The thing is that this will not change unless someone files malpractice suits against them.
If a lab misses finding the parasites, submit again until they do. Once they admit that you do have parasites file a lawsuit asking for a refund of all previous test PLUS punitive damages for continued pain and suffering due them not finding the problem the first time.
After they have been sued a couple of time, win or lose, the insurance companies will force them to fix the problem or lose their liability insurance.