The Guide to Type Approval Procedures for Mobile Preliminary Drug Testing Devices used for Transport Law Enforcement in Great Britain contains a description of the requirements to be met for consideration of type approval for screening device. This is found at the following link : www.gov.uk/government/publications/mobile-preliminary-drug-testing-devices
Roadside drug driving screeners provide an indication of delta-9-THC (the main active component in cannabis) present in a motorist’s system. Delta-9-THC metabolite of cannabis can remain detectable for up to several hours.
Following a positive screening result for suspected drug driving, the police obtain two samples of blood for analysis. One sample is provided to the alleged offender, who can submit their sample for independent testing at a laboratory of their own choice. The police submit the other blood sample to an accredited laboratory for analysis in line with guidance on sample storage and delivery.