Worms, gotta love them...my vet told me that only 25% of wormies show up in samples, or rather, that only 25% of samples show worms...which DOES NOT mean your pup doesn't have worms if his sample is clear. Doesn't mean he does, either. But, why don't you call your vet, and just double check? Ask if it is necessary. It is a rather easy process to worm...