For every educated and honest client, I've met others that are not as well informed and who *seem* to feel the need to lie about whether their pet has been on a preventive (heartworm for example) as prescribed.
Not a 'lie' story, but an honest 'confused' one that I had recently:
We had a client present a dog for his annual checkup. He told me that the dog was getting heartworm meds regularly, every month. Said he didn't need a refill yet. He declined the heartworm test, but he did bring in stool.
The next day, I got a call from the wife. She asked me if the stool test results were back yet, because she wanted to know if she could re-start the dog on his heartworm pills.
Easily recalling the case from the day before, I asked, why did your husband tell me that your dog gets his heartworm pill every month and hasn't missed? (And I told her, yes, stool was fine, but that was checking for intestinal parasites... not heartworm infection).
She said, he didn't know any better. He thought she was giving the dog his pill, and she thought HE was giving the dog the pill.
The truth? The dog hadn't gotten a heartworm pill in 4 or 6 months, I can't recall that part of the story now.
I like to play devil's advocate too. What if I am the vet that authorizes a refill for heartworm meds on a dog that the owner swears he's been giving it for the past two years (which is when, perhaps in this case, the last test was done)? The dog has a reaction and dies.
I don't want to be the vet having to defend my license on that, for such a simple measure as neglecting to require a blood test.
The day we eliminate the risk of positive heartworm infection causing reaction with a preventive is perhaps the day the medication can become OTC. Until then, it's in the pet's best interests, and yes, the prescriber's, to cover all bases.
Not sure how strict other practices are, but in my own, we have the policy of annual heartworm testing, but, like many policies, there's always exceptions made. It's up to the doctor whether making the exception to what is known to be the generally accepted standard is worth the risk in these situations.