Normally functioning kidneys remove waste from the bloodstream. Blood levels of certain waste products are commonly used as indicators that the kidneys are functioning normally. When the kidney's filtration function is compromised, waste products accumulate in the blood. Two waste products commonly used to assess kidney (renal) function are urea and creatinine. Based on extensive studies of persons with normal kidney function "usual clinical ranges" have been established for both these substances. Blood levels higher than the usual clinical range may be a sign that the kidneys are not functioning properly.
Blood urea nitrogen (BUN) is a measure of the level of urea present in the blood. Urea is a waste product produced from the breakdown of food protein. BUN elevation is not specific for kidney dysfunction but can be influenced by non-kidney related factors such as the protein content of a person's diet and state of hydration.
Creatinine is a waste product from meat protein in the diet and normal muscle function. Creatinine levels are less likely to be influenced by diet and non-kidney related factors. The simultaneous elevation of BUN and creatinine beyond the usual clinical ranges is most suggestive of possible abnormal renal function.
A creatinine clearance test provides a more accurate measure of kidney function than BUN and/or blood creatinine levels. Examination of a urine specimen is an additional screen for possible kidney disease. The presence of protein or red blood cells in the urine can be an early sign of kidney disease. For additional and more complete information on Tests of Kidney Function, the following web site is suggested: