A Norwegian registry study found a modest positive association between blood amphetamines and physician-assessed impairment among suspected drugged drivers. It did not establish a methamphetamine-only impairment threshold or a rule that diagnoses every individual driver.
Gustavsen and colleagues examined 878 registry cases in which amphetamine and/or methamphetamine were the only drugs detected in the blood samples. Most drivers had been apprehended after conspicuous driving or accidents. This is observational human primary research in a selected forensic population, not randomized dosing and not a sample of all road users.
Amphetamine alone occurred in 691 cases, methamphetamine alone in 31, and both in 156. The authors added amphetamine and methamphetamine concentrations and called the sum blood amphetamines. A conclusion about that combined measure must not be relabeled a pure methamphetamine concentration-response result.
The narrative reports 643 drivers judged impaired and 235 judged not impaired. Increasing concentration categories were associated with a greater proportion classified impaired, with a plateau around the 0.27–0.53 mg/L category. The relationship persisted after adjustment for age and gender. However, median concentrations in the impaired and non-impaired groups did not differ significantly.
These findings are compatible: a category-based regression and a comparison of group medians are different analyses. The plateau also does not identify a concentration at which everyone is impaired. Younger drivers were more often judged impaired at similar concentrations, further limiting a concentration-only interpretation.
The clinical test of impairment combined observations and performance tasks evaluated by physicians. It was not an observed crash-risk calculation. The authors acknowledged variation in physicians’ knowledge and nonstandardized examination conditions, along with uncertainty about external validity for actual driving safety.
The study lacked dose, administration time, use pattern and prior experience. Fatigue after repeated use, tolerance and other unmeasured characteristics could affect the observed association. The paper also contains a discrepancy between the impaired count in its narrative and Table 1; the narrative total should not silently be presented as an independently reconciled table count.
The paper supports an association in the selected population and contradicts a categorical claim that concentration never relates to clinical findings. It also contradicts treating its grouped results as a validated individual cutoff. Identify the actual analytes, specimen, sampling time and observed signs before applying it to a case.
Our current-user simulator analysis examines a different population and outcome. Different designs can produce different associations without one erasing the other.
Read the original investigation: doi:10.1016/j.aap.2005.11.005.