Kidney function is tested primarily through blood tests and urine tests, most of which require nothing more than a standard blood draw and a urine sample. The most important number your doctor looks at is your estimated glomerular filtration rate (eGFR), which measures how many milliliters of blood your kidneys filter per minute. In some cases, imaging or a tissue biopsy may be needed to find the cause of a problem, but routine screening starts with simple lab work.
The eGFR Blood Test
The cornerstone of kidney function testing is the eGFR. Your kidneys contain tiny filters called glomeruli that remove waste and extra water from your blood. The eGFR estimates how efficiently those filters are working by measuring creatinine, a waste product from normal muscle activity that your kidneys are supposed to clear. When your kidneys aren’t filtering well, creatinine builds up in your blood.
A single blood draw is all that’s needed. The lab measures your creatinine level, then plugs it into a formula along with your age, sex, weight, and height to calculate your eGFR. People naturally produce different amounts of creatinine depending on their body size, diet, and activity level, so the formula adjusts for those differences.
Since July 2022, hospitals in the U.S. have been required to use race-neutral eGFR formulas. Previous calculations included a variable for Black patients that could overestimate their kidney function by as much as 16 percent, potentially delaying diagnosis and access to transplant lists. That variable has been removed from current guidelines.
What eGFR Numbers Mean
Your eGFR result maps directly to a stage of chronic kidney disease (CKD). Here’s how the National Kidney Foundation breaks it down:
- 90 or above: Normal kidney function (Stage 1 if other signs of damage exist, such as protein in urine)
- 60 to 89: Mild loss of function (Stage 2)
- 45 to 59: Mild to moderate loss (Stage 3a)
- 30 to 44: Moderate to severe loss (Stage 3b)
- 15 to 29: Severe loss (Stage 4)
- Below 15: Kidney failure (Stage 5)
A single eGFR reading doesn’t tell the whole story. Your doctor will typically repeat the test and combine it with urine results before reaching any conclusions.
The BUN Test
A blood urea nitrogen (BUN) test is often ordered alongside creatinine. When your body breaks down protein from food, it produces urea as a byproduct, and your kidneys are responsible for clearing it. High BUN levels can signal kidney trouble, but this test is less specific than eGFR because many other factors raise BUN: a high-protein diet, dehydration, stress, burns, or even certain medications. Normal ranges vary by age and sex. For adult women, the typical range is 6 to 21 mg/dL; for adult men, 8 to 24 mg/dL.
Because BUN is so easily influenced by non-kidney factors, doctors don’t rely on it alone. It’s most useful as a supporting data point alongside eGFR and urine tests.
Urine Albumin-to-Creatinine Ratio
Healthy kidneys keep protein in your blood and out of your urine. When the filters are damaged, a protein called albumin starts leaking through. The urine albumin-to-creatinine ratio (uACR) measures how much albumin is escaping. You provide a urine sample, and the lab calculates the ratio.
A normal result is less than 30 mg/g. A reading between 30 and 299 mg/g indicates moderately increased albumin loss and raises your risk of kidney failure, heart failure, and stroke. A result of 300 mg/g or higher, confirmed on a repeat test, typically points to kidney disease. Importantly, you can have a uACR above 30 even when your eGFR looks normal, which is why both tests are recommended together for a complete picture.
The 24-Hour Urine Collection
For certain patients, doctors need a more precise measurement than a single blood draw can provide. A 24-hour urine collection compares the creatinine in your blood to the total amount in all the urine you produce over a full day. This gives a direct creatinine clearance rate rather than an estimate.
The process is straightforward but requires discipline. You’ll receive a collection container and collect every drop of urine for a 24-hour period. Missing even a single trip to the bathroom can throw off the results. This test is sometimes used when eGFR calculations may not be accurate, for example in people with unusual muscle mass, extreme diets, or certain medical conditions.
Cystatin C: A Second Opinion Blood Test
Creatinine-based eGFR has a limitation: it’s heavily influenced by muscle mass. For people whose creatinine levels may not reflect their true kidney function (very muscular individuals, people with muscle-wasting conditions, or those on certain diets), a second blood marker called cystatin C can help. Cystatin C is produced by nearly all cells in the body at a steady rate, making it less affected by body composition.
Guidelines from the Kidney Disease: Improving Global Outcomes (KDIGO) workgroup recommend cystatin C for confirmatory testing when creatinine alone isn’t reliable enough for clinical decisions. An eGFR calculated using both creatinine and cystatin C together is generally considered more accurate than either marker on its own.
Imaging and Kidney Biopsy
When blood and urine tests reveal a problem but don’t explain the cause, your doctor may order imaging. An ultrasound is the most common first step. It shows the size, shape, and structure of your kidneys and can identify blockages, cysts, or stones without any radiation.
If imaging still doesn’t provide a clear answer, a kidney biopsy may be recommended. During this procedure, a doctor uses an ultrasound to guide a needle into the kidney and removes a small sample of tissue. The sample is examined under a microscope to identify the specific type of kidney disease, determine how quickly it’s progressing, or evaluate whether a current treatment is working. Biopsies are also used to monitor transplanted kidneys that aren’t functioning as expected.
Not everyone with abnormal lab results needs a biopsy. The decision depends on your symptoms, the severity of your results, and your overall health. Common triggers include blood in the urine originating from the kidney, excessive or rising protein levels, or unexplained buildup of waste products in the blood.
Preparing for Kidney Function Tests
Most basic kidney blood tests don’t require fasting, but your doctor will let you know if yours does. One thing worth knowing: biotin supplements can interfere with lab results. Doses of 5 mg or higher can cause falsely high or falsely low readings depending on the type of assay the lab uses. If you take a standard biotin supplement (5 to 10 mg), wait at least 8 hours before your blood draw. People on high-dose biotin therapy (100 mg or more per day) should stop taking it at least 72 hours beforehand. This is especially relevant if you have reduced kidney function, since impaired kidneys clear biotin more slowly and the interference may last longer.
For a urine test, you’ll typically be asked to provide a sample in a cup at the office. If a 24-hour collection is ordered, you’ll get specific instructions and collection supplies. The key is capturing every urine output during the full 24 hours so the lab can calculate an accurate clearance rate.