Dasani, you are incorrect in this statement. Serum levels can be done for ANY narcotic. That's not saying that the police actually performed a serum level test at the time of the arrest. In fact, they would first have to identify the intoxicant, then run a serum level test for that substance from a sample taken at the time of the arrest (or shortly thereafter.)

I believe serum levels could be argued by the defense as some people are highly tolerant to opioids and a specific serum level for one person may not be as intoxicating as the same serum concentration in another person.

But then again, all states have BAC thresholds at which a person is considered intoxicated--whether they can pass the roadside sobriety test or not.