At some level, the 'proper' way to treat disease is to first fully determine the biological cause and process of the illness, and then develop some intervention which will certainly reverse this.
In the case of parasites, this is easy - you remove the parasite, whether by surgery, drugs or sprinkling salt on it.
In many fields, the precise mechanism of action is not understood when a treatment is administered - for example, Jenners Variolation for smallpox had no sane provable mechanism of action, and was only really provable sometime shortly after 1950.
Statistics however were a simple, powerful, and inarguable proof - a tiny fraction of people died compared to the untreated group.
https://soundcloud.com/bmjpodcasts/...cognising-depersonalisation-and-derealisation was a good recent podcast that illustrates some of the problems with applying this approach to the brain.
It discusses a condition where though you recognise that it is in fact you doing things, it doesn't feel like you.
The right measure for that at this time is subjective, and there is pretty much no alternative.
It's quite plausible that a full understanding of this condition will not occur even when we can fully and deeply simulate an entire brain.
Ask the patients what their primary problem is.
Asking them how much it's affecting them, and trying to come up with questionaires asking how they feel about it is a good step.
A graph from
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3154208/ was recently posted on asthma and placebos - where subjective improvements with sham and placebo treatment were equal to the real drug, but objective improvement for these were zero.
This shows the problem with only using questionaires about feelings about your condition - it needs to be anchored in reality, or simple questions.
Not 'how do you feel about your maths ability' but 'what is 37*2'. Not 'what tasks can you do', but 'what did you do yesterday'. Not 'do you feel fatigue makes other people like you less'...
This is hard.
Arthritis was brought up earlier, and has lessons. Clinicians assessment of x-ray/MRI/ultrasound images of joints might be seen as the gold standard.
At least in some cases as I understand it though, apparent condition of the joints does not map well to functional changes.
I think it comes down to that you've got to ask the patient what's bothering them, how it's affecting their life, and then come up with measures that correlate well to measures of how it's affecting them.
If they say they can't walk far, that's easy to test.