Roadside oral-fluid testing came to San Diego County in 2017, when SDPD debuted mouth-swab drug screening at a downtown DUI checkpoint — and the technology has been evolving ever since. Here’s the key thing to understand: a swab can suggest a drug is present, but it cannot prove you were impaired. New technology doesn’t change your defense rights. San Diego DUI attorney Joshua Price explains what these tests are, what they can and can’t show, and why a positive swab is the start of the conversation — not the end of your case.
An oral-fluid (saliva swab) test is a roadside screening tool: an officer collects a saliva sample from your mouth, and a machine analyzes it for the presence of drugs. San Diego got its first look at the technology in March 2017, when the San Diego Police Foundation donated two Dräger DrugTest 5000 machines — about $6,000 each — to the San Diego Police Department. They debuted at the St. Patrick’s Day DUI checkpoint downtown, the first technology of its kind used in San Diego County. San Diego police have previously used oral-fluid screening devices, including the Dräger DrugTest 5000, but current deployment and device counts should be confirmed for each case.
At the time, the swab devices were already in use by police departments in more than a dozen states, and were expected to become more popular as marijuana legalization spread. The 2017-era machine — about the size of a mini bookshelf stereo — screened for the presence of seven drugs: marijuana, cocaine, opiates, methamphetamine, amphetamine, methadone, and benzodiazepines.
Since then, the technology has kept evolving. California has continued exploring breath and oral-fluid testing — the CHP has run an Oral Fluid Pilot Program — but as of our 2025 review of marijuana DUI law, these tools were not widely accepted in California courts. Oral-fluid results are generally best described as roadside screening or investigative evidence, not a substitute for properly collected evidentiary blood testing, unless admissibility is established in the specific case. If you’ve been swabbed at a San Diego stop, ask about the current state of the law: what a device can screen for, and what a court will actually accept, are two very different questions.
Every stop is different, and the exact sequence depends on the agency, the officer, and the current state of the technology. But based on how marijuana DUI investigations work in California and how the swab devices were introduced here, a stop involving oral-fluid screening generally moves through stages like these. In broad terms, that sequence tends to run: a traffic stop → officer observations, field sobriety tests, and a PAS or drug-recognition investigation → a possible request for an oral-fluid swab as a screening tool → an arrest decision → an evidentiary chemical test, usually a blood draw in drug-DUI cases.
It starts like any DUI investigation — a traffic stop or a sobriety checkpoint. San Diego’s swab devices made their debut at a downtown DUI checkpoint in 2017. (Where and how often swabs are used today is a question for Josh — technology and deployment keep evolving.)
The officer looks for signs associated with drug use: the odor of cannabis, red or watery eyes, slowed speech, or marijuana paraphernalia in the vehicle. These subjective observations often carry more weight in a marijuana DUI case than any test number.
Coordination tests like the walk-and-turn and one-leg stand were designed to detect alcohol impairment — and are less reliable for cannabis. Read why field sobriety tests matter less than you think.
The officer collects a saliva sample and the device screens it for the presence of drugs — the 2017-era machines screened for seven. Whether an oral-fluid swab is requested before or after arrest, and whether it is presented as voluntary, depends on agency protocol and case facts. Ask counsel before assuming a roadside swab carries the same consequences as a post-arrest implied-consent test — see “Your Rights” below.
Whatever the screen shows, it shows only that a drug is present in your system. The device does not read the level of intoxication — and in California, impairment cannot be determined by chemical test results alone.
For drug levels, a blood test is required — that was true when the swab machines arrived in 2017 and blood remains the most common chemical test in marijuana DUI cases. A Drug Recognition Expert (DRE) evaluation — a 12-step protocol by a specially trained officer — may also be part of the investigation.
A swab screen can indicate that a drug — THC, for example — is present in your oral fluid. That’s the machine’s whole job. When SDPD’s devices arrived in 2017, that was the explicit limitation reported: the device does not read the level of intoxication. Drivers would have to take a blood test for that information.
California law doesn’t punish having THC in your system — it punishes driving while impaired. Under Vehicle Code § 23152(f), prosecutors must prove that you consumed a drug and that, as a result, your mental or physical abilities were so impaired that you could no longer drive with the caution of a sober person, using ordinary care, under similar circumstances. Impairment is based on performance and observation — not just numbers on a test.
With marijuana, the gap between presence and impairment is especially wide. THC is fat-soluble and can remain detectable long after its acute effects subside. And THC affects individuals differently: a casual user may be impaired at a low concentration, while a frequent user may show higher THC with little observable impairment. Some states set numeric thresholds (such as 5 ng/mL of THC in blood), but California has no statutory cutoff — there is no THC equivalent of the 0.08% BAC limit.
“It is not possible to determine marijuana impairment by chemical test results alone.”
— Joshua Price, “DUI and Marijuana: What Is the Law and How Are THC Levels Measured in California?” (2025)
That gap is where the defense lives. A positive swab may tell a story about the past several hours — or days. It does not, by itself, tell a jury anything about how you were driving.
