Oxidative deamination of amino acids results in the formation of amine groups that are released into the tissues in the form of NH4+ (ammonium ion). Urea, or carbonic acid diamide (CH4N2O), is a substance with a molecular weight (MW) of 60.06 Da that is formed in the liver through a cycle of reactions (urea cycle) to eliminate ammonia (NH3), a toxic substance. In humans, ureogenesis is, therefore, an expression of protein catabolism. Urea diffuses freely between intracellular and extracellular fluids and is, therefore, uniformly distributed in body fluids. More than 90% of plasma urea is concentrated and excreted in the urine through the renal excretory system, constituting approximately 80–90% of urinary nitrogen. The concentration of urea in the serum and other biological fluids depends on two factors: production by the liver and elimination by the kidneys. In healthy adults, the reference range for urea is between 3.5 and 9.0 mmol/L (21–54 mg/dL); for those aged more than 60 years, it is slightly higher. At birth and in the first years of life, the reference range is between 1.0 and 5.0 mmol/L (6–30 mg/dL). Recently, pediatric and neonatal reference values have been divided by age groups and analytical methods. Most analytical methods for the determination of urea are based on the enzymatic reaction of hydrolysis of urea by urease (urea amidohydrolase; EC 3.5.1.5) leading to the formation of ammonium ions. There are also electrochemical methods that are based on conductivity measurements in cells containing the reaction mixture (sample + reagent) or on potentiometry measurements using ammonium ion- selective electrodes in contact with a membrane on which urease is distributed. The absolute method for the measurement of urea is mass spectrometry. To convert a urea value from international system of units (SI) (mmol/L) to traditional units (mg/dL), multiply the value by 6.02; conversely, to obtain a value expressed in SI units from a result expressed in milligrams per deciliter, multiply the value by 0.166. Increases in plasma urea concentration are observed during reduced renal function, whereas significant decreases can be observed only in the presence of severe hepatic damage with impaired hepatocyte function. Some extrarenal pathologies can lead to significant increases in blood urea (prerenal hyperazotemia), such as, for example, hypotension, states of dehydration and hypovolemic shock, cardiac insufficiency, sepsis, and hyperthyroidism. Moreover, urea nitrogen is a product of protein catabolism, especially of those of food origin, and, therefore, its plasma levels can be significantly influenced by high or low protein content diets.

The elimination rate of urea is a function of its plasma concentration, of the renal plasma flow (perfusion rate), and of the ability to remove this substance from the plasma by the kidneys (renal function). Indeed, the amount of urea excreted by urine depends not only on glomerular filtration but also on tubular reabsorption; it is estimated that about 40–50% of filtered urea is normally reabsorbed by the tubules. The value of the concentration of urea in the serum can be expressed by either considering the entire molecule or in terms of nitrogen content in the molecule; in this case, we refer to urea nitrogen (blood urea nitrogen (BUN)). Since nitrogen contributes 28/60 to the total weight of urea, starting from a given value of urea nitrogen concentration, urea concentration can be calculated by multiplying this value by 2.14.

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Creatinine test for Diagnosis of Kidney Diseases

Standard Urine Test for Diagnosis of Kidney Diseases

Diagnosis of Ventilator-Associated Pneumonia

protein C, protein S

prostate-specific antigen (PSA), prostate cancer gene 3 (PCA3), MI-prostate score (MiPS)

prolactin levels (PRLs)

Enzyme-Linked Immunosorbent Assay (ELISA)

Agglutination Reaction

Precipitation Reaction

Heparins and Fondaparinux

Activated Partial Thromboplastin Time (aPTT)