While he’s furiously googling, here’s the answer:
The effects of Fentanyl at a given blood concentration are not fixed, particularly in chronic users and outside of therapeutic levels. So the level of fentanyl in the blood that results in death is hard to pin down. Therefore, the quantity of fentanyl in the blood is not very informative in determining whether fentanyl was the cause of death.
Norfentanyl is the metabolite of fentanyl. The body breaks down fentanyl into norfentanyl over time. The presence of norfentanyl can indicate remote use and/or chronic use with higher levels being more consistent with the latter which is in turn consistent with a higher tolerance.
So the ratio of fentanyl to norfentanyl is the more relevant number in an overdose case. That ratio is higher in overdose deaths, I presume because the drug kills too quickly for the body to metabolize it. Apparently, there is no norfentanyl in a significant number of overdose death autopsies.
A lower ratio of fentanyl to norfentanyl is, of course, less consistent with overdose death because presumably the body has survived and metabolized more of the drug and/or the recent dose isn’t as great relative to the body’s tolerance.
In this case, the ratio was pretty low. Lower than the median of all fentanyl DUI cases studied by this lab over the relevant period (which is significant because presumably the people driving were alive) and much much lower (less than a quarter) of the average levels for blood samples tested for autopsies where the autopsied individual had fentanyl concentrations in the same range as Floyd’s.
Now everybody else can be glad that I shortened all that to “the fentanyl level” instead of glazing your eyes over with this **** you will never need to know.