Drug Driving Blood Tests: How Accurate are They and Can the Result Be Challenged?

How Drug Driving Blood Tests Work

Roadside screening helps police identify whether a driver may have taken drugs. A blood sample is then used to establish whether a specified drug exceeds its legal limit.

Seventeen drugs are listed in the relevant regulations, each with its own threshold.

Very low limits apply to several illegal drugs. For example, cannabis has a limit of 2 micrograms per litre of blood, while cocaine has a limit of 10 micrograms per litre.

Higher limits apply to some prescription medicines, including diazepam and temazepam. A statutory medical defence may also apply where prescribed medication was taken in accordance with medical advice.

After arrest, an appropriately qualified healthcare professional takes the blood sample. It is divided, allowing the driver to request a portion for independent testing.

The prosecution’s portion is sent to a forensic laboratory, where the concentration of any specified drug is measured.

Roadside Screening vs Evidential Blood Tests

A roadside drug test is a preliminary screening tool. Approved devices generally screen for cannabis and cocaine, but they do not establish the concentration of either drug in the blood.

A positive result may provide grounds for arrest and further testing. However, the laboratory blood result provides the main evidence for a specified-limits prosecution.

This distinction matters when challenging the evidence. Questioning the roadside result alone may not undermine a case supported by a properly collected and analysed blood sample.

How Accurate Are Drug Driving Blood Test Results?

Forensic laboratories must follow strict testing and quality standards, including relevant accreditation requirements and the Forensic Science Regulator’s Code of Practice.

However, several factors can affect a result:

• Sample handling: Incorrect labelling or gaps in the chain of custody may raise doubts about the sample’s identity.
• Storage conditions: Delays or unsuitable temperatures can affect the stability of certain drugs in blood.
• Laboratory errors: Problems with equipment calibration, analytical methods or quality controls may affect accuracy.
• Measurement uncertainty: Every laboratory measurement has a recognised margin of uncertainty.

Measurement uncertainty is particularly important where the reported drug concentration is close to the legal limit.

For example, a borderline result may require closer examination to establish whether the prosecution can prove that the threshold was exceeded.

These issues do not automatically invalidate a blood test. A forensic expert must assess whether a particular problem could have affected the result.

When Can a Drug Driving Result Be
Challenged?

A drug driving result may be challenged where there are genuine concerns about how the sample was collected, handled or analysed.

A solicitor may examine whether the correct collection procedures were followed, whether the sample was properly identified and whether laboratory records support the reported concentration.

The main evidence includes custody records, medical notes, sample continuity documents and the laboratory’s certificate of analysis.

Where necessary, an independent expert can review the testing methods, measurement uncertainty and relevant quality control records.

Independent Blood Testing

Drivers can request a portion of the divided blood sample for independent analysis.

An accredited forensic laboratory can test the retained sample and provide a separate concentration measurement. This may be useful where the prosecution’s result is close to the legal limit or there are concerns about the original analysis.

Timing matters because drugs and their metabolites can degrade during storage. Delaying independent testing may make the results less useful.

If you have retained a sample, seek legal advice promptly about suitable storage and whether independent analysis is appropriate.

A second result does not automatically overturn the prosecution’s evidence. Any differences must be assessed by a forensic expert.

Penalties for Drug Driving

Drug driving is a criminal offence normally dealt with in the magistrates’ court.

The maximum penalty for the specified-limits or impairment offence is six months’ imprisonment, an unlimited fine or both.

A conviction also normally carries a mandatory driving disqualification of at least 12 months.

Where the driver has a qualifying previous drink or drug driving conviction within ten years, the minimum ban increases to three years.

The court considers the circumstances of the offence, including the drug concentration, driving behaviour, previous convictions and any aggravating or mitigating factors.

In limited cases, special reasons connected to the offence may justify a shorter disqualification or no ban.

A conviction may also affect employment, insurance and travel to certain countries.

How a Solicitor Can Help with a Drug Driving Case

A specialist motoring defence solicitor can review the full body of evidence in a drug driving case from the outset: the custody record, the medical notes, the certificate of analysis, the continuity documents and any available information about the laboratory’s accreditation and quality controls. 

This review identifies whether there are procedural or scientific issues worth pursuing and informs advice on the realistic strength of any potential challenge to the drug driving evidence.

Where a retained sample exists, a solicitor can advise on the urgency of commissioning independent analysis and arrange for it to be carried out by a suitably accredited forensic laboratory. 

Where measurement uncertainty is potentially relevant to the drug driving charge, the solicitor can instruct a defence expert to examine the prosecution’s analytical data and produce a report that can be used at trial or in discussions with the prosecution.

Early advice matters in drug driving cases 

The window for independent analysis is finite, some procedural challenges require prompt action before evidence is lost, and decisions about plea and trial strategy are better made with a full understanding of the evidence rather than after it has been formally served.

Speak to our Motoring Offences solicitors for advice about your drug driving position and next steps.