can detect kidney disease early, track its progression, and help guide treatment decisions.
What These Tests Actually Measure
Kidney function testing isn’t a single test. It’s a panel that typically includes both blood draws and urine samples, each measuring something different about how your kidneys are performing.
Creatinine is a waste product your muscles produce constantly. Healthy kidneys filter it out of your blood efficiently, so when creatinine builds up in a blood sample, it signals that your kidneys may not be keeping up. There’s no single “normal” creatinine level because it varies with your age, sex, and body size. Someone with more muscle mass naturally produces more creatinine, which is why this number is never interpreted on its own.
Blood urea nitrogen (BUN) is another waste product, created when your body breaks down protein. Like creatinine, BUN levels rise when kidney filtering slows down. BUN also tends to increase naturally as you age. On its own, a BUN result isn’t very useful. Your provider will compare it to your creatinine and eGFR to get a clearer picture.
eGFR is the centerpiece of kidney function testing. Rather than being measured directly (which requires a complicated procedure), it’s calculated from your creatinine level along with your age and sex. The result estimates how much blood your kidneys filter each minute. Since 2021, the standard calculation in the United States uses the CKD-EPI equation, which was updated to remove a previously used race-based adjustment. This change was recommended by a joint task force from the National Kidney Foundation and the American Society of Nephrology to ensure the formula doesn’t disproportionately affect any group of patients.
Urine albumin-to-creatinine ratio (uACR) checks for a protein called albumin leaking into your urine. Healthy kidneys keep albumin in the blood, so finding it in urine is an early warning sign. A normal uACR is less than 30 mg/g. Results between 30 and 299 mg/g indicate a higher risk of kidney failure, heart failure, or stroke. A result of 300 mg/g or higher, confirmed on a repeat test, may mean kidney disease is already present and puts you at severe risk of kidney failure or heart attack.
When and Why You Might Need One
Kidney disease often causes no symptoms in its early stages, which is why these tests are so valuable for people with risk factors. Diabetes is the leading cause of kidney disease, and high blood pressure is the second. If you have either condition, your provider will likely order kidney function tests regularly to catch any decline before it becomes serious.
Other reasons a provider might order these tests include a family history of kidney disease, heart disease, obesity, or if you’re taking medications that can stress the kidneys over time. Symptoms that sometimes prompt testing include swelling in your hands or feet, changes in how often you urinate, persistent fatigue, foamy urine, or unexplained nausea.
How eGFR Maps to Kidney Disease Stages
Your eGFR result places your kidney function on a scale that doctors use to stage chronic kidney disease (CKD). The stages, defined by the National Kidney Foundation, look like this:
- Stage 1 (eGFR 90 or above): Normal filtering, but other signs like protein in the urine suggest possible kidney damage.
- Stage 2 (eGFR 60 to 89): Mild loss of function.
- Stage 3a (eGFR 45 to 59): Mild to moderate loss.
- Stage 3b (eGFR 30 to 44): Moderate to severe loss.
- Stage 4 (eGFR 15 to 29): Severe loss of function.
- Stage 5 (eGFR below 15): Kidney failure.
An eGFR above 90 with no albumin in the urine generally means your kidneys are working fine. Notice that Stage 1 can still have a normal eGFR. That’s why the urine albumin test matters so much: it can reveal damage that eGFR alone would miss.
Cystatin C: A More Precise Option
Because creatinine levels are influenced by muscle mass and other non-kidney factors, creatinine-based eGFR isn’t always reliable. In those cases, your provider may order a cystatin C test. Cystatin C is a small protein produced by nearly all cells in your body at a steady rate, making it less affected by things like body composition or diet.
The KDIGO guidelines (an international standard for kidney care) recommend cystatin C as a confirmatory test when creatinine-based results seem off. An eGFR calculated from both creatinine and cystatin C together is generally accepted as more accurate than either marker alone. This combination is especially useful for people with unusually high or low muscle mass, older adults, and anyone whose creatinine-based result seems inconsistent with their overall health picture.
How to Prepare
Whether you need to fast depends on the specific tests ordered. A renal function panel sometimes requires fasting for 8 to 12 hours beforehand, but your provider will tell you. If you do need to fast, that means no food, juice, coffee, soda, or flavored water. Plain water is fine and encouraged.
For the urine portion, there’s usually no special preparation. Drinking an extra glass of water beforehand helps ensure you can provide a large enough sample. If you’re menstruating, let your provider know, since menstrual blood can affect urine test results.
A few other things to keep in mind: if you take a creatine supplement (commonly used for muscle building), you’ll likely need to stop before the test because it can artificially raise creatinine levels. Your provider may also ask you to avoid eating meat for a set period before testing, since dietary protein can influence results. Don’t stop any prescription medications unless your provider specifically tells you to, but do mention all vitamins, supplements, and over-the-counter drugs you’re taking.
What Can Skew Your Results
Creatinine is the test most vulnerable to non-kidney influences. People with high muscle mass, such as athletes or bodybuilders, naturally have higher creatinine levels that don’t reflect kidney problems. Conversely, someone with very low muscle mass, like an older adult or someone with a chronic illness, might have creatinine levels that look normal even when kidney function is declining. This is one of the main reasons eGFR is calculated rather than relying on raw creatinine numbers, and why cystatin C testing exists as a backup.
Diet also plays a role. A high-protein meal the night before a blood draw can temporarily bump creatinine and BUN levels. Dehydration can concentrate waste products in your blood and make kidney function appear worse than it is. Even intense exercise shortly before a test can temporarily elevate creatinine. A single abnormal result doesn’t necessarily mean kidney disease. Providers typically repeat the test or order additional markers before drawing conclusions.
What Happens After Testing
If your results come back normal, your provider may simply recommend retesting at routine intervals, especially if you have diabetes or high blood pressure. If your eGFR is mildly reduced or your uACR is elevated, the first step is usually a repeat test in a few weeks to confirm the finding. A single abnormal reading can reflect temporary factors like dehydration or a recent high-protein meal.
Confirmed abnormal results lead to further evaluation. Your provider may order imaging, additional blood work, or a referral to a nephrologist (kidney specialist) depending on the stage. For early-stage kidney disease, management often focuses on controlling blood pressure and blood sugar, adjusting medications that may stress the kidneys, and dietary changes to reduce the workload on your kidneys. The earlier kidney disease is caught, the more effective these strategies are at slowing its progression.