That's a really simplistic view of a complicated problem. For an example of why that can't work, read about the studies on the relatives of actress Glenn Close. One is bipolar and one scizophrenic. Both were found to have a genetic variant that might be summarised as causing a glycine deficiency (possibly just in the brain IDK). Treating them with high doses of glycine eliminates symptoms in both of them. It seems unlikely that even checking blood levels would indicate a problem. Perhaps a genetic test? But this is apparently not a common genetic marker in those two conditions.
Another solution in some cases is to change a person's environment, relationships, job, etc... Some depression can be relieved by changing ones worldview or other purely psychological interventions.
In short, the notion of objective physical measures indicating specific conditions is not possible in most cases.
SSRIs are used to increase serotonin levels, but they dont even use a serotonin test to diagnose. I have my suspicion that there's not a strong correlation.
All problems that are undefined are "complicated". That doesn't change the fact that defining them properly is a prerequisite to dealing with them productively. Calling such efforts "simplistic" is just calling for surrender.
We know that there is never going to be a straightforward genetic test for mental illness because plenty of studies have been done on identical twins, so it is clear there is no simple mutation out there to test for.
It is possible that there are metabolic abnormalities that show up in cerebrospinal fluid and not in blood tests. Assuming that because we use SSRIs we should therefore test serotonin is quite contrary to what I am suggesting.
It should be treated as axiomatic that there is a physical basis for variations in mood and cognitive function, and we need to be able to objectively quantify it. That's not simple to work out, but just because it's difficult doesn't make it less essential. Making up vague stories about causes is what's simplistic and does not help produce better treatments.
Perhaps functional MRI could help make a diagnosis of depression? Even with different underlying causes, a classification could perhaps be made based on imaging.
You can talk about MRIs, or genes, or serotonin all day, but that doesn't mean you know what depression (or other mental state) is - using the current tools you assume you know what it is and that is the problem.
One way to measure objectively is to look at whether someone is employable and maintains employment. But that is far too low resolution and non specific. People change their mood during a day and from one day to a next, and there must be physical correlates in principle.
There is a huge human cost to the fact that nobody knows what they are doing, because they cannot relate treatment to anything objective. Sometimes a drug seems to be not working well after several years, and the dosage is raised, and all the while the side effect of the drug is what's causing the problem, so it gets worse the higher the dosage goes. In the mean time, that person becomes unable to work, and is caught in a series of catch-22s.
Another solution in some cases is to change a person's environment, relationships, job, etc... Some depression can be relieved by changing ones worldview or other purely psychological interventions.
In short, the notion of objective physical measures indicating specific conditions is not possible in most cases.
SSRIs are used to increase serotonin levels, but they dont even use a serotonin test to diagnose. I have my suspicion that there's not a strong correlation.