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A dog's breed can affect pain sensitivity, but not necessarily the way your vet may think
(www.sciencedaily.com)
tl;dr: A dog may seem to have a low pain tolerance due to reacting quickly to painful stimuli, but the reaction could be caused by increased anxiety, and vice-versa.
It's an interesting study, though it does seem obvious in hindsight. I would anecdotally agree: my one dog has given me no reason to believe that she has a high pain tolerance, but the vet has always made a point to mention how she never so much as flinches when she needs a lab draw or something. I'm pretty certain this is because she adores the vet and all of the vet techs. She has zero anxiety there and, in fact, it's kind of her happy place. She's getting pets and praise, of course she doesn't care that she gets an uncomfortable little poke.