The two core tests used to check kidney function are a blood test that estimates how well your kidneys filter (called eGFR) and a urine test that measures whether protein is leaking into your pee (called uACR). Together, these two tests can detect kidney disease early, even before you feel any symptoms. Depending on the results, your doctor may order additional blood work, imaging, or in rare cases, a biopsy.
eGFR: The Main Blood Test
Your kidneys filter your blood constantly, removing waste and excess fluid. The glomerular filtration rate, or GFR, measures how much blood your kidneys filter per minute. Since GFR can’t be measured directly in a routine visit, labs estimate it using a blood sample. That estimate is your eGFR.
To calculate eGFR, a lab measures creatinine in your blood. Creatinine is a waste product your muscles produce naturally. Healthy kidneys clear it efficiently, so when creatinine builds up in your blood, it signals that your kidneys are filtering less effectively. The lab plugs your creatinine level into a formula (the 2021 CKD-EPI equation, now recommended by the National Kidney Foundation) along with your age and sex to produce your eGFR number. Race is no longer factored into the current equation.
Your eGFR result is reported in mL/min/1.73 m², but the number itself is what matters most. Here’s how it maps to kidney function:
- 90 or above: Normal kidney function (though kidney damage can still exist if protein is found in urine)
- 60 to 89: Mild loss of function
- 45 to 59: Mild to moderate loss
- 30 to 44: Moderate to severe loss
- 15 to 29: Severe loss
- Below 15: Kidney failure
A single eGFR reading below 60 doesn’t automatically mean chronic kidney disease. Your doctor will typically repeat the test after a few months to see if the number persists before making a diagnosis.
What Can Throw Off Your Creatinine Results
Because eGFR depends on your creatinine level, anything that changes creatinine independently of your kidneys can skew the result. Factors that raise creatinine and make your kidney function look worse than it is include eating large amounts of cooked meat, taking creatine supplements, recent high-intensity exercise, and having a very muscular build. Certain medications, including trimethoprim (an antibiotic) and some antiviral drugs, can also bump creatinine up without reflecting real kidney damage.
On the flip side, factors that lower creatinine and make your kidneys look healthier than they are include a vegan or vegetarian diet, low muscle mass, pregnancy, a history of amputation, muscle-wasting conditions, and severe liver disease. If any of these apply to you, your doctor may order an additional marker called cystatin C for a more accurate picture.
Cystatin C: A More Precise Backup
Cystatin C is a protein produced by nearly all cells in your body at a steady rate. Unlike creatinine, it isn’t meaningfully influenced by muscle mass, diet, age, or sex. That makes it especially useful when creatinine-based eGFR might be unreliable.
Current guidelines from KDIGO (a leading kidney disease organization) recommend starting with creatinine-based eGFR for most people. But when factors like unusual muscle mass, extreme body weight, or other conditions compromise accuracy, labs can measure cystatin C and combine it with creatinine in a single equation. This combined approach reduces error compared to either marker alone. It’s specifically recommended for people with a BMI above 40, older adults being evaluated for transplant or dialysis access, and certain cancer patients.
Urine Albumin-to-Creatinine Ratio (uACR)
While the blood test measures how well your kidneys filter, the urine test reveals whether your kidneys are leaking something they shouldn’t be. Albumin is a protein that healthy kidneys keep in your blood. When the kidney’s filtering units are damaged, small amounts of albumin slip through into your urine. The uACR test compares how much albumin and creatinine are in a single urine sample, reported in milligrams per gram (mg/g).
The ranges are straightforward:
- Below 30 mg/g: Normal. Very low risk of kidney failure, heart attack, or stroke.
- 30 to 299 mg/g: Moderately increased albumin. Higher risk of kidney failure, heart failure, or stroke.
- 300 mg/g or higher: Severely increased albumin. If confirmed on a repeat test, this typically indicates kidney disease and a significantly elevated risk of kidney failure.
This test is routinely ordered for people with diabetes or existing chronic kidney disease, since these conditions are the most common causes of kidney damage. But it can be ordered for anyone when kidney problems are suspected. The uACR can detect damage years before eGFR starts to drop, which is why both tests are often run together.
Blood Urea Nitrogen (BUN)
BUN measures the amount of urea nitrogen in your blood. Urea is another waste product your kidneys are responsible for clearing. A high BUN level generally signals that your kidneys aren’t working well, though it can also rise from dehydration, a high-protein diet, or certain medications. BUN levels vary by sex and tend to increase with age. Doctors often look at BUN alongside creatinine rather than relying on it alone, because it’s less specific to kidney function than eGFR.
Kidney Ultrasound
When blood and urine tests come back abnormal, imaging is often the next step. A kidney ultrasound is painless, uses no radiation, and gives your doctor a look at the size and structure of your kidneys. It can reveal blockages such as kidney stones, cysts, tumors, or structural abnormalities that might be causing or contributing to poor function. It also shows whether your kidneys have shrunk, which can indicate long-standing damage. An ultrasound is typically one of the first imaging tests ordered because it’s quick, safe, and widely available.
Kidney Biopsy
A kidney biopsy, where a small piece of kidney tissue is removed with a needle for examination under a microscope, is reserved for situations where the diagnosis can’t be made any other way. It’s not routine. A biopsy is recommended when the tissue sample could change your treatment plan or clarify how far the disease has progressed.
Common reasons a biopsy might be needed include persistent protein in the urine above 1 g/day with no clear cause, protein levels above 3 g/day in someone without diabetes, blood in the urine alongside rising creatinine, or acute kidney injury that doesn’t resolve within 7 to 14 days after the cause is addressed. In someone with kidney disease that suddenly worsens, or who develops new protein or blood in their urine, a biopsy helps determine whether a treatable condition like inflammation is driving the damage.
A biopsy is avoided when the risks (bleeding, infection) outweigh the likely benefit. For people with clearly diagnosed conditions like diabetic kidney disease with a typical pattern of slow decline, the biopsy wouldn’t change management, so it’s usually skipped.
Preparing for Kidney Tests
For the urine test, you’ll typically provide a single sample at your appointment. No special preparation is needed. For blood tests, your doctor may or may not ask you to fast depending on what else is being measured alongside kidney markers. If fasting is required, it usually means no food or drink besides plain water for 8 to 12 hours beforehand. Avoid gum, smoking, and exercise during the fasting window. Staying well hydrated with plain water actually helps, since it keeps your veins plump and makes the blood draw easier.
Let your doctor know about any supplements you take, especially creatine, and ask whether you should pause any medications before testing. Don’t stop prescribed medications on your own. If you recently did an intense workout or ate a large steak dinner, mention it, since either could temporarily raise your creatinine and make your eGFR appear lower than your true baseline.