Kidney Function Test: Creatinine, Urea and eGFR Explained in Plain Language
Creatinine, blood urea and eGFR each measure kidney function differently. Here's how they relate, and why eGFR is often the more useful number.
Kidney function tests get less everyday attention than blood sugar or cholesterol, but they're some of the most consistently informative numbers on a routine report — largely because kidneys filter waste at a fairly predictable rate, which makes deviations meaningful.
Creatinine: a byproduct with a use
Creatinine is a waste product from normal muscle activity, filtered out by the kidneys at a steady rate. When kidney filtering slows down, creatinine builds up in the blood — which is why a rising creatinine, even within the "normal" range, is worth watching over time.
Blood Urea (BUN): more sensitive to outside factors
Blood urea also reflects kidney filtering, but it's more easily nudged by dehydration, high-protein meals, or certain medications — which is exactly why it's interpreted alongside creatinine rather than instead of it.
eGFR: the number that actually answers "how well are my kidneys working?"
eGFR (estimated Glomerular Filtration Rate) is calculated from creatinine, age, and sex, and expresses kidney function as a percentage of expected capacity. Doctors generally think in terms of eGFR stages more than raw creatinine, because it accounts for the fact that a "normal" creatinine means something different for a 25-year-old than a 70-year-old.
Why hydration status matters when interpreting a single reading
A single dehydrated day can nudge both creatinine and BUN upward without reflecting any real change in kidney health. This is one of the clearer examples of why a trend across several reports is more trustworthy than one report taken in isolation.
This is general health information, not medical advice. Always talk to a qualified healthcare professional about your own results.
