Overview of Urea and Creatinine

Jose H. Salazar, MS, MLS(ASCP)CM

Disclosures

Lab Med. 2014;45(1):e19-e20. 

In This Article

Introduction

Urea, commonly referred to as blood urea nitrogen (BUN) when measured in the blood, is a product of protein metabolism. BUN is considered a non-protein nitrogenous (NPN) waste product. Amino acids derived from the breakdown of protein are deaminated to produce ammonia. Ammonia is then converted to urea via liver enzymes. Therefore, the concentration of urea is dependent on protein intake, the body's capacity to catabolize protein, and adequate excretion of urea by the renal system.[1]

Urea accounts for the majority (up to 80%–90%) of the NPNs excreted by the body. The body's dependency on the renal system to excrete urea makes it a useful analyte to evaluate renal function. An increase in BUN can be the result of a diet that is high in protein content or decreased renal excretion.

Creatinine, also a NPN waste product, is produced from the breakdown of creatine and phosphocreatine and can also serve as an indicator of renal function.[2] Creatine is synthesized in the liver, pancreas, and kidneys from the transamination of the amino acids arginine, glycine, and methionine. Creatine then circulates throughout the body and is converted to phosphocreatine by the process of phosphorylation in the skeletal muscle and brain. The majority of the creatinine is produced in the muscle. As a result, the concentration of plasma creatinine is influenced by the patient's muscle mass. Compared to BUN, creatinine is less affected by diet and more suitable as an indicator of renal function.

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