Good kidney numbers generally mean an eGFR (estimated glomerular filtration rate) of 90 or above, a creatinine level within reference range, and no protein in your urine. But “good” depends heavily on your age, sex, and muscle mass, so a single number without context can be misleading. Here’s what each test measures and what your results actually mean.
eGFR: The Most Important Number
Your eGFR estimates how many milliliters of blood your kidneys filter per minute. It’s calculated from your creatinine blood test, your age, and your sex. A result of 90 or higher is considered normal kidney function, and anything from 60 to 89 is mildly reduced but often still healthy, especially as you get older.
In healthy adults under 35, the average eGFR is around 100. After that, kidneys naturally lose filtering power at a rate of roughly 0.5 to 1.2 points per year. By age 50, the lower end of the normal range drops to about 60 for women and reaches that point around age 55 for men. So an eGFR of 68 in a 70-year-old woman may be perfectly normal aging, while the same number in a 30-year-old would be a red flag.
Here are the average eGFR values by age and sex from a large study of healthy kidney donors:
- Ages 20 to 34: ~100 for men, ~99 for women
- Age 50: ~90 for men, ~87 for women
- Age 60: ~83 for men, ~79 for women
- Age 70: ~77 for men, ~71 for women
- Age 80: ~70 for men, ~63 for women
Since 2021, labs in the United States have used a race-free equation to calculate eGFR. The National Kidney Foundation and American Society of Nephrology recommended removing race from the formula after finding it created disparities in care, particularly for Black patients who were sometimes diagnosed later or placed on transplant lists later than they should have been.
CKD Stages and What They Mean
Chronic kidney disease is classified into stages based on eGFR. Understanding these stages helps you interpret where your numbers fall:
- Stage 1 (eGFR 90+): Normal filtering, but other signs of kidney damage like protein in urine may be present
- Stage 2 (eGFR 60 to 89): Mildly reduced function
- Stage 3a (eGFR 45 to 59): Mild to moderate reduction
- Stage 3b (eGFR 30 to 44): Moderate to severe reduction
- Stage 4 (eGFR 15 to 29): Severely reduced function
- Stage 5 (eGFR below 15): Kidney failure, dialysis typically needed
An eGFR below 60 sustained over three months is the standard threshold for a CKD diagnosis. But context matters. Many older adults dip below 60 as part of normal aging without ever developing kidney failure or needing treatment.
Creatinine: What Your Blood Level Should Be
Creatinine is a waste product from muscle metabolism. Your kidneys filter it out, so higher levels in the blood can signal that your kidneys aren’t keeping up. The normal range is 0.7 to 1.3 mg/dL for men and 0.6 to 1.1 mg/dL for women.
The tricky part is that creatinine is heavily influenced by your body composition. People with more muscle mass, including athletes and those who eat high-protein diets, naturally produce more creatinine. A muscular person can have a creatinine of 1.3 or even slightly above with perfectly healthy kidneys. On the flip side, someone who is elderly, frail, or has very low muscle mass might have a creatinine that looks “normal” even though their kidneys are actually impaired. The number can mask real problems in people with smaller frames.
Physical activity also plays a role. People who exercise regularly tend to have higher creatinine levels in both blood and urine, largely because of greater muscle mass and higher protein intake. Even a single intense workout can temporarily bump the number up.
BUN: Blood Urea Nitrogen
BUN measures the amount of nitrogen from urea, a waste product created when your body breaks down protein. The normal range is 5 to 20 mg/dL. Like creatinine, BUN rises when the kidneys struggle to clear waste. But it’s less specific to kidney function because dehydration, a high-protein meal, or even gastrointestinal bleeding can raise it independently.
The ratio of BUN to creatinine can be informative. In kidney disease, both numbers tend to rise together at roughly a 10-to-1 ratio. If your BUN is disproportionately higher than your creatinine, the cause is more likely dehydration or something unrelated to your kidneys. A low ratio can point to things like liver disease or very low protein intake.
Urine Albumin: Catching Damage Early
Your kidneys aren’t supposed to let protein through into your urine. The urine albumin-to-creatinine ratio (uACR) checks for tiny amounts of a protein called albumin that can leak through when the kidney’s filtering units are damaged. A good uACR is below 30 mg/g. Results between 30 and 300 mg/g indicate moderately increased albumin (sometimes called microalbuminuria), which is an early marker of kidney damage. Above 300 mg/g signals more significant damage.
This test is especially important because it can detect kidney problems before your eGFR drops. You can have a perfectly normal eGFR while albumin is already leaking, which is why Stage 1 CKD is defined as normal filtration plus evidence of kidney damage like protein in the urine. If you have diabetes or high blood pressure, annual uACR screening is recommended because these conditions are the two leading causes of kidney disease.
Cystatin C: A More Accurate Alternative
If your doctor suspects your creatinine-based eGFR might be inaccurate, they may order a cystatin C test. Cystatin C is a small protein produced at a steady rate by all cells in your body. Unlike creatinine, its blood level isn’t affected by muscle mass, diet, or physical activity, which makes it a more reliable marker in certain people: the very muscular, the very thin, the elderly, and those with extreme diets.
The normal range for cystatin C is roughly 0.61 to 1.01 mg/L for both men and women. When combined with creatinine, it gives the most accurate eGFR estimate available without invasive testing. The 2021 guidelines from the NKF and ASN specifically recommended expanding cystatin C testing to reduce diagnostic errors.
How Often to Check Your Numbers
If you’re healthy with no risk factors, kidney function is typically checked as part of routine blood work during an annual physical. If you have diabetes, high blood pressure, heart disease, or a family history of kidney problems, guidelines recommend annual screening with both a creatinine blood test and a urine albumin test. Once someone is diagnosed with CKD, testing frequency increases depending on the stage, sometimes every three to six months to track the rate of decline.
A single abnormal result doesn’t necessarily mean kidney disease. Dehydration, intense exercise, certain medications, and even eating a lot of cooked meat before the test can temporarily shift your numbers. If a result comes back outside the normal range, your doctor will typically recheck it before drawing conclusions. The diagnosis of CKD requires abnormal findings sustained over at least three months.