As a kid (early 20s) I used to work in a big city ER...starting out on the night shift. Many patients required a “tox screen” mainly for alcohol but sometimes heroin. The hospital couldn’t do them at night so we sent the sample,by cab,to a 24 hour lab about a mile away. We’d have the alcohol levels called back to us within an hour. Other things take longer. Also,with medical examiners I’ll bet there are all sorts of legal regulations involved.
I get results from my blood tests within a week................