You have a defendant indicted for DWI. Their SFST's are average. Their blood comes back negative for alcohol. Their DPS labs for drugs come back positive with a numerical result of "Alprazolam 0.075 milligrams per liter" (making the drug and amount up as an example). How do y'all tell the jury what that drug in that amount actually means in terms of effect on the defendant? The DPS lab won't testify to effect, just amounts, right?

You're going to want a Drug Recognition Expert testify as to the effects of Xanax on a person's mental and physical faculties in those amounts.

I'm interested in this topic, too, as I have a DWI drugs-only trial coming up next week. I'm wondering what objections might come up and how to answer them if I have a DRE testify to effects on the body when the defendant refused to submit to a DRE evaluation?

I suspect defense counsel may object to speculation, but I can respond that the witness is a DRE and he can testify to his expert opinion, yes? Anything I'm missing there?

DPS lab folks will testify if driving behavior, SFST results, defendant's words/actions correspond to the substance(s) found in their blood. Without a ton of information (likely more than the standard DWI report contains), no one will be able to testify that xxx mg/l of any drug means that defendant was intoxicated. Too many variables. The main one being if the defendant is taking that substance as a recreational drug,or has been prescribed that substance (and if so, how long have they been taking it on a daily basis). Someone who takes Xanax every day for the last eight years may have their normal mental and physical faculties, and have a certain level of Xanax in their blood. That exact same level, in a person who has never taken Xanax could be highly intoxicating. I like the idea of getting a DRE to testify, but unless you have a complete interview with the defendant, your DRE won't likely have enough information either.

Another thing to keep in mind is that "[t]he definition of “intoxicated” in the driving-while-intoxicated statute focuses on the state of intoxication, not on the intoxicant [or quantity thereof]." Salim.

All drugs have a range of "therapeutic level," and if the amount of the drug is in the therapeutic range, the criminalist should be able to testify that the drug is having "some effect" upon the user. Whether the effect rises to the level of intoxication/loss of normal use, is a factual issue to be argued.