[PennysMum]

It isn't incompetence on the part of the lab; it has to do with the life cycle of the worms. At certain points in the life cycle, the parasites may not be detected since they do not continually shed their eggs, which is what the lab checks for. Even using the most advanced equipment and high-speed centrifuges, fecal tests are only about 80% accurate, which is why a lot of veterinarians request multiple stool tests several weeks apart on puppies and kittens even after they have had one test come back negative.

Dispensing dewormer willy-nilly can end up being costly too, if you are worming for things that the dog doesn't actually have and then need to pursue additional treatment down the road when that does not work-- it's more responsible to know what you're targeting before handing out medication, especially since some parasites that are common in puppies do not respond at all to broad spectrum dewormers.