That was my thought also while reading your post.Ps the vet never ever tests for anything here, I'm not sure why. Leaves me feeling like I'm throwing random antibiotics at her hoping one will work
Testing is important to know what you are dealing with.
Random antibiotics can make it even worse as they often have serious side effects and will weaken the birds natural immune defense.
But the problem with treating chickens is that they hide their unwellness so well and pretend to be healthy for such a long time, that when we finally notice that something is off it is often too late for testing and waiting for the results to come in, as this will often take up to 7 days and if we waited until then to treat the bird, it would have died before we could even start the treatment.
This just to explain why vets often don't test prior to prescribe broad spectrum antibiotics for chicken/birds.