Which lab value is filtered out of the blood by the kidneys and is used to assess kidney function?

• A. Blood Urea Nitrogen
• B. Chloride
• C. Glucose
• D. Creatinine

Answer: (D)

Creatinine is a waste product produced by muscle metabolism that is cleared by the kidneys through glomerular filtration. Because its production is relatively constant and it is filtered with minimal reabsorption, the level of creatinine in the blood rises as the glomerular filtration rate (GFR) declines. That direct relationship makes serum creatinine a reliable marker for estimating GFR and overall kidney function.

Blood Urea Nitrogen (BUN) can also rise with kidney dysfunction, but it is affected by many other factors such as hydration, protein intake, and liver function, so it is less specific for measuring filtering capacity. Chloride is simply an electrolyte and doesn’t reflect kidney filtration status. Glucose is filtered and normally reabsorbed in the kidneys; its presence in urine or blood is more about glucose handling and diabetes control than a direct measure of GFR.

So the lab value best used to assess kidney function by filtration is creatinine.