Feasibility Assessment of Chemical Testing for Drug Impairment: Final Report

Willette, Robert E. · 1985 · ROSA P / United States. National Highway Traffic Safety Administration

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Summary

This 1985 report by the National Highway Traffic Safety Administration (NHTSA) assesses the feasibility of using chemical tests on body fluids to detect drug-impaired driving. The study focuses on five substances: marijuana, secobarbital, diazepam, diphenhydramine, and methaqualone. The primary objective was to determine if specific concentrations of these drugs in urine or saliva could serve as presumptive indicators of impairment, thereby justifying further blood analysis. The research relied on a review of existing pharmacokinetic data and behavioral impairment studies, utilizing mathematical modeling to correlate drug levels in body fluids with blood concentrations and subsequent performance deficits. The methodology employed standard pharmacokinetic analysis to relate urine and saliva concentrations to blood levels, which were then linked to measures of behavioral impairment from laboratory tasks. For marijuana, the study analyzed THC and its metabolite (THC-9-acid) in urine and saliva, drawing on extensive data from NHTSA and NIDA studies. For the other four drugs, the analysis focused primarily on saliva concentrations, as urinary metabolite levels often persisted beyond the period of demonstrable impairment, making urine less reliable for detecting acute impairment. The study simulated concentration-time curves to identify threshold levels that would predict impairing blood concentrations with reasonable probability. The findings indicate that marijuana is the only drug for which sufficient data support the use of urine testing as a screening tool. A urine threshold of 100 ng/ml for THC metabolites provides better than a 50% probability of detecting blood THC levels associated with impairment. For the other drugs, saliva appears more promising for presumptive screening. Proposed saliva thresholds include 0.5 µg/ml for secobarbital, 5 ng/ml for diazepam and its primary metabolite, 180 ng/ml for diphenhydramine, and 150 ng/ml for methaqualone. However, the report cautions that these are preliminary estimates based on limited existing data. Blood remains the only body fluid capable of reliably relating drug levels to impaired driving, though current testing is largely restricted to stationhouse or emergency room settings. The significance of this report lies in its identification of potential screening thresholds for drug-impaired driving, highlighting the utility of urine tests for marijuana and saliva tests for other sedatives and stimulants. It underscores the need for further prevalence data and experimental studies to validate these thresholds. The report concludes that while chemical testing for drug impairment is feasible in principle, the current state of knowledge limits its practical application, necessitating more robust data to establish reliable presumptive tests for field use.

Key finding

Urine testing for THC-9-acid at or above 100 ng/ml provides better than a 50% probability of detecting blood THC levels associated with impairment, while saliva thresholds of 0.5 ug/ml for secobarbital, 5 ng/ml for diazepam, 180 ng/ml for diphenhydramine, and 150 ng/ml for methaqualone are proposed as presumptive indicators for blood analysis.

Methodology

modeling

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

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• cannabis
• illicit drugs