4 Specimen RequirementsĬreatinine may be measured using serum, plasma, or urine specimens. Combined with plasma creatinine as a creatinine/BUN ratio, BUN can be a useful analyte in differentiating pre or post renal increase of plasma NPNs. Alone BUN is not the ideal marker for GFR. BUN is not the preferred marker for clearance because it is influenced by factors such as a high protein diet, variables in protein synthesis, and patient hydration status. 3Īlthough not as specific as creatinine, BUN can also be used as an indicator of renal function. In addition, a plasma creatinine level that is within normal reference range does not equate to a normal functioning renal system. Plasma creatinine levels may not be affected until significant renal damage has occurred. The measurement of plasma alone should not be used to assess renal function. The reduced GFR will then lead to an increase in plasma creatinine concentration. If the GFR is decreased, as is in renal disease, creatinine clearance via the renal system is compromised. These characteristics make creatinine a useful endogenous marker for creatinine clearance. The measurement of creatinine concentrations in plasma and urine samples illustrates the filtration capacity of the glomerulus, also known as the glomerular filtration rate (GFR.) Creatinine is produced endogenously within the body and is freely filtered by the glomerulus. Compared to BUN, creatinine is less affected by diet and more suitable as an indicator of renal function. As a result, the concentration of plasma creatinine is influenced by the patient’s muscle mass. The majority of the creatinine is produced in the muscle. Creatine then circulates throughout the body and is converted to phosphocreatine by the process of phosphorylation in the skeletal muscle and brain. 2 Creatine is synthesized in the liver, pancreas, and kidneys from the transamination of the amino acids arginine, glycine, and methionine. An increase in BUN can be the result of a diet that is high in protein content or decreased renal excretion.Ĭreatinine, also a NPN waste product, is produced from the breakdown of creatine and phosphocreatine and can also serve as an indicator of renal function. The body’s dependency on the renal system to excrete urea makes it a useful analyte to evaluate renal function. Urea accounts for the majority (up to 80%–90%) of the NPNs excreted by the body. 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. Ammonia is then converted to urea via liver enzymes. Amino acids derived from the breakdown of protein are deaminated to produce ammonia. BUN is considered a non-protein nitrogenous (NPN) waste product. Urea, commonly referred to as blood urea nitrogen (BUN) when measured in the blood, is a product of protein metabolism. Urea, creatinine, renal, non-protein nitrogen, clinical laboratory science
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