A Cochrane review [Abstract] 1 included two interrupted time-series studies conducted a) in the companies in USA and b) on federal injury data. In these studies two interventions of interest were evaluated: mandatory random drug testing and mandatory random and for-cause alcohol testing programmes.
In reanalysis of the data in one study mandatory random and for-cause alcohol testing was associated with a significant decrease in the level of injuries immediately following the intervention (-1.25 injuries/100 person years, 95% CI -2.29 to -0.21) but the intervention did not significantly affect the existing long-term downward trend (-0.28 injuries/100 person years/year, 95% CI -0.78 to 0.21).
Mandatory random drug testing was significantly associated with an immediate change in injury level following the intervention (1.26 injuries/100 person years, 95% CI 0.36 to 2.16) in one study, but in the second study there was no significant effect (-1.36/injuries/100 person years, 95% CI -1.69 to 0.41). In the long term, mandatory random drug testing was associated with a significant increase in the downward trend of injuries in one study (-0.19 injuries/100 person years/year, 95% CI -0.30 to -0.07) and in the downward trend in fatal accidents in another study (-0.83 fatal accidents/100 million vehicle miles/year, 95% CI -1.08 to -0.58).
Comment: High quality interrupted time-series is a feasible study design but a cluster-randomised trial would be the ideal one. The study was downgraded for quality and design (no control).
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