How reliable are the relaxin tests your vet can do on your dog.?

we have imported this dog from hungary to nz she was breed and she is showing all the signs off a prgnancy she has 12 days to go, but yesterday for the import paper work they did a relaxin test and it came back neg, can this happen, i have read it happends with small litters and in large dogs. she is a dogue de bourdeaux. around 60 kgs and we were thinking 3 to 4 pups. anyone had this happen or are we looking at a false pregnacy.