What Did Mandatory Roadside Drug Testing Measure in Catalonia?

A roadside survey used mandatory participation to reduce one source of selection bias. Its estimated prevalence still depends on where and when drivers were sampled and on the testing method used.

The population and test behind the estimate

The researchers sampled 521 drivers on non-urban roads in Catalonia in autumn 2014. Their analysis used the initial oral-fluid screening results. Although positive screens led to collection for laboratory confirmation, those confirmation results were unavailable for the study’s observations. The reported overall positive prevalence was 16.4%.

What a positive-prevalence figure cannot establish

The estimate describes screening positivity under that survey design. It cannot establish that the same percentage of drivers were functionally impaired. A case-level opinion requires the actual test and confirmation records, collection time and observations relevant to driving.

Source

Alcañiz et al., PLOS ONE · Full text