Is BUN an index of GFR?
Can measurements of blood urea nitrogen (BUN) serve as an index of GFR?
BUN is not a reliable index of GFR. The kidney tubules reabsorb urea in quantities that vary depending on the state of hydration, thus rendering BUN an inaccurate marker for GFR.
BUN concentration is also strongly affected by changes in catabolism and protein intake. Because urea is reabsorbed in the proximal tubule, urea clearance underestimates GFR.
Some people advocate using the mean of the creatinine clearance (overestimates GFR due to tubular secretion of creatinine) and urea clearance (underestimates GFR due to tubular reabsorption) to get a better estimate of GFR.
