Kidney Function Tests: eGFR, Creatinine, BUN & CKD Stages Explained

eGFR (estimated glomerular filtration rate) measures how efficiently your kidneys filter waste from blood, expressed in mL/min per 1.73m² of body surface area. It's calculated from your creatinine, age, and sex using the CKD-EPI equation (updated 2021 to remove race adjustment). Normal eGFR is above 90; below 60 sustained for 3+ months meets the definition of chronic kidney disease. eGFR is more clinically useful than creatinine alone because it accounts for body size.

Creatinine above 4.0 mg/dL generally indicates severe kidney impairment (roughly Stage 4-5 CKD). A sudden rise of 0.3 mg/dL or more within 48 hours, or a doubling from baseline, suggests acute kidney injury and warrants urgent evaluation. However, context matters — a creatinine of 1.5 in a small elderly woman is much more concerning than 1.5 in a muscular young man. Always look at eGFR rather than creatinine alone.

CKD has five stages based on eGFR: Stage 1 (eGFR ≥90, kidney damage present), Stage 2 (60-89, mild decrease), Stage 3a (45-59, mild-to-moderate), Stage 3b (30-44, moderate-to-severe), Stage 4 (15-29, severe), and Stage 5 (<15, kidney failure). Stages 1-2 usually have no symptoms. Stage 3 is when monitoring and treatment intensify. The 2024 KDIGO guidelines recommend considering SGLT2 inhibitors starting at Stage 3 for kidney protection.

High BUN (blood urea nitrogen, above 20 mg/dL) most commonly indicates dehydration — check the BUN/creatinine ratio. If the ratio is >20:1, dehydration or GI bleeding is likely. If BUN and creatinine rise proportionally (ratio 10-20:1), kidney impairment is more likely. Other causes include high-protein diet, GI bleeding (digested blood becomes BUN), congestive heart failure, and catabolic states (burns, fever, surgery).

eGFR (estimated glomerular filtration rate) is calculated from a routine blood creatinine level using an equation (the CKD-EPI formula) that factors in age and sex — no urine collection is needed, which is why it's reported automatically on nearly every metabolic panel today. Creatinine clearance is an older, more direct method that measures how much creatinine actually leaves the blood over a defined period, traditionally via a 24-hour urine collection paired with a same-day blood draw (or estimated with the Cockcroft-Gault equation, which factors in body weight). Creatinine clearance tends to run slightly higher than true GFR because a small amount of creatinine is also secreted by the kidney tubules, not just filtered — so it isn't simply a more accurate version of eGFR, each has trade-offs. In practice, eGFR is used for CKD staging in the vast majority of patients because it only requires a blood draw; creatinine clearance is mainly reserved for specific situations — pregnancy, extremes of body size, amputees, or medication dosing that needs a more direct clearance measurement. The two typically agree within 10-20% for most adults, and diverge more when muscle mass or diet is unusual.

No — creatinine, BUN, and eGFR do not require fasting on their own. You can eat and drink normally before the draw. The one exception is a combined draw: if your doctor has ordered your kidney panel alongside tests that do require fasting, like a lipid panel or fasting glucose, follow the fasting instructions for that combined blood draw rather than the kidney markers specifically.

The core kidney function test — creatinine, BUN, and the eGFR calculated from them — is a blood test, drawn from a vein like any other routine bloodwork. It's often paired with, but is a separate test from, a urine albumin-to-creatinine ratio (UACR), which checks for protein leaking into the urine — an early sign of kidney damage that a blood-only panel can miss. A complete kidney workup typically includes both: the blood panel for filtration rate, and the urine test for albumin/protein.

Yes. KFT (kidney function test), RFT (renal function test), "renal panel," and "kidney panel" are different names for the same core blood panel — creatinine, BUN, and eGFR. The terminology varies mainly by region and lab: KFT/RFT are more commonly used outside the US, while US labs and clinicians more often say "basic metabolic panel" or "kidney function panel." Some versions bundle in electrolytes (sometimes labeled U&E, for urea and electrolytes, in UK usage) — if your report includes sodium, potassium, and chloride alongside creatinine and BUN, that's the same test with electrolytes added, not a different one.

Diabetes and high blood pressure are the two leading causes of chronic kidney disease worldwide, and both damage the kidneys gradually and silently — early-stage kidney damage from either condition typically causes no symptoms. This is why major guidelines recommend at least annual eGFR and urine albumin testing for anyone with diabetes or hypertension, even when they feel completely well: catching a declining eGFR or rising albumin early opens the door to treatments (including the SGLT2 inhibitors mentioned in the CKD stages above) that can meaningfully slow further kidney damage, whereas waiting for symptoms usually means significant kidney function has already been lost.

Yes — creatine monohydrate supplementation reliably raises serum creatinine slightly, and it can make your calculated eGFR look lower than your true filtration rate. A 2025 systematic review and meta-analysis of randomized controlled trials in BMC Nephrology found creatine supplementation raised serum creatinine by about 0.13 mg/dL on average with no significant change in true glomerular filtration rate — the bump reflects extra creatine being broken down into creatinine, not reduced kidney filtration. If you take creatine and get a routine kidney panel, mention the supplement to your doctor; a cystatin C-based estimate (see the marker table above) isn't affected by creatine intake and can confirm true kidney function if a creatinine-based result looks borderline.

Not directly — creatinine, BUN, and eGFR measure how well your kidneys filter blood, not whether a tumor is present. Kidney (renal cell) cancer often doesn't reduce overall filtration until it's advanced or affects both kidneys, since one healthy kidney can usually keep eGFR in the normal range even if the other has a tumor. According to the Canadian Cancer Society, there's no blood test that diagnoses kidney cancer on its own — diagnosis relies on imaging (ultrasound, CT, or MRI), often triggered by visible blood in the urine (hematuria) or a mass found incidentally on a scan ordered for something else. A normal kidney function test doesn't rule out kidney cancer, and symptoms like blood in the urine, unexplained flank pain, or a palpable abdominal mass warrant imaging regardless of how your kidney panel reads.

A basic kidney panel (creatinine, BUN, eGFR) typically costs $10-$30 through direct-pay lab networks like LabCorp or Quest without insurance, and is usually fully covered with no out-of-pocket cost when ordered by a doctor and billed through insurance after the deductible. Adding a urine albumin-to-creatinine ratio for a complete kidney workup typically adds another $10-$25 cash-pay. Prices vary by lab, region, and whether the panel is bundled into a larger comprehensive metabolic panel, which is often the same or only marginally more expensive than the kidney markers alone.

At-home kidney test kits (typically a urine dipstick or a finger-prick blood sample mailed to a lab) can give a reasonable estimate but are generally less precise than a standard venous blood draw processed at a clinical lab, and dipstick-only kits usually screen for protein in the urine rather than measuring creatinine or eGFR directly. They're reasonable for general screening or tracking a trend between doctor visits, but an abnormal or borderline at-home result should be confirmed with a standard lab-drawn kidney panel before drawing any conclusions, since factors like hydration status and sample handling affect at-home results more than a controlled venous draw.

Iodinated contrast dye (used in CT scans) and, less commonly, gadolinium contrast (used in some MRIs) are cleared by the kidneys, and in patients with reduced kidney function, contrast can occasionally trigger contrast-induced nephropathy — a temporary or, rarely, lasting further decline in kidney function. Radiology departments check a recent eGFR before contrast administration specifically to catch reduced kidney function in advance, since the imaging protocol, contrast dose, or need for extra hydration may be adjusted (or an alternative non-contrast study chosen) when eGFR is below the safety threshold typically used by most radiology departments.