"Indeed, you can. What you cannot do, however, is remain alive to get the results."

If we're broadening our minds to all possibilities, it's worth pointing out that you can expose a third party to your as-yet-unconfirmed prion disease and then destructively test them, which would then allow you to test yourself while remaining alive.

Unless they are negative?

No, he is making a joke that you can only test by looking at slices of brain tissue (which require you to be dead to do)

You don't have to be dead when they start, but you will be when they finish.

I don't understand. Aren't biopsies of the brain possible? I still remember that episode of House MD, Euphoria.