Which measurement estimates GFR in adults?

• A. Urinalysis for protein
• B. Serum bilirubin
• C. Serum creatinine with eGFR calculation (CKD-EPI)
• D. Blood urea nitrogen alone

Answer: (C)

Estimating GFR in adults relies on a serum creatinine–based calculation because creatinine is a waste product filtered by the kidneys, and its level, when interpreted with patient factors, provides a reliable estimate of filtration rate. Using serum creatinine with an eGFR equation like CKD-EPI accounts for differences in muscle mass, age, sex, and race, giving a standardized measure (in mL/min/1.73 m²) of kidney function that is more accurate than creatinine alone. Other approaches either detect damage or reflect other organ function rather than filtration: urinalysis for protein shows kidney injury but not the current rate of filtration; serum bilirubin reflects liver function; and blood urea nitrogen is influenced by hydration and protein intake, making it an unreliable stand-alone estimator of GFR. Therefore, combining serum creatinine with an eGFR calculation is the measurement that estimates GFR in adults.