While there is an ISO standard for pet chips, there are several competing versions that are also in use. I think my vet said there were 3 or 4 different kinds and each had a different kind of reader.
There's three different frequencies commonly used in microchips for pets: 125, 128, and 134.2 KHz (the last being the ISO standard). Hope your local animal shelter has a universal chip reader, because I doubt your vet knows whether the chips they use are standards-compliant or not.
ISO standards would be the least of my concerns given how common it is for implanted pet chips to migrate around the body and end up in the dog's leg etc.