Urea is produced as a waste product of protein metabolism and is readily found within the blood. The kidneys are responsible for filtering this urea out of the blood into the urine. Consequently, a urea test is commonly performed to determine how well the kidneys are functioning, with elevated levels of urea in the blood suggesting impaired kidney function. However, consuming large amounts of protein or the presence of dehydration may also result in elevated levels of urea. In comparison, low amounts of urea may indicate liver failure, malnutrition, excessive hydration or the consumption of a low protein diet.

Creatinine is a breakdown product of creatine phosphate that is found in muscle. Serum creatinine is an important indicator of renal health because it is an easily measured by-product of muscle metabolism that is excreted unchanged by the kidneys. Causes of elevated serum creatinine can be attributed to by a variety of lifestyle factors including, consuming creatine supplements, high amounts of animal protein, or performing high intensity exercise. Whilst other causes include impaired kidney function. On the other hand, low serum creatinine can be an indication of the kidneys functioning efficiently. In addition, a low protein diet or reduced muscle mass may also result in a low reading.

The estimated glomerular filtration rate is one of the best tests used to assess how well your kidneys are functioning. The kidneys are composed of thousands of glomeruli which act as tiny filters that are capable of filtering waste from the blood. The eGFR is an estimation of how much blood passes through these glomeruli with each passing minute and can be calculated based on blood creatinine levels, gender, age and weight. A high eGFR indicates optimal functioning of the kidneys. In contrast, a low value for eGFR means your kidneys may not be working to their full potential and can be a sign of kidney disease. eGFR can also be a useful tool in determining the stage of kidney disease which is important in identifying the best treatment for you. Early detection of kidney disease means there could be a better chance of slowing or stopping the progression of the disease. A low eGFR may also be observed in individuals from certain ethnic backgrounds or those with increased muscle mass, in which case impaired kidney function may not be the case.