Urea is one of the most important indicators of kidney function. Urea is the end product of protein metabolism. It is synthesised by the liver, passes easily through all membranes and quickly enters all the body’s media. The concentration of Urea in the body depends on how fast the liver produces it and how fast the kidneys excrete it. Urea synthesis is rarely impaired in the liver and is rapidly filtered in the kidneys (40-50% is reabsorbed and then actively secreted by tubular cells). Therefore, this parameter is mainly used to assess kidney function (along with creatinine). Unlike creatinine, it reflects total kidney function. Creatinine, and especially cystatin-C, is more reflective of filtration. Creatinine is a more specific kidney test, but can be ne such a sensitive indicator in some cases of kidney disease. Both indicators are complementary. The urea reduction ratio during kidney dialysis is an important prognostic indicator.

Urea and creatinine do not necessarily increase or decrease together in the same way. When kidney function is compromised, urea levels rise early. Only when the serum creatinine concentration increases by more than 250 µmol/l can it be assumed that the accompanying increase in urea concentration is related to renal failure. On the other hand, if the urea concentration rises above 30mmol/l, an extrarenal cause can almost always be excluded. Urea depletion occurs during liver dysfunction, when there is insufficient urea production.

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