Jose H. Salazar, MS, MLS(ASCP)CM, Overview of Urea and Creatinine, Laboratory Drugs, Quantity 45, Concern 1, February 2014, Pages e19–e20, https://doi.org/10.1309/LM920SBNZPJRJGUT
Urea, generally known as blood urea nitrogen (BUN) when measured within the blood, is a product of protein metabolism. BUN is taken into account a non-protein nitrogenous (NPN) waste product. Amino acids derived from the breakdown of protein are deaminated to supply ammonia. Ammonia is then transformed to urea by way of liver enzymes. Due to this fact, the focus of urea relies on protein consumption, the physique’s capability to catabolize protein, and ample excretion of urea by the renal system.1
Urea accounts for almost all (as much as 80%–90%) of the NPNs excreted by the physique. The physique’s dependency on the renal system to excrete urea makes it a helpful analyte to guage renal operate. A rise in BUN could be the results of a weight-reduction plan that’s excessive in protein content material or decreased renal excretion.
Creatinine, additionally a NPN waste product, is produced from the breakdown of creatine and phosphocreatine and also can function an indicator of renal operate.2 Creatine is synthesized within the liver, pancreas, and kidneys from the transamination of the amino acids arginine, glycine, and methionine. Creatine then circulates all through the physique and is transformed to phosphocreatine by the method of phosphorylation within the skeletal muscle and mind. The vast majority of the creatinine is produced within the muscle. Because of this, the focus of plasma creatinine is influenced by the affected person’s muscle mass. In comparison with BUN, creatinine is much less affected by weight-reduction plan and extra appropriate as an indicator of renal operate.
Medical Significance
The measurement of creatinine concentrations in plasma and urine samples illustrates the filtration capability of the glomerulus, also called the glomerular filtration fee (GFR.) Creatinine is produced endogenously throughout the physique and is freely filtered by the glomerulus. These traits make creatinine a helpful endogenous marker for creatinine clearance. If the GFR is decreased, as is in renal illness, creatinine clearance by way of the renal system is compromised. The lowered GFR will then result in a rise in plasma creatinine focus. The measurement of plasma alone shouldn’t be used to evaluate renal operate. Plasma creatinine ranges might not be affected till important renal injury has occurred. As well as, a plasma creatinine stage that’s inside regular reference vary doesn’t equate to a standard functioning renal system.3
Though not as particular as creatinine, BUN may also be used as an indicator of renal operate. BUN just isn’t the popular marker for clearance as a result of it’s influenced by elements comparable to a excessive protein weight-reduction plan, variables in protein synthesis, and affected person hydration standing. Alone BUN just isn’t the perfect marker for GFR. Mixed with plasma creatinine as a creatinine/BUN ratio, BUN generally is a helpful analyte in differentiating pre or submit renal enhance of plasma NPNs.4
Specimen Necessities
Creatinine could also be measured utilizing serum, plasma, or urine specimens. Components comparable to fluoride and ammonium heparin shouldn’t be used as a result of interference with the strategy of measurement. Measuring creatinine clearance requires the gathering of a 24-hour timed urine specimen and a plasma pattern gathering throughout the 24-hour urine assortment interval.
Serum or plasma could also be examined for BUN. Components comparable to fluoride and citrate shouldn’t be used due interference with the strategy of measurement.1
Testing Strategies
Creatinine could also be measured both chemically or enzymatically. The chemical methodology referred to as the Jaffe response entails creatinine reacting with picric acid in an alkaline setting to supply an orange-red coloured compound. The drawback to this methodology is that the response is non-specific for creatinine. Different sources within the affected person pattern comparable to ascorbic acid, acetone, or cephalosporins might erroneously produce an orange-red coloration as they react with the picric acid. The chemical methodology is time consuming and never extensively utilized in automated devices.5
Creatinine could also be additionally measured enzymatically. A number of enzymatic strategies using creatiniase have been used to measure creatinine. These enzymatic strategies make use of using a spectrophotometer to measure NADH to NAD+ or H2O2 to H2O.
BUN is most regularly measured utilizing enzymatic strategies. Step one entails the enzyme urease to hydrolyze urea, thereby producing ammonium. The second step entails the quantitative measurement of ammonium utilizing a wide range of strategies to find out the quantity of urea within the pattern. The varied strategies embody the glutamate dehydrogenase (GLDH) methodology which measures NADH to NAD+, measurement of ammonium ion conductivity, or an indicator dye that reacts with ammonium ions.6
Abbreviations
References – “creatine waste product”
“creatine waste product”