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How does GFR actually measure kidney function?
Your kidneys hold about a million tiny filters called glomeruli. Glomerular filtration rate (GFR) is an estimate of how many milliliters of blood those filters clean each minute, normalized to body size.
Most labs don't measure it directly. They estimate it (eGFR) from a waste product — usually creatinine, sometimes cystatin C — plus your age and sex. Creatinine comes from muscle, so more muscle can make eGFR look lower than your kidneys really are, and very low muscle can make it look better.
That's why a single number matters less than the trend across several tests, read alongside your urine albumin result.
About the author
Swetha Raju
Columbia M.S. Candidate in Clinical Human Nutrition · AKF Certified Kidney Health Coach · NKF Medical Advisory Committee (MAC) Member · NKF peer mentor · CKD patient advocate · Published chronic disease and nutrition researcher
Swetha Raju is the founder of NephroNourish. As a published researcher and lifelong chronic disease patient, she translates renal nutrition and metabolic science into practical guidance people can actually use.
A note on scope. This article is educational and not individual medical advice. Always discuss changes with your nephrologist, dietitian, or care team.