Which lab value is most indicative of renal function and what is a normal range for adults?

• A. Serum creatinine; 0.6–1.3 mg/dL
• B. BUN; 7–20 mg/dL
• C. Estimated GFR; 90–120 mL/min/1.73 m2
• D. Serum potassium; 3.5–5.0 mEq/L

Answer: (A)

When evaluating kidney function, the most direct indicator is the substance the kidneys clear from the blood: serum creatinine. Creatinine is produced at a fairly steady rate from muscle metabolism, and the kidneys excrete it mainly through glomerular filtration. If the glomerular filtration rate drops, serum creatinine rises, making it a reliable signal of reduced kidney function. The normal range for adults is about 0.6 to 1.3 mg/dL, though exact values can vary slightly by lab.

BUN can be influenced by hydration status, protein intake, and liver function, so it doesn’t track kidney function as specifically. Serum potassium can be affected by many factors and isn’t a direct measure of filtration. Estimated GFR is a calculated value that uses creatinine along with age, sex, and race; it’s useful, but it’s an estimate dependent on creatinine, whereas serum creatinine itself directly reflects kidneys’ filtering capacity.