The most common blood test for kidney function is called a renal function panel (also known as a kidney function panel). It measures several markers in your blood, with the three most important being creatinine, blood urea nitrogen (BUN), and estimated glomerular filtration rate (eGFR). Your doctor may also order a basic metabolic panel, which includes kidney markers alongside other measurements like blood sugar and electrolytes.
The Three Core Kidney Markers
Creatinine is the workhorse of kidney testing. It’s a waste product created by the normal breakdown of muscle tissue and the digestion of protein. Healthy kidneys filter creatinine out of your blood and get rid of it through urine. When your kidneys aren’t working well, creatinine builds up. Normal serum creatinine ranges from 0.74 to 1.35 mg/dL for adult men and 0.59 to 1.04 mg/dL for adult women, according to the Mayo Clinic.
Blood urea nitrogen (BUN) is another waste product that comes from protein breakdown. Like creatinine, it’s removed by the kidneys, so rising BUN levels can signal that your kidneys are struggling. BUN can also rise from dehydration or a high-protein diet, so it’s rarely interpreted on its own.
The estimated glomerular filtration rate, or eGFR, is the number your doctor pays closest attention to. It isn’t measured directly from your blood. Instead, it’s calculated using your creatinine level, age, and sex. The result estimates how efficiently your kidneys are filtering waste, measured in milliliters per minute. A normal eGFR is 90 or above. The lower the number, the worse your kidneys are functioning.
What Your eGFR Number Means
Doctors use eGFR to classify chronic kidney disease into five stages, each tied to a specific range:
- Stage 1 (eGFR 90+): Kidney damage is present (often detected through protein in the urine), but the kidneys still function normally.
- Stage 2 (eGFR 60–89): Mild loss of kidney function.
- Stage 3a (eGFR 45–59): Mild to moderate loss of function.
- Stage 3b (eGFR 30–44): Moderate to severe loss of function.
- Stage 4 (eGFR 15–29): Severe loss of function.
- Stage 5 (eGFR below 15): Kidney failure.
A single eGFR result doesn’t automatically mean kidney disease. Your doctor will typically retest over weeks or months to see whether low numbers persist before making a diagnosis.
Cystatin C: A More Accurate Alternative
Creatinine levels naturally vary based on how much muscle you have, which can skew eGFR results. If you’re a bodybuilder, older, or your initial results were borderline or unclear, your doctor may order a cystatin C blood test instead. Cystatin C is a protein produced steadily by cells throughout your body, and it doesn’t fluctuate with muscle mass the way creatinine does. Your eGFR can be recalculated using cystatin C alone or in combination with creatinine for a more reliable estimate.
The current recommended formula for calculating eGFR, adopted in 2021, uses creatinine, age, and sex. Earlier versions included a race-based adjustment that has since been removed from the standard equation recommended by the National Kidney Foundation.
Additional Markers on a Kidney Panel
Beyond creatinine, BUN, and eGFR, a renal function panel often includes electrolytes that your kidneys help regulate. Phosphate is one of the most telling. Healthy kidneys filter extra phosphate out of your blood, so when kidney function declines, phosphate levels climb. High phosphate pulls calcium out of your bones over time, weakening them. That’s why calcium, vitamin D, and parathyroid hormone levels are frequently checked alongside phosphate in people with known kidney disease.
Potassium and sodium are also commonly included because damaged kidneys lose the ability to keep these electrolytes in balance. Dangerously high or low potassium, in particular, can affect your heart rhythm, which is why it’s monitored closely.
How to Prepare for the Test
A kidney blood test is a simple blood draw, typically from a vein in your arm. Whether you need to fast beforehand depends on what your doctor orders. A standalone renal function panel may not require fasting, but if it’s bundled with a basic metabolic panel or other tests, your doctor might ask you to skip food for 8 to 12 hours. The safest approach is to ask when the test is scheduled.
Let your provider know about any prescription medications, over-the-counter drugs, vitamins, and supplements you’re taking. Some can temporarily affect creatinine or other markers. Don’t stop any medications unless your doctor specifically tells you to.
Renal Panel vs. Basic Metabolic Panel
You might see either name on your lab order, and the overlap can be confusing. A basic metabolic panel (BMP) is a broader test that includes kidney markers (creatinine and BUN) along with glucose, calcium, and electrolytes. A renal function panel is more focused, grouping together the specific waste products and substances tied to kidney health. Both give your doctor useful kidney information, but the renal panel is more targeted when kidney disease is the primary concern or is already being monitored, especially in people with diabetes.