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Despite the efforts by the scientific community to find a test that would enable an accurate diagnosis to be made with objectivity, depressive disorders, at the moment, do not have any specific technique available useful for making this diagnosis.
At the moment, none of the imaging tests that are used to “see” the brain (CT scan, scanner, magnetic resonance, radiography, SPECT, PET, functional magnetic resonance, the electroencephalogram [EEG], blood analysis—including serotonin and/or lithium—, or genetic studies using saliva or mucosa from the mouth) are useful for diagnosing depressive disorders.
The only relevant information source for the diagnosis of this illness, is the clinical interview with the patient (and, often, with a close relative). It is also important to highlight that the use of scales and clinically structured interviews cannot ensure the infallibility of the diagnosis.
Is it useful to measure blood serotonin levels?
For several decades, it has been suggested that the cause or aetiology of depressive disorders (particularly, depressive episodes—major depression) could be due to a decrease in the levels of serotonin in the brain. However, it has never been able to be demonstrated, and for this reason, it is currently not justified to measure serotonin levels in blood with the aim of making or ruling out the diagnosis of a depressive disorder.
On the other hand, to quantify the levels of serotonin in blood adds a significant confounding factor or, better said, error. The margins of “normality” that are shown in the results report of the analysis make reference to “normal ranges” for individuals on experimental treatment with tryptophan (a molecule that the body uses to make serotonin). The experimental treatment with tryptophan gives rise to artificially (and uselessly) elevated blood levels of serotonin; if evaluated with this “artificial” normal range, in patients that are not ingesting tryptophan supplements, the levels of serotonin in the blood appear “abnormally” low (when in reality, they are normal).