Urea and Creatinine

Usually urea and creatinine changes are proportional to each other, however there are exceptions. Ruminants excrete urea through the digestive tract and urea is a significant source of protein; therefore, creatinine is probably a more sensitive indicator of renal disease in cattle and it may be increased when urea is within or below the RI. Animals with reduced muscle mass may have lower creatinine concentrations than expected, even with renal disease. In this situation, urea concentration may be increased while creatinine concentration is within or below the RI. Animals that are experiencing intestinal hemorrhage may have disproportionately high urea concentration relative to creatinine. The digested blood provides a rich source of amino acids which, in turn, results in increased ammonia within the intestinal tract. The ammonia load is transported to the liver and converted to urea, thus increasing its serum concentration. Since urea concentration may also be influenced by the amount of dietary protein, high protein diets can cause increased urea in patients in the absence of renal disease. Conversely, animals consuming low protein diets may have serum urea concentrations disproportionately lower than creatinine concentrations even when renal disease is present.

With automated chemistry analyzers, serum urea concentration is measured from release of ammonium ions by the enzyme urease, therefore the term blood urea nitrogen (BUN) is no longer applicable for urea testing performed in diagnostic laboratories; however BUN is still often used in clinic settings where different methodologies are used. A test known as symmetric dimethylarginine (SDMA), has recently been promoted as detecting renal disease in dogs and cats at an earlier stage than traditional tests and not being affected by nonrenal factors that can alter creatinine concentration (e.g. muscle mass). However, independent research studies to fully evaluate SDMA’s usefulness have yet to be conducted.