Kidney Disease - What Are the Diagnostic Tests?

In addition to removing soluble waste products from the body, the kidneys help to regulate water balance and also produce erythropoietin, a hormone which controls red blood cell production in the bone marrow. With kidney disease erythropoietin production is often decreased which can result in chronic anaemia.

What tests are required for the diagnosis of kidney disease?

Primarily all that is required is a small blood sample and a urine sample obtained at about the same time. The blood sample can be used for haematology investigation, i.e. complete blood count (CBC) investigation and also a biochemical profile. The urine will be subjected to general urine analysis. 

What will these tests show if kidney disease is present?

CBC- This analyses the red blood cells, the white blood cells and the platelets in the blood sample. A decrease in red blood cell production and reduction in haemoglobin levels may indicate possible renal problems.

If the problem is due to infection or is inflammatory in nature, the white blood cell count is likely to be increased. Sometimes platelet numbers are reduced if a clotting problem has occurred as a result of the kidney disease. 

Serum-  This is the liquid part of the blood and contains many substances including enzymes, proteins, fats, glucose and metabolic waste products. 

Biochemistry- Two of these tests are urea (blood urea nitrogen – BUN) and creatinine. BUN is one of the waste products of protein breakdown in the body and creatinine is produced as part of the normal muscle metabolism. Most of these waste products are normally cleared from the bloodstream by the kidneys and excreted in the urine. Thus increases in BUN and creatinine are often seen with kidney disease. They can also occur with dehydration and other causes, which results in reduced blood flow to the kidneys.

Electrolytes- Other substances whose blood concentrations are regulated by the kidneys are also measured. These include electrolytes such as sodium, potassium, chlorides, bicarbonate, phosphorus and calcium. An indication of the level of these electrolytes is important since wide variations from normal can result in other problems, e.g potassium which is often increased with kidney problems can cause heart problems (cardiac arrhythmias).

Urine analysis

A compete urine analysis is important for proper interpretation of the BUN and creatinine values of the serum biochemistry. In addition, analysis of the urine leads to clues with respect to the underlying cause of the kidney problem. Urine specific gravity (USG) gives an indication of the concentration of the urine. With kidney disease the kidneys' ability to concentrate urine tends to be lost so that the USG is usually low. This, combined with a serum biochemistry result showing a high BUN and creatinine indicates that this is not the result of dehydration but due to a problem with the kidneys.

The presence of blood or increased protein in the urine sample can be an indication of possible urinary tract infection since some kidney diseases are characterised by a large loss of protein into the urine.

"The presence of blood or increased protein in the urine sample can be an indication of possible urinary tract infection since some kidney diseases are characterised by a large loss of protein into the urine."

Evaluation of urine sediments using special techniques is also important in the diagnosis of kidney disease. Blood cells, bacteria, crystalline material and tissue cells can all be found in urine from diseased kidneys. Crystals will often indicate the presence of urinary tract stones or, in some cases, the ingestion of toxins such as antifreeze (ethylene glycol) which has resulted in the kidney problem.

Are these tests specific?

Occasionally changes seen in the screening tests will point to an inflammatory or toxic cause for the kidney disease. However, there are many causes of kidney problems and in addition the kidneys have huge inbuilt reserves. Therefore an increase in BUN or creatinine is frequently not detected until over 70-75% of kidney tissue has been damaged. By this time many kidney diseases have progressed to a chronic phase and the original cause may not then be apparent.

Nevertheless these tests are still extremely useful particularly in the case of kidney problems and can, in some cases, indicate the need for further more specialised or more specific tests.