Kidney function tests (sometimes called renal function tests or RFT) measure how well your kidneys are filtering waste from your blood. The key markers are creatinine, BUN (blood urea nitrogen), and the calculated eGFR (estimated glomerular filtration rate).
Chronic kidney disease (CKD) affects an estimated 37 million Americans — and about 90% don't know they have it. Catching declining kidney function early makes a real difference. Estimate yours with our eGFR calculator.
Kidney Function reference ranges (US standard)
These ranges are aligned with Quest Diagnostics and LabCorp. Your own report prints the exact range your lab used — always defer to that range, since methods differ slightly between labs.
| Parameter | Normal Range | What it measures |
|---|---|---|
| Creatinine | Men 0.7–1.3 · Women 0.5–1.1 mg/dL | Muscle waste filtered by kidneys |
| BUN | 7–20 mg/dL | Protein waste (urea) |
| eGFR | 90+ mL/min/1.73m² normal | Overall filtration rate |
| BUN/Creatinine Ratio | 10:1 – 20:1 | Helps locate the problem |
| Uric Acid | Men 3.4–7.0 · Women 2.4–6.0 mg/dL | Gout & kidney marker |
| Albumin (urine) | Under 30 mg/g | Early kidney damage marker |
Understanding your kidney numbers
eGFR and CKD stages
eGFR is the best single measure of kidney function. The stages (KDIGO, US guidelines) are: 90+ = G1 (normal), 60–89 = G2 (mildly decreased), 45–59 = G3a, 30–44 = G3b, 15–29 = G4 (severely decreased), and under 15 = G5 (kidney failure). A single low eGFR isn't a diagnosis — CKD requires the reduction to persist for 3 months.
Creatinine
Creatinine rises when kidneys filter less effectively. But it's also affected by muscle mass — bodybuilders and very muscular people naturally run higher without any kidney problem. That's why eGFR (which adjusts for age and sex) is more useful.
BUN
BUN rises with dehydration, high-protein diets (including keto), GI bleeding, and kidney disease. The BUN-to-creatinine ratio helps your doctor tell dehydration apart from true kidney injury.