For forensic toxicology testing, a number of bodily sources are tested for drugs. These include examination of stomach contents (which may remain even if the victim vomited), heart blood, peripheral blood (femoral vein), urine (often enough is still present in the bladder/ ureters even if the victim voided), and vitreous fluid of the eyeball. For really chronic poisonings (arsenic comes to mind), some substances can also be tested by taking hair samples (usually pubic hair, not head hair, BTW).
Initially, the screens aren't looking for specific drugs (Benadryl/ diphenhydramine mentioned above, which is an antihistamine with sedative effects). Drug categories screened for include broad categories of benzodiazepines, opioids, amphetamines, and cannabinoids, as well as some others-- all of the specific drugs in those categories share similar chemical signatures and metabolic pathways.
The vitreous, by the way, demonstrates drug concentrations over time that have diffused into the eye fluids-- the vitreous humor is not a vascular compartment (receiving blood flow). It's not as useful for acute poisonings. For example, Michael Jackson's vitreous humor showed very high concentrations of propofol component molecules, adding to the evidence and testimony that he had been receiving intravenous propofol in very high amounts, regularly over time. His body was literally "soaked" in propofol.
If the Medical Examiner says Cooper's tox was negative, I'm going to believe that.
With the evidence of the scratches to the face and the head abrasions, as well as the physical appearance of the body (eyes partially open, tongue protruding), all of that point to a child that was not even slightly sedated. Sadly, everything points to this child being fully conscious and suffering terribly as he died.
This is a pretty good article.
http://www.webmd.com/mental-health/addiction/features/the-truth-about-toxicology-tests?page=2
''The first thing we would do is a basic screen for drugs in the urine and in the blood," Magnani says. The search would be for drugs such as opiates, amphetamines, marijuana, alcohol, and barbiturates, she says.
The basic toxicology screen typically uses an immunoassay, Robin says. This type of test looks for drugs in the blood using specific antibodies that detect various classes of drugs.
If something shows up, a more sophisticated test is done, using techniques such as mass spectrometry, which can identify chemicals in substances by their mass and charge.
"These confirmatory methods are actually more sensitive," Robin says. "You can find lower quantities [of the substance]."
The more sophisticated tests can tell experts the exact concentration of the drug or other substance, says Hall, who is also clinical assistant professor of public health at Weatherford College in Weatherford, Texas.
Experts also can determine if two drugs found together may have had a synergistic effect -- which happens when two drugs similar in their actions produce an exaggerated effect when taken together. It's akin to ''one plus one equals five," Robins says.
If joypath comes by, perhaps she would be willing to comment more on toxicology testing.