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.