Assessment of Relative Crash Risk Following Marijuana

Studies that have assessed crash responsibility offer more insight into the quantitative relationship between marijuana usage and crash involvement. An excellent review of culpability studies has recently been published (17). The general design of these studies is to compare rates of drug use in at-fault drivers vs no-fault drivers and compute the ratio, with values greater than 1.0 indicating increased rates of risk. The 95% confidence interval is also computed, and when the range includes 1.0, the difference in responsibility rates is not significant at the p = 0.05 level.

In most of these studies, authors validate their data set and methodology by assessing odds ratios for alcohol. The relationship between alcohol and risk of crash involvement has been well established, most famously in the 1960 Grand Rapids Study. In each case the method showed the expected significant relationship at the p = 0.05 (95% confidence interval) level between alcohol positivity and greater odds of crash involvement.

The data from studies that made odds ratio assessments based on the presence of the inactive THC-COOH metabolite uniformly failed to show significant differences at the p = 0.05 level in rates of accident involvement for the drug-positive drivers. This can be rationalized in terms of the fact that the metabolite is inactive and that in most cases urine was being tested. Bearing this in mind, together with the fact that urine can test positive for the metabolite for many hours or even days after the effect has passed, its detection in urine is not a good surrogate for impairment, and the negative findings are not surprising.

Studies assessing crash risk based on parent THC in blood are more informative. One study of 2500 injured drivers (18,19) showed a trend towards increasing odds ratio with increasing THC concentration (although not significant at p = 0.05) and found that culpable drivers had a higher mean THC concentration (p = 0.057). This suggests a dose-dependent increase in risk, with the threshold for significance being somewhere above 2 ng/mL THC. One limitation of the Hunter study is the lack of control of the interval between driving and when the sample was collected. Intervals of an hour or less between the driving and the time the sample was collected would cause appreciable decreases in THC concentration.

In a cohort of 3398 fatally injured drivers (20), the authors avoid this limitation because absorption of THC will stop at the time of death. Those data showed an odds ratio of 2.7 in cases in which THC was detected and 6.6 when the THC concentration was greater than 5 ng/mL.

Several studies have evaluated crash risk in drivers positive for both alcohol and marijuana (THC or THC-COOH). Table 4 shows that irrespective of whether the parent drug or metabolite was measured, when combined with alcohol the odds ratio for crash involvement was between 3.5 and 11.5 (significant in all cases, p = 0.05) and compared to alcohol positive cases was still significant, with an odds ratio of 2.9.

Taken together, these data represent strong evidence for a concentration-dependent (and consequently dose-dependent) relationship between THC and risk of crash involvement and enhanced risk for any use of marijuana when combined with alcohol.

Summary of Odds Ratio of Becoming Involved in Fatal or Injurious Traffic Accidents Under the Influence of Cannabis, Alcohol, or Their Combination as Reported in Culpability Studies

Substance

Authors

Odds ratio

95% CI

Drug-free cases

1.0

Alcohol

Terhune and Fell (21)

5.4*

2.8-10.5

Williams et al. (22)

5.0*

2.1-12.2

Terhune et al. (23)

5.7*

5.1-10.7

Drummer (24)

5.5*

3.2-9.6

Hunter et al. (18)

6.8*

4.3-11.1

Lowenstein and Koziol-Mclain (25)

3.2*

1.1-9.4

Drummer et al. (20)

6.0*

4.0-9.1

THC-COOH

Terhune and Fell (21)

2.1

0.7-6.6

Williams et al. (22)

0.2

0.2-1.5

Terhune et al. (23)

0.7

0.2-0.8

Drummer (24)

0.7

0.4-1.5

Hunter et al. (18)

0.9

0.6-1.4

Lowenstein and Koziol-Mclain (25)

1.1

0.5-2.4

TCH (range: ng/mL)

<1.0

Hunter et al. (18)

0.35

0.02-2.1

1.10-2.0

0.51

0.2-1.4

>2.0

1.74

0.6-5.7

1-100

Drummer et al. (26)

2.7*

1.02-7.0

5-100

6.6*

1.5-28.0

Alcohol/THC or

Williams et al. (22)

8.6*

3.1-26.9

THC-COOH

Terhune et al. (23)

8.4*

2.1-72.1

Drummer (24)

5.3*

1.9-20.3

Hunter et al. (18)

11.5*

4.6-36.7

Lowenstein and Koziol-Mclain (25)

3.5*

1.2-11.4

Significant changes in OR indicated as follows: *<0.05. THC-COOH, 11-carboxy-THC; THC, A9-tetrahydrocannabinol. From ref. 1 .

Significant changes in OR indicated as follows: *<0.05. THC-COOH, 11-carboxy-THC; THC, A9-tetrahydrocannabinol. From ref. 1 .

0 0

Post a comment