I doubt the ME was involved as Donnie was taken to a hospital for treatment. The hospital assuredly ran a toxicology panel as they do with all suicide attempt victims. They would have results for common drugs of abuse within the hour. Prescription drugs (ie antidepressants) can be measured within a short time too. Donnie was considered as an organ donor so I doubt his tox panel was that bad or the family would have been told he wasn't eligible.
IIRC the family consented when asked about organ donation, but in the end his organs were not harvested as he had been in intensive care for too long. Traumatic brain injury often results in other organs failing.
Not to mention they were probably roasted from drugs, alcohol, poor diet and who knows what else.
ETA - I see I was a day late and a dollar short on my thoughts on his organs.