Kidney function is tested primarily through blood tests and urine tests, with the two most important measurements being your estimated glomerular filtration rate (eGFR) and your urine albumin-to-creatinine ratio (uACR). Together, these tests reveal how well your kidneys filter waste and whether they’re leaking protein they shouldn’t be. Most people get results within a day or two from a standard lab draw and urine sample.
The eGFR Blood Test
The eGFR is the single most important number for assessing kidney function. It estimates how many milliliters of blood your kidneys filter per minute, calculated from a simple blood draw that measures creatinine, a waste product your muscles produce at a steady rate. Healthy kidneys remove creatinine from your blood at roughly the same pace your body generates it, so when creatinine builds up, it signals your kidneys are falling behind.
Your lab uses your creatinine level along with your age and sex to calculate an eGFR score. A normal eGFR is 90 or above. Here’s how the stages break down:
- 90 or higher: Normal kidney function
- 60 to 89: Mildly decreased function
- 45 to 59: Mild to moderate decrease
- 30 to 44: Moderate to severe decrease
- 15 to 29: Severely decreased function
- Below 15: Kidney failure
An important detail: an eGFR between 60 and 89, or even 90 and above, doesn’t automatically mean you have chronic kidney disease. Those levels only count as CKD if there’s also evidence of kidney damage, such as protein in your urine or structural abnormalities on imaging. The number alone isn’t the whole picture.
The Urine Albumin Test
Your kidneys normally keep albumin, a protein in your blood, from spilling into your urine. When the tiny filters inside your kidneys are damaged, albumin leaks through. The urine albumin-to-creatinine ratio (uACR) measures exactly how much is escaping, and it can catch kidney damage earlier than a blood test alone.
A normal uACR is less than 30 mg/g. A result between 30 and 299 mg/g means your kidneys are leaking small amounts of protein, which raises your risk of kidney failure, heart failure, and stroke. A uACR of 300 mg/g or higher, confirmed on a repeat test, points to kidney disease. Because uACR findings can be the earliest indicator of chronic kidney disease, catching elevated albumin early opens a window for lifestyle changes and treatments that slow progression.
The test itself is straightforward. You provide a urine sample, often first thing in the morning. No catheter, no special collection container. Your doctor may repeat the test to confirm the result, since temporary factors like intense exercise, fever, or urinary tract infections can cause a one-time spike.
Blood Urea Nitrogen (BUN)
BUN measures another waste product your kidneys are supposed to clear. Your liver produces urea when it breaks down protein, and your kidneys filter it out. A high BUN level generally signals that your kidneys aren’t keeping up, though it’s less specific than eGFR. Dehydration, a high-protein diet, and certain medications can all push BUN up without any actual kidney damage. BUN is most useful when interpreted alongside creatinine and eGFR rather than on its own.
Cystatin C: A More Precise Option
Standard creatinine-based testing has a blind spot. Because creatinine comes from muscle breakdown, people with unusually high or low muscle mass can get misleading results. Someone with significant muscle loss, a limb amputation, or a spinal cord injury may appear to have better kidney function than they actually do, simply because their body produces less creatinine.
Cystatin C is an alternative blood marker that isn’t affected by muscle mass or diet. It’s more sensitive to early changes in kidney function and can detect both acute and chronic kidney disease sooner than creatinine alone. An eGFR calculated from both creatinine and cystatin C together provides the most accurate assessment. If your doctor suspects your creatinine-based eGFR doesn’t reflect your true kidney function, or if you have liver disease, prolonged immobility, or malnutrition, a cystatin C test may give a clearer answer.
At-Home Kidney Tests
Home test kits now let you check for albumin in your urine using a test strip and a smartphone. The National Kidney Foundation evaluated a home testing program that screened nearly 1,500 people at risk for chronic kidney disease, and half showed elevated uACR. These kits measure the same albumin marker that a lab test does, and in the NKF program, results were automatically sent to patients’ medical records so providers could coordinate follow-up.
Home kits are useful for screening, especially if you have diabetes, high blood pressure, or a family history of kidney disease and haven’t been tested recently. They don’t replace a full lab workup. A home test can flag a potential problem, but confirming kidney disease requires blood work for eGFR and a repeat urine test through your provider.
How to Prepare for Kidney Tests
Basic kidney blood panels sometimes require fasting, sometimes not. It depends on what else your doctor orders alongside the kidney markers. If fasting is required, you’ll need to avoid food and drinks other than plain water for 8 to 12 hours beforehand. During that fasting period, also skip gum, smoking, and exercise, all of which can affect results.
Don’t stop any prescription medications before testing unless your provider specifically tells you to. Some drugs can influence creatinine levels or other kidney markers, and your doctor needs to account for that when reading results. Do mention any vitamins and supplements you take, since certain ones can skew test values. For a urine test, no fasting is needed, but your provider may ask you to collect your first urine of the morning for the most reliable reading.
When Testing Goes Beyond Blood and Urine
In most cases, blood and urine tests tell the full story. But when results are confusing or point to something more serious, a kidney biopsy may be needed. This involves removing a tiny piece of kidney tissue with a needle for examination under a microscope. Biopsies are reserved for specific situations: unexplained sudden kidney injury, nephrotic syndrome (when your kidneys dump large amounts of protein), or when a doctor suspects an inflammatory condition attacking the kidneys. A biopsy identifies the exact type of kidney disease, which matters because different diseases require different treatments.
Imaging tests like ultrasound can also play a role. An ultrasound shows the size and shape of your kidneys and can reveal blockages, cysts, or stones that might be causing problems. It’s painless and typically used when there’s a structural question that blood and urine tests can’t answer.
Who Should Get Tested and How Often
Kidney disease is often silent in its early stages, producing no symptoms until significant damage has occurred. Routine screening is especially important if you have diabetes, high blood pressure, heart disease, or a family history of kidney failure. These are the biggest risk factors, and catching a decline at stage 2 or 3a gives you far more options than discovering it at stage 4.
If you’re in a high-risk group, an annual eGFR and uACR is a reasonable baseline. If you’ve already been diagnosed with chronic kidney disease, your provider will likely test more frequently to track the rate of change. A single abnormal result doesn’t mean your kidneys are failing. Trends over time matter more than any individual number, which is why repeat testing is part of the process.