The most common test for kidney function is a blood test that measures creatinine, a waste product your muscles produce at a steady rate. From that single blood draw, a lab calculates your estimated glomerular filtration rate (eGFR), a score that tells you how well your kidneys are filtering. A score below 60 suggests kidney disease, while 90 or above is normal. Most people get this test as part of routine bloodwork, but doctors also use urine tests, electrolyte panels, and sometimes imaging to build a fuller picture.
eGFR: The Core Kidney Score
Your kidneys filter about 50 gallons of blood every day, removing waste and excess fluid. The eGFR estimates how efficiently they’re doing that job, measured in milliliters per minute. Labs calculate it using your creatinine level along with your age and sex. Since 2021, the standard formula (called CKD-EPI 2021) no longer includes a race coefficient, following recommendations from the National Kidney Foundation and the American Society of Nephrology.
The eGFR is the number doctors use to stage chronic kidney disease:
- 90 or above: Normal kidney function (Stage 1 if other signs of damage exist)
- 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)
Creatinine on its own can vary quite a bit from person to person. Someone with more muscle mass naturally produces more creatinine, which is why the eGFR calculation exists: it adjusts for individual differences and gives a more reliable reading than raw creatinine alone.
Urine Albumin Test
Healthy kidneys keep protein in your blood where it belongs. When the filters are damaged, a protein called albumin starts leaking into your urine. A urine albumin-to-creatinine ratio (ACR) catches this early, often before your eGFR drops at all. You can provide a single urine sample for this test, no 24-hour collection needed.
The thresholds are straightforward. Less than 30 micrograms per milligram of creatinine is normal. Between 30 and 300 indicates moderately increased albumin, sometimes called microalbuminuria. Above 300 is clinical albuminuria, a sign of more significant kidney damage. This test is especially important if you have diabetes or high blood pressure, since both conditions damage the kidney’s filtering units over time. Doctors often order an eGFR and a urine albumin test together because the two complement each other: eGFR tells you how well the kidneys are working right now, while albumin in the urine can signal damage before function noticeably declines.
Blood Urea Nitrogen (BUN)
BUN measures how much urea, another waste product, is circulating in your blood. Your liver produces urea when it breaks down protein, and your kidneys are responsible for clearing it. The normal range is roughly 6 to 24 mg/dL.
A high BUN can point to kidney problems, but it’s less specific than eGFR. Dehydration, a high-protein diet, certain medications, heart failure, gastrointestinal bleeding, and even severe burns can all push BUN up without any kidney disease being present. That’s why BUN is typically interpreted alongside creatinine and eGFR rather than on its own. If your BUN is elevated but your eGFR is normal, the cause is more likely something other than kidney damage.
Electrolyte Panels
Your kidneys regulate the balance of minerals in your blood, so when kidney function declines, those levels shift. Potassium and sodium are the two electrolytes doctors watch most closely.
Normal blood sodium falls between 135 and 145 milliequivalents per liter. Low sodium can mean the kidneys are retaining too much water. High sodium may reflect dehydration or a more serious underlying problem. Potassium is especially important because high levels (hyperkalemia) can change your heart rhythm. In advanced kidney disease, the kidneys lose their ability to clear excess potassium efficiently, making regular monitoring essential. Electrolyte testing won’t diagnose kidney disease on its own, but abnormal values often prompt further investigation and help guide treatment once a diagnosis is established.
Cystatin C: A More Precise Option
Creatinine-based eGFR works well for most people, but it has blind spots. Because creatinine comes from muscle, the results can be misleading in people with unusually high or low muscle mass. This includes people with amputations, spinal cord injuries, severe weight loss from cancer or prolonged hospitalization, or those who take creatine supplements or eat very high-protein diets.
Cystatin C is an alternative blood marker produced by nearly all cells in the body at a constant rate, making it less influenced by muscle mass. Current guidelines from the kidney disease organization KDIGO recommend combining creatinine and cystatin C for the most accurate eGFR in several situations: people with a BMI above 40, elderly patients being evaluated for transplant or vascular access, and anyone on medications that are cleared by the kidneys where precise dosing matters. The combined equation is more accurate than either marker alone. About 80 to 90 percent of eGFR values calculated from either single marker fall within 30 percent of the true measured filtration rate, so adding the second marker tightens that estimate.
Your doctor may also order cystatin C as a confirmatory test if your creatinine-based eGFR falls between 45 and 59 with no albumin in your urine. In that borderline zone, confirming the result can determine whether you actually have Stage 3a kidney disease or whether the creatinine reading was simply off.
Imaging and Biopsy
When blood and urine tests flag a problem, imaging often comes next. A kidney ultrasound is noninvasive and can reveal structural issues like cysts, stones, blockages, or kidneys that have shrunk from chronic damage. It’s painless and takes about 20 to 30 minutes.
A kidney biopsy, where a small tissue sample is taken with a needle, is reserved for situations where the diagnosis remains unclear after other testing. Common reasons include unexplained kidney failure, significant protein or blood in the urine without an obvious cause, or suspected autoimmune conditions affecting the kidneys. For people who have received a kidney transplant, a biopsy is often necessary if the transplanted kidney shows signs of dysfunction, such as rising creatinine, swelling, or decreased urine output. Biopsies aren’t routine, but they provide a level of detail that blood and urine tests simply can’t.
How to Prepare for Testing
Most kidney blood tests require minimal preparation, but a few details matter. If your eGFR is being measured, avoid eating cooked meat for four to six hours beforehand, since meat temporarily raises creatinine levels and could skew your results. Some labs may ask you to fast, so check with your doctor ahead of time. If you’re also giving a urine sample, there may be additional instructions about timing or collection. Staying normally hydrated (not overhydrating) is generally fine unless you’re told otherwise.
Results typically come back within a day or two. If your eGFR is normal and your urine shows no albumin, your kidneys are functioning well. If either test is abnormal, your doctor will likely repeat it in a few weeks to confirm the finding before moving on to further workup. A single abnormal result doesn’t always mean kidney disease, since temporary factors like dehydration, illness, or medication changes can affect the numbers.