What does estimated glomerular filtration rate primarily estimate?

• A. Urine protein excretion
• B. Glomerular filtration rate
• C. Renal blood flow
• D. Kidney size on imaging

Answer: (B)

Estimated GFR is a measure of how well the kidneys' filtering units, the glomeruli, are functioning. It’s calculated from serum creatinine along with factors such as age, sex, and sometimes race because creatinine production varies with muscle mass and demographics. This makes eGFR a practical proxy for the rate at which filtrate is produced per minute, without needing invasive clearance testing.

Urine protein excretion reflects how much protein leaks into the urine, signaling potential glomerular injury, but it does not quantify the overall filtration capacity. Renal blood flow speaks to perfusion of the kidney, not the actual filtration rate. Kidney size on imaging is an anatomical measurement and does not directly indicate how much filtrate the glomeruli can produce. Thus, the concept being tested is that estimated GFR primarily estimates the glomerular filtration rate.