Policing the State’s DUI Science

A breath-testing machine produces a number. A forensic laboratory issues a report. The prosecution presents those results as evidence of intoxication or impairment. But how reliable are those results, and what do they actually establish?

Breath-testing instruments and forensic laboratories can produce valuable scientific evidence. But science is only as reliable as the methods, equipment, procedures, and assumptions on which it depends. When the government uses scientific evidence to prosecute someone for a crime, that evidence deserves careful examination.

At the Dalton Law Office, PLLC, we do not place blind faith in technology, particularly when our clients’ freedom, livelihoods, and reputations are at stake. Ms. Dalton has spent years examining the scientific evidence used in Washington DUI prosecutions and litigating problems with testing procedures and forensic conclusions. We believe that careful investigation and a working understanding of the underlying science are essential parts of effective DUI defense.

Washington Breath-Testing Instruments: How Reliable Are the Results?

Washington law enforcement agencies use breath-testing instruments approved by the State Toxicologist, including the Dräger Alcotest 9510 and DataMaster instruments. These machines analyze alcohol in a person’s breath rather than directly measuring the alcohol concentration in their blood.

Although the instruments are designed to produce reliable measurements when properly operated and maintained, their accuracy depends on several factors, including the testing process, the condition and maintenance of the instrument, the performance of required checks, and compliance with established procedures.

A printed result may appear precise and conclusive. But precision in the number displayed does not, by itself, establish the accuracy of the measurement or the reliability of the process that produced it.

We examine the instrument’s testing history, inspection and maintenance records, calibration and accuracy checks, testing procedures, and other information that may bear on the reliability of the State’s evidence.

Breath-Testing Procedures and the Fifteen-Minute Observation Period

Washington law establishes specific foundational requirements for the admission of evidentiary breath-test results. These include requirements concerning the qualifications of the operator, a fifteen-minute pretest observation period, foreign substances in the person’s mouth, internal and external instrument checks, and agreement between two separate breath samples.

These procedures are not meaningless formalities. They are intended to help ensure that the testing process produces reliable evidence.

Even seemingly minor details may become important. Did the officer follow the required observation procedures? Were there circumstances that could have affected the breath sample? Did the instrument perform its required checks? Do the testing records support the officer’s account? Were there irregularities in the equipment’s maintenance or operation?

Washington courts continue to examine these questions. In State v. Sliger (2026), the Washington Supreme Court considered whether residual chewing tobacco in a driver’s mouth affected the admissibility of a Dräger breath test. Although the Court upheld admission of the results on the record presented, the litigation illustrates how particular details of the testing procedure can raise substantial questions of law and scientific reliability.

We examine those details because the reliability of scientific evidence is not something that should simply be presumed.

Blood Testing, Toxicology, and Forensic Laboratory Evidence

DUI investigations may also involve blood samples submitted for laboratory analysis. Blood testing presents its own scientific and evidentiary issues, including the collection and preservation of samples, chain of custody, analytical methods, laboratory quality controls, measurement uncertainty, and interpretation of the reported results.

The detection of alcohol or drugs is only part of the inquiry. Depending on the allegations, the timing of the sample, the substance involved, its concentration, the person’s physiology, and other evidence may also become significant.

Drug DUI cases can be particularly complicated. The presence of prescription medications or other substances in a blood sample does not necessarily establish that they impaired a person’s driving. We examine whether the scientific evidence actually supports the conclusions the prosecution seeks to draw.

For more information about these issues, see our Drug, Marijuana, and Prescription Medication DUI Defense page.

Admissibility Is Not the Same as Reliability

Washington law recognizes an important distinction between whether scientific evidence may be admitted in court and how much weight that evidence deserves.

Under RCW 46.61.506, the State may establish the statutory foundation necessary for admission of a breath test. But the law expressly preserves the defense’s ability to challenge the accuracy and reliability of the test, the functioning of the instrument, and its maintenance procedures. Once the statutory foundation for admission has been established, those challenges may remain relevant to the weight the jury gives the evidence.

That distinction matters. The fact that a breath-test result is admitted into evidence does not make it scientifically unassailable.

Our approach includes careful review of testing records, examination of scientific literature and methodologies, consultation with qualified experts when appropriate, and litigation of evidentiary issues through pretrial motions, hearings, and trial.

We also recognize that not every testing irregularity renders evidence inadmissible or unreliable. The task is to identify problems that actually matter, understand their legal and scientific significance, and present them effectively.

Looking Behind the Laboratory Report: What the State’s Own Records Can Reveal

A forensic toxicology report tells only part of the story. The Washington State Patrol maintains extensive technical records, including current and archived testing procedures, quality assurance manuals, laboratory protocols, measurement uncertainty calculations, and information about the qualifications of forensic personnel. These materials can help us evaluate whether testing was performed properly and whether the reported conclusions are scientifically supported.

Perhaps even more revealing are the State Patrol’s own notifications concerning laboratory problems and changes in testing practices. Its publicly available records include notifications concerning evidence-storage refrigerator outages and lists of potentially affected cases, blood-collection vial advisories, changes to THC measurement uncertainty, environmental testing results, and potential impeachment disclosures involving methamphetamine testing and discrepancies identified in laboratory results. These are not hypothetical issues invented by defense attorneys. They are matters the State Patrol itself has documented and disclosed.

Depending on the case, we may also examine underlying analytical data, chromatograms, calibration and quality-control records, chain-of-custody documentation, and the laboratory’s historical procedures. Not every irregularity affects a particular case, but determining whether one does can be critical.  In appropriate cases, we recommend retaining independent forensic experts to examine the State laboratory’s work or the breath-testing evidence. We work closely with those experts to identify weaknesses in the prosecution’s scientific conclusions and develop persuasive challenges to the evidence against our clients.

A laboratory report is the conclusion of a scientific process, not a substitute for examining that process. We believe the State’s evidence deserves that scrutiny.

For additional information, see the Washington State Patrol Toxicology Laboratory’s technical records and notifications.

Challenging the State’s Science in Washington DUI Cases

We are not interested in scientific evidence as theater. We want to know what the evidence can actually prove, what it cannot prove, and whether the prosecution is asking a judge or jury to accept conclusions that go beyond what the science supports.

When testing procedures are questionable, instruments malfunction, laboratory methods are inadequately supported, or expert conclusions exceed the available evidence, we are prepared to challenge them.

Dalton Law Office represents clients facing DUI charges throughout Southwest and Western Washington, including Clark, Cowlitz, Skamania, Pacific, Thurston, Mason, and Jefferson Counties.

If you are facing DUI charges, call 360-213-0013 to speak with Ms. Dalton or another attorney at Dalton Law Office, PLLC, or to schedule a consultation. For urgent matters outside regular business hours, our recorded telephone message provides emergency contact information.