Which laboratory value best estimates overall kidney function?

• A. Serum potassium
• B. Blood urea nitrogen
• C. Glomerular filtration rate
• D. Serum creatinine

Answer: (C)

Estimating overall kidney function is best done with glomerular filtration rate because it directly reflects how much filtrate the kidneys produce each minute, i.e., their power to clear waste and maintain fluid and electrolyte balance. GFR captures the combined filtering ability of all functioning nephrons, so it tracks kidney health across a range from normal to advanced disease and guides staging and management.

The other values don’t measure filtration capacity as directly. Serum potassium shows how well the kidneys excrete potassium, but potassium balance can be influenced by hormones and shifts between body compartments. BUN can rise for reasons other than reduced filtration, like dehydration or high protein intake, so it’s not specific to GFR. Serum creatinine is influenced by muscle mass and age and, while helpful, does not by itself give a complete picture of filtration, especially in individuals with unusual muscle mass or body size. In practice, we estimate GFR using creatinine-based equations to get a clearer assessment of overall kidney function.