When I asked my vet about doing this he said it would be a waste of time, as the bitch produces the same hormones if she is having a phantom or a true pregnancy. This was a few years ago, so maybe things have changed and they can test for something else?
I know how frustrating it is for a bitch to 'miss'. Chloe hasn't caught the last 2 times I've tried to mate her.
I hope things work out for you.