Which waste product is most accurate for measuring GFR, and why?

The correct choice among the options is that creatinine is the most accurate waste product for measuring glomerular filtration rate (GFR) because it is not reabsorbed by the kidneys. Creatinine is a byproduct of muscle metabolism and is produced at a relatively constant rate in the body, depending on muscle mass. When it is filtered through the glomeruli, it is not significantly reabsorbed back into the bloodstream, making it a reliable indicator of kidney function.

Since creatinine is only filtered and not reabsorbed, its concentration in serum is inversely related to GFR; if GFR decreases, creatinine levels in the blood rise, indicating impaired kidney function. This characteristic makes creatinine a useful marker in clinical practice for estimating GFR and assessing kidney health.

In contrast, other substances mentioned do not serve as accurate measures of GFR. For instance, urea can be reabsorbed in the kidneys, which can alter its concentration depending on hydration status and other factors. This variability makes urea less reliable as a sole indicator of glomerular function.

Glucose is typically reabsorbed in the renal tubules until renal threshold levels are exceeded, meaning that high levels of glucose in the urine often indicate diabetes or