Your vet is suggesting blood work before surgery and this is done quite commonly though many vets do it more with senior dogs.
The reason they want to do this test is to make sure the dog will process anesthesia properly. The liver and kidneys work at removing anesthesia from the system. If they are working normally the vet gives the med to keep the dog asleep at the usual rate but if they are not then doing that might overdose the dog. So making sure the organs are working right is a good thing and routinely done before humans have surgery.
The reason your vet might particularly want to do this with your pup is because some Shih Tzu have a problem called a liver shunt. You can read about that here
http://www.vet.utk.edu/clinical/sacs/shunt/faq.php
Sounds like you found a nice cautious vet for Bella!
Hope this helps you!
Actually I think pre anesthetic blood work is a good choice to check on liver function as well as kidney function etc.
Putting the dog under anesthesia for a liver test wasn't mentioned in your question.
I'd suggest you ask exactly what the test is and why the vet thinks its required.
If they are doing a liver biopsy for example, which is one reason for putting a dog under anesthesia, you should know why they feel the need to do that before agreeing to it. Same if they want to do an ultrasound.
Your vet may be seeing signs in your dog of a health issue suggesting a biopsy would be a good idea.
I'd suggest you go back to your vet and ask for a full explanation of what they want to do and why. If its blood work I'd be inclined to suggest you do that for maximum safety for your dog. If its something else you want to know exactly why they think this is needed.
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