Which lab value best reflects kidney function?

• A. Albumin
• B. Potassium
• C. Creatinine (and estimated GFR)
• D. BUN

Answer: (C)

Kidney function is best judged by how well the kidneys are filtering waste from the blood. Creatinine is a waste product cleared by the kidneys, so when filtration declines, serum creatinine rises. Because creatinine is produced at a roughly constant rate, its level serves as a reliable proxy for filtration rate, and the estimated GFR combines creatinine with age, sex, and race to quantify how much blood is filtered each minute. This makes creatinine and eGFR the most direct reflection of kidney function among the options.

Albumin doesn’t measure filtration; it reflects nutritional status or protein loss. BUN can be influenced by hydration, dietary protein, and catabolic state, so it isn’t as specific for filtration. Potassium levels can be affected by shifts between body compartments and acid–base status, not just kidney function.