BLOOD urea nitrogen (BUN), or plasma creatinine, or both, are commonly used in clinical practice as routine procedures for the evaluation of renal function. These simple tests are also useful in following impairments in renal function brought about by disease.
Not always clearly recognized is the fact that BUN or plasma creatinine levels are the result of the interplay of two independent factors: production or synthesis, and excretion. Excretion is accomplished almost exclusively via the kidneys. Production of urea will depend on the many factors which affect human nitrogen metabolism. Rates of production of creatinine are a function of muscle mass.
In the steady state, the rate of excretion of metabolic end products, such as creatinine and urea, must equal the rate of production. Therefore, if changes in plasma creatinine or urea are to reflect changes in excretion or clearance alone, rates of production must also be constant.