Renal Function

The kidneys regulate the amount of water and salts that we have in our bodies. They do this by filtering the blood through millions of structures called nephrons. The kidneys also pass out certain waste products from the body. Urine is made up of the excess water, salts and waste products passed out by the kidneys down to the bladder.

The usual blood test which checks that the kidneys are working properly measures the level of urea, creatinine, and certain dissolved salts. 

 

Urea is a waste product formed from the breakdown of proteins and is passed out in the urine.  A high blood level of urea ('uraemia') indicates that the kidneys may not be working properly, or that you are dehydrated (have a low body water content).

 

Creatinine is a waste product made by the muscles.  A high blood level of creatinine indicates that the kidneys may not be working properly.  Creatinine is usually a more accurate marker of kidney function than urea.

 

Estimated glomerular filtration rate (eGFR) also provides a guide to kidney function.  Although the level of creatinine in the blood is a useful guide to kidney function, the eGFR is a more accurate measure. Blood creatinine can be used to estimate the eGFR using age, sex, and race. This is often calculated by computer and reported with the creatinine blood test.  The eGFR usually becomes lower with increasing severity of kidney damage.

Dissolved salts that are routinely measured are sodium, potassium, chloride and bicarbonate. They are sometimes referred to as 'electrolytes'.  Abnormal blood levels of any of these may be due to a kidney problem, however some other conditions may also alter the salt balance in the blood.