A Comparison of High Drug Concentrations in Impaired Driving and Postmortem Casework in Harris County, TX

Drug-impaired driving is a growing public safety issue. Addressing impairment due to drugs other than ethanol can be challenging for forensic toxicologists as many factors need to be considered including the type of drug(s), drug-drug interaction, the dose(s) and the individual's physiological condition and drug use history. Interpretation of blood drug test results is additionally difficult as drug concentrations in impaired driving cases may overlap levels typically viewed as toxic. This study compares blood concentrations of drugs in impaired driving cases to those in postmortem cases in Houston, TX, from 2014 to 2020. Blood drug concentrations from driving while intoxicated (DWI) or driving under the influence of drugs (DUID) cases submitted to Houston Forensic Science Center (HFSC) and Harris County Institute of Forensic Sciences (HCIFS) were compared to postmortem blood test results from HCIFS. Eight DWI/DUID cases had drugs that exceeded impaired driving concentrations reported in the literature. These drugs included fentanyl (220 ng/mL), oxycodone (680 ng/mL), hydrocodone (310 and 490 ng/mL), clonazepam (330 ng/mL), methamphetamine (3,500 and 7,100 ng/mL) and tetrahydrocannabinol (THC) (160 ng/mL). For oxycodone and hydrocodone, the presented DWI/DUID cases exceeded 91% and 96% of postmortem concentrations, respectively. The 7,100 ng/mL methamphetamine DWI/DUID result was greater than 98% of postmortem cases. The presented DWI/DUID concentrations were higher than all but one postmortem case for clonazepam and higher than all postmortem cases for THC. This study demonstrates that extremely high drug concentrations in DWI/DUID casework blur the line between therapeutic/recreational and toxic concentrations.