Cat & Dog - Kidney Disease Tests

The kidneys remove soluble waste products and help to regulate fluid balance in the body.  They also produce erythropoietin, a hormone which controls red blood cell production in the bone marrow.  With kidney disease erythropoietin production is often decreased resulting in chronic anaemia in addition to the more common signs of kidney problems, which include increased thirst and signs of toxaemia (uraemia) due to failure to eliminate nitrogen waste.

What tests are required for the diagnosis of kidney disease?

All that is required is a small blood sample and a urine sample obtained at about the same time.  The blood sample provides a complete blood count (CBC) and also a biochemical profile.  The urine is used for 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 will 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.  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 electrolytes, enzymes, proteins, fats, glucose and metabolic waste products.  

Biochemistry

Two important biochemistry tests are urea (blood urea nitrogen - BUN) and creatinine.  BUN is one of the waste products of protein breakdown in the body.   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.  Increases in BUN and creatinine are often seen with kidney disease.  They can also occur with dehydration and other causes, which result in reduced blood flow to the kidneys.

Other substances whose blood concentrations are regulated by the kidneys are also measured.  These include electroylytes such as sodium, potassium, chlorides, bicarbonate, phosphorus and calcium.  These values are important since wide variations from normal can result in other problems, such as increased potassium levels which often occur with kidney problems can cause heart irregularities (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 full analysis of the urine can indicate 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 indicator of possible urinary tract infection.

Evaluation of urine sediments is also important.  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 kidney damage.

Are these tests specific?

Occasionally changes seen in these 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% of kidney tissue has been damaged.  By this time many kidney diseases have progressed to a chronic phase and the original cause may not be apparent.

Nevertheless, these tests are still extremely useful and can, in some cases, indicate the need for further more specific tests.

Recently, a very sensitive test that can detect minute quantities of albumin (microalbuminuria) has become available which has revolutionised the detection of very early kidney disease. See handout on Early renal disease.

Please feel free to contact us if you wish to discuss any specific problem.