Urea vs creatinine -explained

Two important biochemical parameters that are measured in blood in order to assess the function of the kidney are blood urea and creatinine. Blood urea is synthesized in the liver from the ammonia produced in the intestinal tract as a by product for the digestion of protein. So, ammonia produced in the intestinal tract

is transported to the liver and then converted into urea.The urea is excreted through the kidney. So, this why blood urea is used as a test for the function of the kidney.

The second parameter is creatinine. Creatinine is a metabolic product produced from the muscles. First, creatine is synthesized in the liver and then transported to the muscles, combined with phosphorus and result in the formation of phosphocreatine

and then, when the body needs energy,the phosphorus is used for synthesis and the creatine is converted into creatinine. The creatinine produced from the muscles is excreted only through the kidney. So, here, these two parameters excreted through the kidney and this is why used to assess the function of the kidney.

However, urea is considered sensitive for renal diseases

but not specific. Creatinine is considered specific but not sensitive.So, the presence of mild renal disease, will result in increase the level of urea in the blood.Why urea is considered none specific for renal diseases is because urea is affected by the levels of proteins produced in the intestinal tract when large amount of protein produced. This means that more ammonia is produced and transported to the liver and will result in increase the level of urea which is not related to a disease in the kidney.

Also, in presence of abnormalities in protein metabolism

will result in reduction in the production of ammonia

and affect the level of urea in the blood. The presence of disease in the liver will affect the production of urea and may result in

reduction in the level of urea in the blood. 

Another factor that affect the level of urea in the blood is dehydration. Urea increased in the blood circulation in cases of dehydration So, cases associated with diarrhea or vomiting

or any other causes of dehydration will result in elevation of urea level in the blood.  So, renal disease is not the only cause for elevation of urea. However, if we excluded other causes that will result in elevation of urea, the kidney will be the cause, at that time.So, urea is considered sensitive it increase in mild renal diseases but, urea is not specific, because there are other factors

affect the level of urea in the blood.

Regarding creatinine, It produced in muscles.

However, creatinine excreted only through the kidney . Creatinine is not affected by the diet. Creatinine is not increased in cases of dehydration. So, creatinine is specific for kidney diseases

but, it increased in late renal diseases.Not like urea, it is specific. Once creatinine increased, the renal disease is confirmed. 

Usually increase of creatinine is associated with elevation of blood urea. However, elevation of blood urea is not

is not essentially associated with elevation of creatinine levels in the blood.  Creatinine level in the blood can decrease in cases of wasting of, in cases of emaciation or wasting of muscles due to the decrease the muscle mass that will result in decrease the production of creatinine and affect the level of creatinine in the blood. So, creatinine level can be decreased in cases of liver diseases or in case of wasting condition, or decrease the muscle mass in the body.