Swab shows: presence of drugs (the 2017 device screened for seven)
Swab doesn’t show: level of intoxication or impairment
For levels: a blood test is required
To convict: the DA must prove actual impaired driving under VC § 23152(f)
THC limit in CA: none — no per se threshold exists
Swabbed at a stop? (858) 289-2624
Oral-fluid testing sits at a moving intersection of technology and law, and the honest answer to several of the questions below is: it depends on the current state of the law — ask. Roadside oral-fluid testing is generally best understood as a screening or investigative step, distinct from the compelled, post-arrest chemical testing governed by California’s implied-consent rules. These are exactly the questions Josh answers in a free consultation, with the current rules in hand.
This is the first question people ask, and it deserves a current, careful answer rather than a generic one. How a roadside swab is legally characterized — and what declining one does or doesn’t cost you — can differ from the rules for post-arrest chemical testing, and the technology’s legal status has been evolving. Refusing a roadside screening request may affect the officer’s investigation, but the most serious statutory refusal consequences generally attach to post-arrest chemical testing under implied-consent rules — and the case facts matter. Before you rely on anything you read online (including this page), ask about the current state of the law. For how California’s implied-consent rules treat post-arrest chemical tests, see our test refusal guide.
No — this one the sources answer directly. The screening device reads presence, not level of intoxication, and California law requires the prosecution to prove actual impairment: that you could no longer drive with the caution of a sober person using ordinary care. It is not possible to determine marijuana impairment by chemical test results alone.
No. A positive screen is one piece of an investigation, not a verdict. Prosecutors in marijuana DUI cases often rely heavily on officer testimony, driving patterns, and physical symptoms — sometimes more than numeric test results — precisely because the numbers don’t map cleanly onto impairment. Every link in that chain can be examined and challenged.
Typically the investigation continues — in 2017 the reported next step for measuring levels was a blood test, and blood remains the most common chemical test in marijuana DUI cases. A roadside swab result is generally best understood as screening only, or as support for probable cause — not as standalone proof of impairment — and its trial admissibility and weight depend on foundation, device reliability, protocol, and court rulings. Ask about how it is being used in your case.
San Diego wasn’t a bystander to this technology — it was an early adopter. SDPD received the county’s first oral-fluid screening machines in 2017, donated by the San Diego Police Foundation, and put them to work at a downtown DUI checkpoint the same week. When a police department invests in drug-screening technology and debuts it at a checkpoint, that tells you where enforcement attention is going.
Legal recreational cannabis makes the stakes wider. Many people are surprised to learn that you can be arrested for driving under the influence of marijuana even as a legal, recreational user — and unlike alcohol, there’s no bright-line number that separates legal driving from a DUI. That combination — evolving screening technology, no per se limit, and enforcement that leans on subjective observations — is exactly why a swab-initiated stop needs a defense lawyer who understands both the law and the science.
Joshua Price has defended San Diego DUI cases through the rise of drug-DUI roadside screening and can evaluate whether testing was properly requested, administered, and interpreted — and the defense fundamentals haven’t changed: challenge the reliability of the testing and its handling, highlight the absence of a reliable impairment threshold, and scrutinize officer training and procedures. The same fundamentals anchor every DUI defense he handles.
“The science of measuring cannabis impairment is evolving, which means there may be opportunities to challenge the evidence in a marijuana DUI case — whether by disputing test reliability, contesting observations, or identifying procedural errors.”
— Joshua Price, San Diego DUI Attorney
The machines SDPD received in 2017 — Dräger DrugTest 5000 units — screened oral fluid for the presence of seven drugs: marijuana, cocaine, opiates, methamphetamine, amphetamine, methadone, and benzodiazepines. The drug panel depends on the device in use, so the exact panel today may differ — ask about the current equipment in your case.
No. The device reads only the presence of drugs — it does not measure the level of intoxication. Measuring levels requires a blood test, and even a blood number doesn’t settle the question: in California, impairment cannot be determined by chemical test results alone.
No. Some states set numeric thresholds — for example, 5 ng/mL of THC in blood — but California has no statutory cutoff. Under Vehicle Code § 23152(f), prosecutors must prove you consumed a drug and that your mental or physical abilities were actually impaired: that you could no longer drive with the caution of a sober person, using ordinary care, under similar circumstances.
As of our 2025 review, breath and oral-fluid drug tests were still emerging tools — being explored, including through a CHP Oral Fluid Pilot Program, but not widely accepted in California courts. Even where such results are allowed in, they are not automatically accepted as courtroom proof of impairment — they remain subject to challenge on reliability, protocol, timing, and foundation. The law here is evolving, so what a court will accept in your case is a question to ask about the current state of the law.
Yes. Because California ties marijuana DUI to proven impairment rather than a number, these cases turn on evidence that can be challenged: the accuracy of the testing and how samples were handled, the absence of a reliable impairment threshold, the officer’s training and procedures, and the subjective observations the prosecution leans on. The evolving science is itself a defense opportunity.
A positive screen is not a conviction — and the law around oral-fluid testing is still evolving. Call for a free consultation, explain the particular facts of your stop, and Josh will tell you where the current law stands and what to do next. We are receiving calls 24/7.
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