In kidney failure with a low GFR, which lab value is most concerning?

• A. Hemoglobin
• B. Potassium
• C. BUN
• D. Sodium

Answer: (C)

When the glomerular filtration rate drops, the kidneys lose their ability to clear waste products from the blood. BUN, or blood urea nitrogen, directly reflects this reduced clearance, so it rises as kidney function worsens. A high BUN indicates azotemia and the buildup of nitrogenous wastes, which can lead to uremic symptoms if not addressed. This makes BUN the most informative indicator of overall kidney excretory failure among the listed labs. While potassium can become dangerously high and sodium or hemoglobin changes can occur, BUN specifically mirrors how well the kidneys are filtering waste in this low-GFR scenario, signaling systemic impact of the renal impairment.