The Reliability and Limitations of Roadside DrugWipe Screening Devices

Introduction

Roadside Drugwipe devices are used in the UK as preliminary screening tools to detect the presence of specific drugs in an individual's oral fluid. These devices are commonly deployed by police to test for cocaine and THC (the psychoactive component of cannabis). A positive result can serve as justification for a subsequent blood test under Section 5A of the Road Traffic Act 1988, which tests for 17 different controlled substances.

While Drugwipe devices provide a rapid and convenient means of assessing drug use, their reliability is not infallible. Solicitors should be aware of the potential for both false positives and false negatives, as these can have significant implications for individuals accused of drug driving.

Efficacy of Drugwipe Devices

Several studies have examined the accuracy of Drugwipe tests, with mixed findings. Pehrsson et al. evaluated the effectiveness of these devices for roadside screening and found notable discrepancies:

• Of the 28 individuals who tested positive for THC in their blood, 13 tested negative on the Drugwipe – a false negative rate of 46.4%.

• Out of 266 participants, 4 tested positive on the Drugwipe for cocaine, despite having no detectable cocaine or BZE in their blood – a false positive rate of 1.5%.

These figures indicate that Drugwipe tests are not foolproof, particularly in detecting THC, where nearly half of confirmed positive blood samples were missed by the screening device.

Causes of False Positives and False Negatives

False positives and false negatives in Drugwipe testing can arise due to a variety of factors:

False Negatives

• Threshold Sensitivity: The Drugwipe test may not be sensitive enough to detect lower concentrations of THC in oral fluid, even when present in the bloodstream.

• Time Since Use: THC levels in oral fluid decline rapidly after consumption, leading to missed detections if sufficient time has passed.

• Environmental and Physiological Factors: Saliva composition, hydration levels, and recent food or drink intake can all impact the test's ability to detect substances.

False Positives

• Cross-Reactivity: Structurally similar compounds, including certain medications, can sometimes cause the test to incorrectly indicate the presence of an illicit substance.

• Environmental Contamination: Traces of drugs present on surfaces, or in second-hand smoke exposure scenarios, may lead to a false positive result.

• User or Administration Error: Inconsistent sample collection techniques, improper storage of the device, or deviations from manufacturer instructions can also contribute to erroneous results.

Legal Considerations

Drugwipe results are non-evidential—they merely provide an indication that a substance may be present in an individual's system. As such, these preliminary findings should not be considered definitive proof of impairment or drug use.

Given the potential for inaccuracies, factors such as the administration of the test, environmental conditions and the possibility of false positives or negatives should be closely examined.

Conclusion

Roadside Drugwipe devices are a useful tool for law enforcement but are not without limitations. Solicitors handling drug-driving cases should be acutely aware of the potential for false positives and false negatives and challenge reliance on these tests where appropriate. Ultimately, Drugwipe results should be treated as an initial screening step rather than irrefutable evidence of drug use or impairment.

Understanding the science behind these devices, their limitations, and the legal framework in which they operate is crucial for effectively representing clients facing drug-related driving charges.