The most important kidney number is your eGFR, and any result below 60 signals a meaningful loss of kidney function. But eGFR isn’t the only number that matters. Your lab results likely include creatinine, BUN, and possibly a urine protein measurement, each telling a different part of the story. Here’s what each number means and where the cutoffs fall.
eGFR: The Main Number to Watch
Your estimated glomerular filtration rate, or eGFR, measures how well your kidneys filter waste from your blood. It’s reported as a number from 0 to over 100, and it roughly corresponds to the percentage of kidney function you have left. A normal eGFR is 90 or above. Below 60, you’re in the range of chronic kidney disease that typically needs monitoring and treatment.
Doctors classify kidney disease into five stages based on eGFR:
- Stage 1 (eGFR 90+): Normal filtering, but other signs of kidney damage exist, like protein in the urine
- Stage 2 (eGFR 60–89): Mildly reduced function
- Stage 3a (eGFR 45–59): Mild to moderate loss
- Stage 3b (eGFR 30–44): Moderate to severe loss
- Stage 4 (eGFR 15–29): Severe loss of function
- Stage 5 (eGFR below 15): Kidney failure
Stage 3 is where most people first learn something is wrong, because stages 1 and 2 rarely cause noticeable symptoms. By stage 4, your kidneys are struggling enough that you’ll likely feel the effects: fatigue, swelling, nausea, or changes in urination. Stage 5 typically means dialysis or a transplant is on the table.
One thing worth knowing: eGFR is calculated from a blood test, not measured directly. Until 2021, the formula included a race-based adjustment that could give Black patients artificially higher results, potentially delaying diagnosis. The current standard formula, adopted after a National Kidney Foundation and American Society of Nephrology review, removes race from the calculation entirely. If your lab results are from before this change, your eGFR may have been reported differently.
Creatinine: Higher Is Worse
Creatinine is a waste product your muscles produce constantly. Healthy kidneys filter it out efficiently, so it stays at low levels in your blood. When kidney function declines, creatinine builds up. Unlike eGFR, where a lower number is bad, a higher creatinine is the warning sign.
Normal creatinine ranges are:
- Adult men: 0.74 to 1.35 mg/dL
- Adult women: 0.59 to 1.04 mg/dL
A result above these ranges doesn’t automatically mean kidney disease. Creatinine levels are influenced by muscle mass, diet (particularly high-protein meals), hydration, and certain medications. A muscular 25-year-old man might have a creatinine of 1.3 mg/dL with perfectly healthy kidneys, while an older woman with the same reading could have significantly reduced function. That’s why doctors use creatinine as an input to calculate your eGFR rather than interpreting it on its own. If your creatinine is rising over time, even within the “normal” range, that trend matters more than any single reading.
BUN: Context Matters
Blood urea nitrogen measures another waste product your kidneys clear. Normal ranges are 6 to 21 mg/dL for women and 8 to 24 mg/dL for men. BUN is less reliable than creatinine as a kidney-specific marker because it fluctuates with protein intake, dehydration, liver function, and even bleeding in the digestive tract.
Doctors don’t use BUN alone to define kidney failure. But when BUN is elevated alongside high creatinine, the combination points strongly toward a kidney problem rather than one of those other causes. A BUN that’s high while creatinine stays normal is more likely to reflect dehydration or a high-protein diet than actual kidney damage.
Urine Albumin: Early Damage Detection
Your kidneys shouldn’t let significant amounts of protein leak into your urine. When they do, it’s a sign the filtering units are damaged, sometimes before eGFR drops at all. The standard test for this is the urine albumin-to-creatinine ratio, or uACR, measured from a single urine sample.
- Below 30 mg/g: Normal
- 30 to 299 mg/g: Moderately increased, an early warning sign
- 300 mg/g or above: Severely increased, indicating significant kidney damage
This test is especially important if you have diabetes or high blood pressure, the two leading causes of kidney disease. You can have a normal eGFR and still show elevated albumin in your urine, which is why stage 1 kidney disease exists as a category. Catching protein spillage early gives you the best window to slow or stop further damage through blood pressure control and other treatments.
Phosphorus and Potassium in Advanced Disease
In earlier stages of kidney disease, your body compensates well enough that blood levels of minerals like phosphorus and potassium stay normal. But once eGFR drops below about 30 (stage 4 territory), your kidneys lose the ability to keep these in balance.
Phosphorus normally stays between 3.5 and 5.5 mg/dL. As kidney function declines, phosphorus climbs because the kidneys can no longer excrete it efficiently. Levels above 6.5 mg/dL are associated with a meaningfully increased risk of death in people on dialysis. High phosphorus pulls calcium from bones, weakens them over time, and can cause calcium deposits in blood vessels and soft tissues. If your phosphorus is running high, it’s a sign your kidney disease has progressed to a point where dietary changes or medications to bind phosphorus in the gut become necessary.
Potassium follows a similar pattern. Healthy kidneys tightly regulate it, but failing kidneys let it accumulate. High potassium is dangerous because it can cause irregular heart rhythms, which is why it’s monitored closely in stages 4 and 5.
Cystatin C: A More Precise Alternative
Most eGFR calculations are based on creatinine, but creatinine has limitations. It’s influenced by muscle mass, diet, and body composition, which means it can overestimate kidney function in people with low muscle mass (older adults, those with chronic illness) or underestimate it in very muscular individuals.
Cystatin C is a protein produced by nearly every cell in the body at a steady rate. It’s filtered by the kidneys and isn’t affected by muscle mass or diet, making it a more consistent marker of actual kidney function. Normal cystatin C levels fall roughly between 0.61 and 1.01 mg/L for adults. Current guidelines recommend using cystatin C alongside creatinine when more precision matters, such as when someone is borderline between kidney disease stages or when creatinine-based eGFR seems inconsistent with the clinical picture. If you’ve been told your eGFR is borderline, asking about a cystatin C test can give you a clearer answer.
How to Read Your Results Together
No single kidney number tells the full story. An eGFR of 55 with normal urine albumin is a very different situation from an eGFR of 55 with a uACR of 250 mg/g. The first suggests stable, mild to moderate loss that may stay steady for years. The second suggests active, ongoing damage that’s more likely to progress without intervention.
Trends matter more than snapshots. A single creatinine reading of 1.4 mg/dL in a man could be meaningless. But if that same man’s creatinine was 1.0 six months ago and 1.2 three months ago, the upward trajectory is concerning regardless of whether each individual number falls within the “normal” range. If you’ve had kidney labs done before, comparing your current results to previous ones is the single most useful thing you can do when trying to understand what your numbers mean.