Has your pet been in for a procedure, and the option of a pre-op, or pre-anaesthetic blood test has been offered? Often offered as an extra prior to your pet’s procedure, it can be a useful tool to give your veterinarian a general overview of your pet’s health prior to their anaesthetic and procedure. However, it can be difficult to know whether it is the right thing for you and your pet. This article aims to outline what a pre-op blood test actually involves, what is tells your veterinarian, and whether it’s the right thing for you and your pet.  

What is a pre-op blood test?  

A pre-op blood test is run prior to your animal undergoing a general anaesthetic for a procedure, like surgery or dentistry.  

A sample will most likely be taken in the days leading up to, or on the day of the procedure. A blood sample will typically be taken from the jugular vein (in the neck), or the cephalic vein (in the front leg). Other veins may be used if these ones aren’t cooperating! Most of the time, this blood sample can be tested in-house, meaning the results will come back quickly. 

The results of the blood test are broken down into 2 parts:  

Haematology  

This tells your vet about the body’s blood cells, including red blood cells, white blood cells and platelets 

Biochemistry 

A basic biochemistry will give tell the vet about kidney and liver function, blood glucose and some enzymes. A more extensive blood test may also include electrolyte levels. In senior cats, it is often recommended to check the levels of a hormone called total T4, which can be diagnostic for hyperthyroidism.  

Why run a pre-op blood test?  

The biggest benefit is using the results to screen for any underlying disease or abnormalities. Examples of these might be:  

  • Anaemia  
  • Dehydration  
  • Kidney dysfunction  
  • Liver dysfunction  
  • High or low blood glucose levels  

Finding out about these abnormalities prior to anaesthesia will allow the patient to be stabilised first, and allow the veterinarian to change their anaesthetic protocol, if appropriate or required, or may even lead to the procedure being postponed. There are even occasions where an underlying condition can be picked up in an outwardly healthy patient that could severely impact on that animal’s safety under anaesthesia. 

It does also provide peace of mind, for both veterinarian and owner. That added layer of knowledge and care that is taken prior to anaesthesia can be very valuable in its own right. 

It is also a great way to get a baseline of your pet’s health, and have an idea of what is normal for them. These results can also be referred back to in the future, if your pet is ever poorly, to compare new blood results with.  

Pre-anaesthetic blood tests are more important for some patients 

The general recommendation is that animals over 8 years old should have pre-anaesthetic bloods performed. This is because they are far more likely to have organ dysfunction than a younger dog, which may greatly affect their anaesthetic, and what medications should be used. One study showed that of 101 senior dogs going for an elective procedure, 30 had new diagnoses based on changes on their bloods, resulting in 13 cancelling their procedure, and 6 having further diagnostic performed. 

Patients with previous concerning changes on blood tests would benefit from a repeat test prior to a procedure to see if there’s been any further change or deterioration.  

Animals that are long-term medication are also recommended to have had a recent blood test to ensure that their body is coping well with the medication. Some medications may affect the liver and kidneys, both of which also metabolise the anaesthetic drugs that we give, so it’s important to confirm this prior to proceeding with an anaesthetic. 

A good pre-operative assessment involves more than a blood test  

An evaluation of every patient undergoing an anaesthetic is essential. This would mean taking a good history of the patient to understand their basic bodily functions, any pre-existing conditions, and any medications they may be on.  

A thorough physical exam should also be performed, by checking the patient’s weight, checking perfusion parameters (mucous membrane colour, capillary refill time and peripheral pulse strength), performing a cardiac auscultation to check the heart rate, and for arrythmias or a murmur, recording the respiratory rate and listening to the lungs, and taking a body temperature.  

There may be something found in the history or physical examination that raises suspicion for your veterinarian, in which case a pre-operative blood test would be important to proceed with. They may also recommend collecting a urine sample for evaluation.  

Bloods might not be appropriate for every patient and owner 

A combination of factors may influence an owner’s decision to approve pre-op bloods. 

For example, in the young, otherwise healthy patient, the chance of picking up a concerning change on a blood test is low. There have been several studies that show that performing a pre-op blood test in an outwardly healthy patient rarely changes a patient’s anaesthetic protocol, and the benefits of performing bloods in these patients hasn’t really been proven. One study showed changes on bloods resulted in a protocol change in only 3% of dogs and 4% of cats. 

Another reason is finances. As tricky as the topic of money is, it’s an important one to have with your veterinarian. There may be cases when, due to financial limitations, pre-op bloods might not be the highest priority, even if that pet would be in theory be an ideal candidate to have one. A good example of this would be an old dog in need of a dental procedure. If there is a budget that needs to be stuck to, the decision may be made to focus on improving quality of life by removing painful teeth, rather than running pre-op bloods. This is a decision that would be made with together by vet and owner. 

For some animals, taking a blood sample might be a very stressful procedure, and some patients might need to have sedative medications prior to being examined. That stress may even affect the blood results (for example showing a falsely high blood glucose result). 

So, are pre-op bloods worth it?  

In general, yes, but it’s not as simple as running them for every patient undergoing an anaesthetic. If a thorough history is taken and pre-operative examination performed that doesn’t ring alarm bells for the veterinarian, the evidence tells us a pre-op blood test is unlikely to change the anaesthetic protocol chosen by the veterinarian. If your pet is a senior patient or has a known medical condition, a pre-op blood test should be strongly considered. The financial implication of running a pre-op blood test should also be taken into consideration.  Listen to your vet’s recommendations, and make the decision together as to whether it’s the right thing for you and your pet.