Why the Accuracy of Breath and Blood Tests Matters in DWI Cases

A woman I know blew a 0.09 on a roadside breath test outside Houston. The legal limit in Texas is 0.08. She was arrested, charged with DWI, and spent the next several months assuming the number was the case and the case was over.

Her attorney looked at something different. When was the Intoxilyzer 8000 she’d been tested on last calibrated? Who certified the officer who administered the test? Was the required 15-minute observation period actually documented?

The calibration records showed the machine had been flagged for a maintenance issue six weeks before her test. The officer’s certification had lapsed. The observation period wasn’t documented in a way that was defensible.

The case resolved without a DWI conviction.

A 0.09 reading that looks airtight at arrest can look very different when someone examines the conditions under which it was produced.

What Breath Tests Actually Measure

The Intoxilyzer 8000 is Texas’s standard evidentiary breath testing device. It doesn’t directly measure alcohol in your blood. It measures alcohol in your breath and uses a conversion formula to estimate blood alcohol concentration.

That conversion is based on an assumed partition ratio of 2100:1, meaning the device assumes 2100 milliliters of breath contains the same amount of alcohol as 1 milliliter of blood. The actual partition ratio varies from person to person and ranges roughly from 1700:1 to 2400:1 across the population. Someone whose actual ratio is 1700:1 will register a higher BAC on a breath test than their actual blood alcohol level because the machine is applying a ratio that doesn’t match their physiology.

This isn’t a minor theoretical concern. It’s a documented source of variation that can produce readings above the legal limit for people whose actual BAC is below it. Defense attorneys retain forensic toxicologists to address this in cases where the reading is close to the 0.08 threshold.

The machine itself has to work correctly.

Breath testing devices require regular calibration using reference samples of known alcohol concentration. If the calibration is off, the readings are off. Texas requires Intoxilyzer 8000 devices to be calibrated and maintained according to specific protocols, and those maintenance records are discoverable in a DWI case.

A machine that hasn’t been properly calibrated, that shows variance from reference standards outside acceptable limits, or that has a maintenance history showing recurring issues produces results that can be challenged in court. These records exist. They get subpoenaed. What they show either supports the reliability of the reading or creates legitimate questions about it.

The officer administering the test also has to be certified. Certification lapses. Officers who administered tests while their certification was expired have produced results that were excluded from evidence in Texas courts. Checking the certification status of the officer who tested you sounds like a minor procedural point. In practice it’s a document check that takes an hour and occasionally ends a prosecution.

The 15-Minute Observation Period

Texas requires that a subject be continuously observed for 15 minutes before an evidentiary breath test. The purpose is to ensure the subject doesn’t ingest anything, belch, or regurgitate during that window, all of which can introduce mouth alcohol that doesn’t reflect actual BAC and can produce artificially elevated readings.

The observation period has to be actual observation. An officer who was writing paperwork, talking to a colleague, or intermittently monitoring rather than continuously watching during those 15 minutes didn’t complete the required procedure. If the observation period wasn’t documented properly or can be shown to have been inadequate, the breath test result is challengeable.

This matters most in cases where the reading is close to the legal limit. A 0.09 reading produced after an inadequate observation period might represent an actual BAC below 0.08 if mouth alcohol contaminated the sample.

Blood Tests and Their Vulnerabilities

Blood tests are generally considered more accurate than breath tests, which is why many DWI attorneys advise clients to request a blood test rather than a breath test when given the choice. But blood test results have their own categories of challengeable issues.

Chain of custody is the first one. The blood sample travels from the phlebotomist who drew it to a laboratory analyst who tests it, passing through storage, transport, and handling at each step. Every person who had custody of the sample needs to be documented. Breaks in that chain create questions about whether the sample that was tested was the same sample that was drawn, whether it was contaminated during transport, and whether storage conditions affected the result.

Fermentation is a real concern. Blood samples that aren’t preserved with the right chemicals or that are stored improperly can produce alcohol through fermentation, meaning the sample itself can develop alcohol content that wasn’t present when it was drawn. A sample with inadequate preservative levels stored at improper temperature for an extended period before testing can show a higher BAC than the person’s actual blood alcohol at the time of the draw.

The laboratory analyst’s qualifications and the laboratory’s accreditation matter. An analyst whose certification wasn’t current, a lab that failed proficiency testing, or a testing procedure that deviated from accepted protocols all create grounds for challenging the result.

FindLaw’s guide to choosing a DUI/DWI attorney covers what to look for when selecting representation for a DWI case, including the specific experience and technical knowledge that makes a meaningful difference in these cases. DWI defense requires attorneys who understand the science behind breath and blood testing, not just the law, because challenging test accuracy is technical work that a generalist attorney won’t approach the same way.

The technical nature of these challenges is part of why having the right representation matters so much early. Defense experts who examine calibration records, maintenance logs, chain of custody documentation, and laboratory protocols need time to do that work properly. An attorney who gets involved a week before trial is too late to build a technically credible challenge to testing accuracy.

Texas DWI Charges and What’s at Stake

A first-offense DWI in Texas is a Class B misdemeanor carrying a minimum of 72 hours in jail, fines up to $2,000, and license suspension of 90 to 365 days. That’s before mandatory surcharges that can add thousands of dollars over three years, required alcohol education programs, and the ignition interlock device requirement.

A second offense is a Class A misdemeanor. A third offense is a felony.

A DWI conviction also affects auto insurance rates significantly and shows up on background checks in ways that affect employment in certain industries. The collateral consequences follow a conviction for years past the sentence itself.

The testing accuracy questions aren’t abstract. They’re the factual foundation of a prosecution. In a case where the chemical test result is the primary evidence of impairment, challenging the reliability of that result is challenging the primary evidence of the crime.

What Happened at the Stop Also Matters

The DWI case doesn’t start with the breath test. It starts with the traffic stop, continues through the field sobriety tests, and reaches the breath or blood test as a final step.

If the stop itself was unlawful, everything that came from it, including the chemical test result, is potentially suppressible as fruit of the poisonous tree. If the field sobriety tests were administered incorrectly or under conditions that affected their reliability, they can be challenged. If there were medical conditions that affected the driver’s performance on those tests, that context matters.

The chemical test result has to be evaluated in context. A 0.09 reading from a properly maintained device, administered by a certified officer after a documented 15-minute observation period, following a lawful stop and correctly administered field sobriety tests, is much stronger evidence than the same number produced under questionable circumstances. Both look identical on the charging document. They’re not the same case.

If you’re facing DWI charges in Houston and a breath or blood test is part of the prosecution’s evidence, the accuracy and reliability of that test deserves examination before you decide how to handle the case.

A DWI defense attorney in Houston Texas with specific experience in challenging chemical test evidence understands how to obtain and analyze calibration records, maintenance logs, and chain of custody documentation, and how to retain the forensic experts needed to evaluate whether the results are defensible. Review the LawInfo profile for background on the firm before reaching out.

The woman outside Houston didn’t contest that she had been drinking. She contested whether the number on the machine accurately reflected her BAC at the time of the test, given the machine’s maintenance history and the officer’s lapsed certification.

Those aren’t excuses. They’re the specific legal and scientific questions that determine whether the prosecution can prove its case with the evidence it has. The answer in her case was no.

A number that looks final on a police report is sometimes the beginning of a technical analysis, not the end of one.