Core Module Course Block (CMCB) 3 Practice Test

Glomerular filtration rate is the amount of plasma the kidneys can filter per minute, reflecting how much functioning filtration surface is available. We don’t measure this directly in everyday practice; instead, we estimate it because direct measurement is more invasive and complex. Creatinine is used for estimation because it is a waste product produced relatively steadily by muscle and is freely filtered by the glomeruli. Most of it is filtered and not reabsorbed, so its level in the blood inversely tracks how well the kidneys are filtering. The eGFR uses serum creatinine along with individual factors such as age, sex, and body size (often race) in a formula—most commonly the CKD-EPI equation—to estimate GFR. This accounts for differences in muscle mass and metabolism between people, giving a more accurate sense of kidney function than creatinine alone. Keep in mind that creatinine-based estimates have limitations (muscle mass, diet, certain medications can affect creatinine production), and in some cases other markers like cystatin C may be used to improve accuracy.