Positive Saliva Drug Test: How It Is Challenged Under Art. 379.2 CP

A positive saliva test at a roadside check triggers immediate alarm, but it is rarely the conclusive proof many drivers fear. The device the officer places in the mouth is a preliminary screening test: it indicates possible drug use, but it neither confirms the substance with laboratory rigour nor measures the impairment of driving required by Article 379.2 of the Criminal Code (CP). Between that first roadside positive and a criminal conviction lies a path full of safeguards, technical requirements and driver rights which, used properly, make it possible to challenge the evidence. This article explains how two-stage detection works, why a positive does not amount to a crime, and what the real lines of challenge are. You can read more on our page about a positive saliva drug test.

What the saliva test is and what weight it carries

The saliva test is a screening device that reacts to the possible presence of certain substances in saliva. Its purpose is to single out the drivers who must move on to a second verification stage, not to declare anyone guilty. In evidentiary terms, therefore, its result is preliminary: a lead that opens the inquiry, not conclusive evidence for the prosecution.

Roadside drug detection in Spain is designed, precisely, as a two-stage system: a preliminary roadside test (the saliva test) and a later confirmatory laboratory analysis. This structure, governed by the traffic legislation (Articles 14 to 14 quater of Royal Legislative Decree 6/2015, the Traffic Act) and by the General Traffic Regulations, is the key to understanding why the first positive, on its own, does not close the case.

The two-stage detection procedure

It is worth separating clearly what each stage contributes:

• Preliminary stage (roadside saliva test): the officer collects a saliva sample and the device returns a qualitative result —positive or negative— for certain substances. It is fast but imprecise: it neither quantifies the amount nor rules out interference.
• Confirmation stage (laboratory): the sample is analysed using highly reliable techniques, such as chromatography and mass spectrometry, in an accredited laboratory. This stage does allow the substance to be identified and quantified rigorously.

The practical consequence is direct: when the prosecution seeks to sustain the offence solely on the screening positive, without the confirmatory analysis, the evidence has a fundamental weakness. The technical reliability of a screening test is not that of a laboratory, and the defence can insist that any conviction rest on the analytical confirmation rather than on an indicative reading.

Presence versus influence: the core of Art. 379.2 CP

Article 379.2 CP does not punish having drugs in your body, but driving under the influence of toxic drugs, narcotics or psychotropic substances. That word —influence— makes all the difference. Unlike alcohol, for drugs the Criminal Code sets no automatic objective threshold above which driving is an offence "in any event". The only route to the offence is proving actual impairment of driving.

This is where the decisive distinction between two often-confused concepts comes in:

• Presence: the mere analytical finding of the substance or its metabolites in the body. This is what the test detects.
• Influence: the actual impairment of the faculties needed to drive at the moment of the check. This is what the offence requires.

The reason the two do not coincide is pharmacological: the substance and its metabolites remain detectable long after the psychoactive effect has worn off. In habitual users, a positive may appear days after the last use, when the person is driving in perfect condition. Settled Supreme Court case law therefore requires more than the positive: proof of actual impairment.

False positives and residual metabolites

Saliva screening devices are not infallible. There are several situations in which a positive does not reflect criminally relevant use:

• Prescribed medication: certain drugs can trigger the test or show up in the analysis; a medical prescription allows the result to be put in context and the criminal reproach ruled out.
• Legal CBD products: some lawfully marketed cannabidiol products may contain traces that interfere with an indicative test.
• Cross-reactions: a screening test can confuse structurally similar substances, producing a false positive that only the laboratory can disprove.
• Residual metabolites: detecting traces of use from days earlier proves past use, not that someone drove under its influence.

That is why the test result must always be cross-checked against the confirmatory analysis, and that analysis, in turn, weighed together with the rest of the evidence. An isolated figure, without clinical context or signs of impairment, does not establish influence.

It is also worth remembering that the screening device only signals that a substance may be present; it does not measure how much, when it was taken, or whether it was still acting at the wheel. A driver who lawfully takes prescribed medication, or who used a legal product the night before, may face exactly the same roadside positive as someone genuinely impaired. The job of the defence is to make that difference visible to the court, with documentary and expert support rather than mere assertion.

Chain of custody and verification of the device

The validity of the evidence also depends on its formal regularity. Two fronts are especially fertile ground for a challenge:

• Chain of custody: the saliva or blood sample must be collected, identified, preserved and transferred to the laboratory without any break. Any undocumented gap —poor labelling, inadequate refrigeration, time lapses— casts doubt on whether what was analysed is what was collected, and weakens the result.
• Device approval and verification: the screening device must be in proper technical condition. The defence can request the type-approval, calibration and maintenance records of the device used.

When these safeguards fail, the evidence may be declared invalid, whatever the result. The challenge is not about "denying the obvious", but about requiring that what reaches trial meets the legal conditions needed to ground a conviction.

Right to a confirmatory test and toxicological expert evidence

Faced with the preliminary result, the driver has defence tools that should be activated as soon as possible:

• Confirmatory blood test: after the saliva positive, a blood analysis can be requested at a healthcare centre. It is a driver's right; its refusal or irregularities in the sampling are grounds for challenge.
• Private toxicological expert: a defence expert can review the official analysis, assess the correlation between the amount found and possible impairment, and question the laboratory methodology.
• Examination of the police report: checking whether the officers describe real signs of impairment or merely record the positive. Without signs and without erratic driving, influence remains unproven.

The combination of these elements often makes it possible to separate the positive from impairment and steer the conduct back to a simple administrative traffic penalty, with no criminal record. Two parallel tracks should never be confused: the administrative penalty punishes the mere presence of the drug and brings a fine and loss of points, while the offence under Article 379.2 CP punishes driving under the influence and carries imprisonment, a fine or community service, plus disqualification and a criminal record. Where impairment is not proven beyond reasonable doubt, the same roadside check can end as nothing more than a traffic fine.

Refusing the test: the offence under Art. 383 CP

Some drivers believe that refusing the saliva test is the "safe" way out: with no substance measured, they reason, there is no evidence. It is exactly the opposite. Refusing to undergo the tests lawfully required by the officer is a stand-alone offence, under Article 383 CP, punishable by six months to one year in prison and disqualification from driving for more than one and up to four years.

Its penalty is usually heavier than Article 379.2 CP itself and, moreover, does not allow a fine as an alternative to imprisonment. The offence is complete on the refusal alone, regardless of whether any influence was later measured. For it to exist, the officer's request must be lawful and the driver must have been informed of the duty to take the test and of the criminal consequences of refusing. Refusing, in short, does not reduce liability: it aggravates it. For other road offences you can visit our practice area on road safety offences.

Specialist criminal defence in road safety

A positive saliva test is not a conviction. It is the starting point of a procedure in which the reliability of the screening, the laboratory confirmation, the chain of custody and, above all, the proof of actual impairment can each be contested. Acting early, keeping the documentation of the check and examining the police report and the information record usually make the difference between an offence and a simple fine.

Alonso Sala is a firm devoted exclusively to criminal law, based at Calle Velázquez 27 in Madrid and acting throughout Spain. We study every traffic check, the device used and the proof of influence before taking any decision on defence or a guilty plea.