Is it worth treating asymptomatic Giardia?

I found out something interesting about Giardia from a cat specialist one time. I had adopted 2 kittens to a friend in another state, and when she moved them there, they almost immediately developed digestive issues and were diagnosed with giardia. Apparently, giardia exists in nearly every part of the world, most people/pets are exposed to it, but because our bodies adapt to the giardia in our own area, most show no signs of it. It is not until you move a cat, particularly if they have a lowered immunity (the kittens were orphaned at a very early age), that the new strain of giardia will affect them. When we adopted a third kitten to the same friend a year later (separated from its mom too early), that kitten also got giardia almost immediately. Giardia is in most water systems across the country.

What I learned from this is that most cats will have exposure to giardia and most will have no issues with it because their healthy immune systems can fight it off. This explains your vets comments about hard to get rid of for good. If your cat has symptoms, then she does need the medications. The question is how severe those symptoms are and how long it will take the lab to confirm the results of the test. If this were me, if she is having issues, I'd start the treatment now and monitor the other cats for illness. You might have another discussion with your vet on the topic to see if you really need to treat everyone. But this should raise a red flag about the girl showing symptoms. If you haven't moved or changed your source of water, is there something else going on that is lowering her immunity right now?

IMO, it doesn't make a lot of sense, but I say that with reservation. If your vet is pushing for you to do this, they know your situation better than I do, and I will never question another person's vet. You, on the other hand can question them.