There is a recent paper about assessment of a suicidal risk in psychotic patients https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4759104/
One of the important points is that the authors compare gene expression levels between alive patients and post-mortem suicidal completers (RNA extracted from blood <24h post-mortem).
In result, they identify suicide-specific blood biomarkers that also have "biological roles in immune and inflammatory response, growth factor regulation, mTOR signaling, stress, and overall the switch between cell survival and proliferation vs apoptosis".
Do you think that one may really perform this type analysis if we know that post-mortem biochemistry is very different? Plus they killed themselves via different means (gunshot, hanging, electrocution, slit wrist etc.)