Preanaesthetic Blood Tests

In addition to a physical exam on the day of your pet’s procedure, preanaesthetic blood testing is typically recommended for most animals that are undergoing anaesthesia. Preanaesthetic blood testing allows us to make a valuable assessment of your pet’s overall health, including hydration status, the function or status of some internal organ systems, and circulating red and white blood cells and platelets. Abnormal results can indicate the presence of disease which may not be evident on a physical exam.

If preanaesthetic bloodwork shows any abnormalities, these can be addressed by making adjustments to your pet’s treatment plan. In some cases, the results will help us to determine the safest and most appropriate anaesthetic drugs to use.

There are two primary components of a preanaesthetic blood test; a complete blood count (CBC) and serum biochemistry.

Complete Blood Count (CBC)

The CBC assesses three types of cells in your pet’s blood.

Red blood cells (RBC) – These very important cells carry oxygen through your pet’s blood to their tissues and organs. Abnormalities in your pet’s red blood cell count can indicate the presence of several diseases, including anaemia (low red blood cells) and polycythaemia (increased red blood cells).

White blood cells (WBC) – Elevated white blood cells can indicate infection or inflammation. Abnormally low white blood cells can indicate a more serious infection or immunodeficiency. Less commonly, substantial abnormalities in white blood cell counts may also indicate cancer.

Platelets – Platelets are responsible for blood clotting. A low platelet count reading can suggest an increased risk of blood loss and bleeding during surgery.

Serum Biochemistry

The serum biochemistry assesses the level of various chemicals and enzymes in the blood that are associated with the function of key internal organs, such as the kidneys and liver.

Kidney function – This is assessed by measuring three key markers – urea, creatinine, and SDMA – all of which are normally cleared and excreted by the kidneys. Elevations in these markers can indicate that the kidney is not effectively excreting these substances, either due to dehydration of kidney disease.

Why is kidney function important to understand before anaesthesia?

In order to perform their role effectively, the kidneys receive a large proportion of the blood flow coming from the heart. During anaesthesia, even with the use of intravenous fluids and careful drug selection, a period of reduced blood pressure is anticipated. Where we have good kidney function, this is well tolerated and not a problem. However, if there is any sign of compromised kidney function on the preanaesthetic blood test results, the treatment plan may need to be adjusted.

The kidneys play a role in the excretion of certain substances including metabolic waste products for removal in the urine. This includes the excretion and removal of drugs that are used for anaesthesia, so for this reason, understanding kidney function prior to anaesthesia is also naturally important.

Liver function – This is assessed by measuring a number of markers. These include the liver enzymes alkaline phosphatase (ALKP) and alanine transferase (ALT). Elevations of these values can indicate various types of liver disease and may have implications for the metabolism of anaesthetic drugs by the liver.

Other important markers – Measurement of the total serum protein level provides a good overall marker for hydration status. Low total protein levels may also be seen with a number of medical conditions. Measurement of serum glucose, as well as serum albumin (an important protein produced by the liver) also have implications for assessing the status and function of the liver. Measurement of the globulin levels also takes place. Globulins are a type of protein produced by the immune system, and serve as a marker for inflammation or infection, and some types of cancer.

What happens if there are abnormalities with my pet’s blood tests?

If your pet’s blood tests show abnormalities, your veterinarian will take appropriate steps depending on the type of abnormality found and the significance of the change.

Some mild abnormalities that occur can be easily explained and the treatment plan may continue as normal. An example of this could be mildly increased white blood cell counts or glucose, which may indicate stress. Other abnormalities less serious in nature that are sometimes encountered may indicate mild dehydration. This can be corrected prior to anaesthesia through the use of intravenous fluids for several hours.

However, if more significant abnormalities are found which could indicate the presence of disease, the vet will contact you to discuss the best way forward for your pet. In some cases, the procedure may go ahead with a recommendation to simply recheck (or monitor) the abnormal values in the next 1-3 months. In other cases, the vet may recommend postponement your pet’s procedure until further investigations are carried out.

My pet had a blood test recently. Do we need to do another blood test now?

This will depend on the situation and is something that should be discussed with your vet. Performing blood tests on the day of your pet’s surgery or procedure is always ideal. The various markers in your pet’s blood which are measured can change rapidly. Red blood cell counts change very rapidly with blood loss, and this can be something that goes unnoticed. White blood cell counts change within hours when infection is present. Liver and kidney disease or insults can manifest over days or weeks as well.

If your pet has had blood tests recently, the decision about whether or not to do a blood test on the day of the procedure is a risk-benefit discussion and needs to take into account a range of factors. These include the age and health status of your pet, just how recently any blood tests took place (and what was measured), and whether or not they are on any ongoing medications.

In some cases, prior blood tests may have shown abnormalities, and preanaesthetic blood testing may be appropriate and an opportunity to monitor or recheck any abnormal values. For many pets on ongoing medications, regular monitoring of liver and kidney values is recommended also.

There may be situations where it is reasonable not to repeat a test. This may include situations where blood testing has been done in the last few weeks during consultations, where the procedure was in the process of being discussed and planned. It may include younger animals where testing very recently has shown excellent results with no abnormalities. In all cases, it is best that this decision be made in discussion with your vet, who will take into account the specific circumstances that affect your pet.